At the end of a deep breath, about 80% of the lung volume is air, 10% is blood, and only the remaining 10% is tissue. Because this small mass of tissue is spread over an enormous area – nearly the size of a tennis court – the tissue framework of the lung must be extraordinarily delicate. It is indeed remarkable that the substance of the lung manages to maintain its integrity in the face of the multitude of insults that inevitably accompany a lifetime of exposure to ambient air and the complex necessity of keeping air and blood in intimate contact, but separate, for the sake of gas exchange.
Part of this success is undoubtedly attributable to the unique design of the lung, which ensures mechanical stability as well as nearly optimal conditions for the performance of the lung's primary function: to supply the blood with an adequate amount of oxygen even when the body's demands for oxygen are particularly high, as during heavy work.
At total lung capacity, the lung fills the entire chest cavity and can reach a volume, in the adult human, of some 5 to 6 L, largely depending on body size. Upon expiration, the lung retracts, most conspicuously from the lower parts of the pleural cavity, the posterior bottom edge of the lung moving upward by some 4 to 6 cm. This preferential lifting of the bottom edge is caused by retraction of the tissue throughout the entire lung, the surfaces of which are freely movable within the thoracic cavity.
The structural background for this mobility of a healthy lung is the formation, during morphogenesis, of a serosal space that is lined on the interior of the chest wall and on the lung surface by a serosa, the parietal and visceral pleurae, respectively (Fig. 2-1). However, this serosal space is minimal, since the visceral pleura is closely apposed to the parietal pleura, with only a thin film of serous fluid intercalated as a lubricant between the two surfaces.1 Both pleural surfaces are lined by a squamous epithelial layer, often called mesothelium (due to its mesodermal origin), whose surface is richly endowed with long microvilli. The apical microvilli increase the surface area available, suggesting that pleural mesothelial cells are capable of participating in active transserosal transport of solutes. The total volume of pleural fluid is about 15 to 20 mL, with approximately 1700 cells/mm3 (75% macrophages, 23% lymphocytes, 1% mesothelial cells). The volume and composition of the pleural fluid have to be tightly controlled to ensure an efficient mechanical coupling between chest wall and lung. Pleural fluid originates from pleural capillaries through microvascular filtration. Drainage occurs partly via lymphatic stomata in the parietal pleura. Transcytosis through mesothelial cells in both directions represents another mechanism involved in pleural fluid homeostasis.2–6
Frontal section of chest and lung showing pleural space. Single arrows indicate retractive force. Double arrows show the excursion of the lung bases and periphery between deep inspiration and expiration.
The connective tissue of the visceral pleura consists of three layers. A superficial layer of predominantly elastic fibers follows the mesothelium, thereby forming an elastic “bag” that enwraps each lobe. A deep sheet of fine fibers follows the outline of alveoli and extends into the depth of the lung. Between these sheets lies a bed of loose connective tissue, containing free cells (histiocytes, plasma cells, and mast cells), that is often close to lymphatics and systemic arterial branches from the bronchial arteries.
The lung is maintained in a stable position within the chest by the hilum, where airways and blood vessels enter from the mediastinum, and by the pulmonary ligament, a long, narrow band of attachment between visceral and mediastinal pleura that extends downward from the hilum. Because of these attachments, a pneumothorax causes the lung to retract and form a lump of tissue that is attached to the mediastinal wall of the thoracic cavity.
The shape of the lung is congruent with that of the fully expanded pleural cavity. This shape is preformed in lung tissue and is hence also evident if an excised lung is inflated, revealing its three faces: the convex thoracic face apposed to the rib cage, the concave diaphragmatic face modeled by the diaphragmatic dome, and the mediastinal face, on which the contours of the heart are impressed beneath the hilum.
As the lung retracts during deflation, the acute edges between the thoracic face and the diaphragmatic and (anterior) mediastinal faces of the lung withdraw; the thoracic and diaphragmatic leaflets of the parietal pleura become apposed, thereby forming a costodiaphragmatic recess on each side (Fig. 2-1). Similarly, as the ventral edge of the lung retracts, the costal and mediastinal pleurae form a recess on each side, corresponding topographically to the borders of the sternum.
The port through which airways and blood vessels enter the lung is the hilum, that is, the attachment of lung tissue to the mediastinum (Fig. 2-1). The airways reach the two hili by the mainstem, or principal, bronchi (Figs. 2-1 and 2-2). The left mainstem bronchus is longer than the right because it must pass under the aortic arch before it reaches the lung. The two principal bronchi course downward and begin to divide sequentially shortly after entering the lung, first releasing the lobar bronchus to the upper lobe (Fig. 2-2). Since a middle lobe is formed only on the right side, there is no middle lobe bronchus on the left; instead, the corresponding parts form the lingula, which receives its airways from the superior bronchus of the upper lobe (Fig. 2-2). The last branch of the stem bronchus goes to the lower lobe.
Bronchopulmonary segments of human lung. Left and right upper lobes: (1) apical, (2) posterior, (3) anterior, (4) superior lingular, and (5) inferior lingular segments. Right middle lobe: (4) lateral and (5) medial segments. Lower lobes (6): superior (apical), (7) medial–basal, (8) anterior–basal, (9) lateral–basal, and (10) posterior–basal segments. The medial–basal segment (7) is absent in the left lung. (Note: The lungs are represented as turned inward slightly to display part of the lateral face.)
The branching pattern of the human bronchial tree and of the pulmonary artery and veins are shown in a resin cast in Figure 2-3. The pulmonary artery joins the bronchi while still in the mediastinum (Fig. 2-4A); its trunk lies to the left of the ascending aorta, and the right pulmonary artery turns dorsally to course between ascending aorta and right principal bronchus. In the hilum, the right pulmonary artery lies anterior to the right principal bronchus; the left pulmonary artery, however, “rides” on the principal bronchus and crosses over the superior lobar bronchus to the posterior side. From there on, the pulmonary artery branches in parallel with the bronchi; characteristically, each bronchus is associated with one closely apposed pulmonary artery branch, and this relationship is strictly maintained to the periphery, that is to the respiratory bronchioles.
A resin cast of the human airway tree shows the dichotomous branching of the bronchi from the trachea and the systematic reduction of airway diameter and length with progressive branching. In the left lung the pulmonary arteries (red) and veins (blue) are also shown.
Schematic diagrams of the relation of the main branches of pulmonary arteries (A) and pulmonary veins (B) to the bronchial tree. The arteries follow the airways. Two mainstems of pulmonary vein penetrate independently into the lung on each side. LA, left atrium; RV, right ventricle.
In contrast, the pulmonary veins (Fig. 2-4B) follow a course independent of the bronchial tree; rather, they lie about midway between two pairs of bronchi and arteries; this position is maintained to the periphery of the airway system. In the hilum, these veins are collected into at least two main veins on either side, which lead into the left atrium located at the back of the heart.
The airways systematically branch over an average of 23 generations of dichotomous branching,7,8 ending eventually in a blind sac (Fig. 2-5). The last nine generations of these airways are connected to tightly packed alveoli, airway chambers in which gas exchange takes place, whereas the central airways serve the function of conducting the air to the gas-exchange parenchyma. In such a system of sequential branching, the unit of lung parenchyma could be defined according to the portion of parenchyma that is supplied by a particular branch of the bronchial tree, and it is possible to conceive of as many types of units as there are generations unless clear definitions for such units are proposed. However, two units appear to be natural:
Model of airway branching in human lung by regularized dichotomy from trachea (generation z = 0) to alveolar ducts and sacs (generations 19–23). The first 14 generations are purely conducting; transitional airways (generation 15) lead into the acinar airways with alveoli that branch over 8 generations (z′). (Modified with permission from Weibel ER: Morphometry of the Human Lung. Heidelberg: Springer-Verlag; 1963.)
The lobes, which are demarcated by a more or less complete lining of pleura. There are three lobes on the right (superior, middle, and inferior lobes), and two on the left (superior and inferior lobes).
The acinus, which is defined as the parenchymal unit in which all airways have alveoli attached to their wall and thus participate in gas exchange. Along the airway tree, the acinus begins with a transitional bronchiole (Fig. 2-5).9,10
Since all other units are somewhat arbitrarily defined, it is not surprising that some ambiguity exists in the literature about their meanings. Nonetheless, a certain convention has been adopted with respect to the following:
The lung segments, which are considered as the first subdivisions of lobes. Figure 2-2 shows the location and distribution of the segments to the various lobes. The symmetry is imperfect because on the left the two segments corresponding to the right middle lobe are incorporated into the superior lobe as the lingula (segments 4 and 5) and because the medial–basal segment of the lower lobe is generally missing on the left (segment 7).
The secondary lobule, an old anatomic unit. It was introduced in the 19th century because “lobules” of about 1 cm3 are visible on the surface of the lung. These lobules are delineated by connective tissue septa that are connected to the pleura. The secondary lobule is difficult to define in terms of the bronchial tree, but it does seem to comprise about a dozen acini. With reference to bronchograms, secondary lobules are supplied by airway branches that are about 1 mm in diameter.
The pulmonary blood vessels show a characteristic relationship to these units (Figs. 2-3 and 2-4). The pulmonary arteries, following the airways, course through the centers of the units and finally fan out into the capillaries located in the delicate alveolar septa of lung parenchyma. In contrast, the veins lie in the boundary between units and collect the blood from at least two or three adjacent units. This arrangement applies to acini and secondary lobules as well as to lung segments.
Therefore, it is evident that the units of lung parenchyma are bronchoarterial units, which share their venous drainage with neighboring units. This architecture has important functional and practical consequences. Except for the lobes, none of the units is separated from each other by complete connective tissue septa.
Organization of Lung Tissue
Basic Structural Elements
While looking at the tissue organization of the lung, we must first consider that the airways and the blood vessels each have their own lining by an uninterrupted cell layer. These layers extend all the way out to the gas-exchange region, but they show different properties in conducting as compared with respiratory structures. Likewise, the connective tissue forms a continuum throughout the lung all the way out to the pleura, but it, too, will be differently organized in the different functional zones; whereas it is reduced to a minimum in the alveolar walls, it contributes a number of different ancillary structures to the wall of conducting airways and blood vessels, such as smooth muscle sheaths or cartilage. This connective tissue space also houses the nutritive vessels and nerves as well as the elaborate defense system related to lymphatic vessels. In the gas-exchange region, however, very few of these accessory structures are found.
The complexity of lung structure is also reflected at the cell biologic level. There is no such thing as a standard “lung cell.” Instead, we find some 40 different cell types, highly specialized both structurally and functionally, in the lung.11–13
A word of caution is also necessary with respect to the extrapolation of structural findings in experimental animals, especially rodents, to the human lung. Noteworthy species differences include the bronchial circulation, the presence of respiratory bronchioles, the ultrastructural composition and distribution of nonciliated bronchiolar epithelial cells and their protein expression pattern, the frequency of certain cell types like alveolar brush cells and lipid-containing interstitial cells (lipofibroblasts), and the ultrastructural organization of lamellar bodies in type II alveolar epithelial cells. All these structural elements have features characteristic of the human lung that are not found in rodents.14
Wall Structure of Conducting Airways
The wall of conducting airways consists of three major components (Figs. 2-6 and 2-7): (1) a mucosa composed of an epithelial and a connective tissue lamina; (2) a smooth muscle sleeve; and (3) an enveloping connective tissue tube partly provided with cartilage.15
Airway wall structure at the three principal levels. The epithelial layer gradually becomes reduced from pseudostratified to cuboidal and then to squamous but retains its organization as a mosaic of lining and secretory cells. The smooth muscle layer disappears in the alveoli. The fibrous layer contains cartilage only in bronchi and gradually becomes thinner as the alveolus is approached.
Light micrographs of bronchial wall. A. The layers from epithelium (EP) to cartilage (CA) with elastic fibers (ef), smooth muscle bundles (SM), and glands (G). B. Higher power of pseudostratified epithelium with cilia (Ci). C. Details of gland with acini (GA) associated with groups of plasma cells (PC). BM, basement membrane; GC, goblet cell.
Although derived from same anlage,16,17 the airway epithelium modifies its differentiation characteristics as we proceed from large bronchi over bronchioles to the alveolar region (Fig. 2-6). A simple epithelium exists as a lining of smaller bronchioles: As we move upward toward larger bronchi, the epithelium becomes higher and some basal cells appear, making the epithelium pseudostratified. At the point of transition into the gas-exchange region – that is, at the entrance into the complex of alveoli – the epithelium abruptly becomes extremely thin. Figure 2-6 also shows that the epithelium is not made of a uniform cell population but that it is, at each level, rather a mosaic of at least two cell types, in that secretory cells as well as some rarer special cells are interspersed into the complex of lining cells.15,18
If we first have a closer look at the epithelium of larger conducting airways, we see that the lining cells are provided with a tuft of kinocilia at their apical cell face, whereas the secretory cells are goblet cells that produce and discharge to the surface a sticky mucus (Figs. 2-7–2-9). This mucus spreads out as a thin blanket on top of the cilia, which are embedded in a periciliary layer containing a dense network of mucins and mucopolysaccharides tethered to the cilia.19 The mucus layer is capable of trapping dust particles that are still contained in the air entering the lung. Kinocilia (Fig. 2-10) are motile cell extensions that are known to beat rhythmically in a given direction and at a frequency of about 12 to 20 Hz.20,21 In the airway epithelium, the cilia are oriented in such a fashion that their beat is directed outward. It is interesting that the cilia of airway epithelia develop at their tip fine claws with which they can grasp the mucus blanket in the phase of their forward beat, whereas on their return to the upright position they glide past the mucus blanket. The result of this is that the mucus blanket, together with trapped foreign material, moves outward or “up the airways” in a steady stream, a feature appropriately called the mucociliary escalator. Since the lining by ciliated cells is uninterrupted from the bronchioles, up the bronchi, to the trachea, this mucociliary escalator ends at the larynx, so that the normal fate of bronchial mucus is to be steadily discharged into the pharynx, whence it is swallowed, usually unnoticed. Only when an excessive amount of mucus accumulates in the trachea or in larger bronchi do we have to assist the system by coughing.
Electron micrograph of section across human bronchial epithelium made of high-columnar cells, most of which are ciliated (Ci). A goblet cell (GC) is cut lengthwise; note mucus droplets in process of accumulating at cell apex (arrow) and leukocyte (LC) caught in epithelium in process of diapedesis. BM, basement membrane; L, lumen.
Surface view of bronchiolar epithelium shows tufts of cilia (Ci) forming on individual ciliated cells and microvilli (MV) on other cells. Note secretion droplet in process of release from goblet cell (arrow).
Cilia (Ci) from human bronchial epithelium seen on sections of epithelial cells in scanning electron micrograph (A), and on thin sections in longitudinal (B), and oblique cross section (C). They are implanted in the epithelial cell by a basal body (BB). Cross-sectioned cilium at high power (inset, C) reveals its membrane, which is enveloping a typical set of two axial tubules and nine peripheral duplex tubules with dynein arm (DY) attached. Note abundant short microvilli (MV) interspersed between cilia.
The secretory cell population shows a number of specialized features. In the bronchi of all sizes and in larger bronchioles, one finds goblet cells interspersed between the ciliated cells; they form the mucus in their endoplasmic reticulum and Golgi complex, store it as droplets in their apical part, and discharge it in bulk (Figs. 2-8 and 2-9). In larger bronchi, one finds, in addition, small mucus glands located in the connective tissue; they are connected to the bronchial surface by long and narrow ducts (Figs. 2-6 and 2-7). In the normal bronchus, the glandular acini are relatively small and composed of serous and mucus cells; enlargement of the acini and a relative increase of mucus cells are characteristics of chronic bronchitis.
Finally, a special nonciliated secretory cell appears in the smaller bronchioles, the club cell (Clara) (Fig. 2-11).22,23 This cell population is very heterogeneous, thus displaying both interspecies and intraspecies variations.24–28 In the human lung, club cells account for about 11% and 22% of the total epithelial cell number in terminal and respiratory bronchioles, respectively.29 Besides the absence of cilia, club cells in conventional preparations are characterized by their dome-shaped apex that protrudes into the airway lumen. In contrast to that in rodents, where this cell is rich in smooth endoplasmic reticulum, club cells in the human lung lack significant amounts of smooth ER. They possess short lateral cytoplasmic extensions while their basal surface that rests on the basement membrane is practically free of infoldings. Membrane-bound electron-dense granules of about 500 to 600 nm diameter are present, which underlines their secretory activity. Our understanding of the functions of club cells is still incomplete. In many aspects, they appear to be functionally related to the secretory cell type of the alveoli, the type II alveolar epithelial cell; ultrastructural features and expression patterns of lung adenocarcinoma cells show characteristics of both club and type II cells. Club cell secretions add to the lining layer of the distal lung. Club cells synthesize and secrete the club cell secretory protein (CCSP),22 which has been shown to be structurally similar to rabbit uteroglobin. The exact function of CCSP in the human lung still remains to be elucidated. CCSP levels in BAL fluid are decreased in smokers and in patients with COPD or interstitial lung diseases.30 Animal studies suggest immunomodulatory functions for CCSP.28 Within the lung, the club cell is the primary site of cytochrome P450 monooxygenase activity. Thus, they are heavily involved in detoxification of xenobiotics. Normal bronchiolar epithelial homeostasis is maintained by proliferation of club cells, whereas a cell population termed “variant Clara cells” or “variant CCSP-expressing cells,” which is associated with neuroepithelial bodies or localized at bronchioloalveolar duct junctions, appears to act as progenitor cells for the bronchiolar epithelium under certain pathologic conditions.28,31
Club cells from human bronchiolar epithelium contain dense secretion granules (g) at apex. Note abundant cytoplasmic organelles such as mitochondria (MI), Golgi complex (GO), or endoplasmic reticulum (ER) as well as microvilli (MV) at surface. Cell membranes are closely apposed and form tight junctions (J) at apical edge. Ci, cilia; N, nucleus; PM, plasma membrane.
There are also some additional rarer cells. Neuroendocrine cells are capable of secreting mediators (amines and neuropeptides) into subepithelial capillaries. Prior to secretion, the bioactive substances are stored in dense-cored vesicles (Fig. 2-12). Occasionally, but only rarely in the adult human lung, these cells are organized in extensively innervated groups, and then termed “neuroepithelial bodies.” Although it seems clear that neuroepithelial bodies have sensory, most likely oxygen-sensing, properties, their exact physiologic function is still poorly understood.32–36 Another rare cell type of the airway epithelium is the brush cell. These cells are characterized by the presence of an apical tuft of blunt, broad, and straight microvilli with root-like structures composed of filaments extending into the cytoplasm (Fig. 2-13). Glycogen granules, vesicles, and smooth endoplasmic reticulum are usually present as well. There is species variation in the occurrence of brush cells. While common in rodents (in rats even present in the proximal alveolar epithelium37) they are only rarely found in the human lung. Their function is only partly explored. Owing to their ultrastructure and their strategic localization in the airways and at alveolar duct bifurcations, sensory/chemoreceptor as well as sentinel/immune surveillance functions have been proposed.38,39 Recent evidence suggests that brush cells “taste” the chemical composition of the airway lining fluid.40,41
Basal part of neuroendocrine cell of human bronchiolar epithelium showing dense-cored vesicles (v). (Reproduced with permission from Weibel ER. Lung cell biology, in Fishman A, Fisher AB, eds. Handbook of Physiology. Section 3: The Respiratory System. vol 1. Bethesda, MD: American Physiological Society; 1985:47–91.)
Brush cell from small bronchiole of rat lung containing broad microvilli (MV). (Reproduced with permission from Weibel ER: Lung cell biology, in Fishman A, Fisher AB, eds. Handbook of Physiology. Section 3: The Respiratory System. vol 1. Bethesda, MD: American Physiological Society; 1985:47–91.)
The layer of connective tissue in the bronchial mucosa consists predominantly of elastic fibers that are oriented longitudinally; these fibers serve to maintain a smooth outline of the longitudinal profile of the bronchial lumen no matter how much the bronchi are stretched as the lungs are inflated. In this connective tissue lamina there are foci of lymphoid cells; often they form small lymphoid follicles.42 However, bronchus-associated lymphoid tissue (BALT) is usually absent in normal adult human lungs and develops only after stimulation when inducible BALT might organize local immune responses.43–46
Smooth muscle bundles form a continuous sleeve in the connective tissue underlying the epithelial tube that extends from the major bronchi to the respiratory bronchioles; beyond the respiratory bronchioles, the bundles extend into the wall of alveolar ducts where the muscle fibers lie in the alveolar entrance rings. The bundles have an oblique course and encircle the mucosal tube in a criss-cross pattern; hence, their contraction results primarily in a narrowing of the lumen.
In the small bronchioles there is little else to the airway wall; the smooth muscle layer is ensheathed by a layer of delicate connective tissue that is in direct contact with adjacent alveoli (Fig. 2-6). In the larger bronchioles and even more in the bronchi, the outer connective tissue sheath forms a strong layer of fibers; in the bronchi, rings or plates of cartilage are incorporated into this layer.
The wall structure in the respiratory bronchioles is identical to that of terminal bronchioles except that in some regions the cuboidal epithelium is replaced by an alveolar epithelium of squamous cells (type I cells) closely apposed to capillaries. Very often, these single alveoli constitute outpouchings in these regions; sometimes simple “respiratory patches” form in the bronchiolar wall (see below).
Wall Structure of Conducting Blood Vessels
The endothelial lining of pulmonary arteries and veins differs from that of capillaries by some site-specific structural and functional differences.47–49 The endothelium of conducting blood vessels is thicker, and parts of its cytoplasm are richly endowed with organelles of various kinds (Fig. 2-14). Clearly, these cells are metabolically more active than those of the capillary endothelium. They are particularly rich in membrane-bound rod-shaped granules termed Weibel–Palade bodies,50,51 which represent the regulated secretory organelles of endothelial cells (Fig. 2-14). The lumen of Weibel–Palade bodies is filled with longitudinally arranged tubules. These tubules represent von Willebrand factor,52 packed in a highly organized spiral that allows rapid secretion into the blood. Other components of Weibel–Palade bodies include tissue-type plasminogen activator, endothelin-1, the leukocyte adhesion receptor P-selectin, interleukin-8, the tetraspanin CD63/LAMP-3, and the small GTPase Rab27a. Thus, Weibel–Palade bodies are actively involved in hemostasis as well as in vasoactive and inflammatory responses.53–56
Part of wall of pulmonary artery from human lung. Endothelial cells (EN) form thick layer; their cytoplasm is rich in organelles. Specific granules of endothelium (arrows), a cross-section of one of which is shown at high power in the inset, are enveloped by a membrane and contain tubules. The arterial wall is of the elastic type, formed of alternating layers of smooth muscle (SM) and elastic fibers (ef). EC, erythrocyte.
Many of the nonrespiratory metabolic functions of the lung – particularly the transformation of certain bioactive substances, such as angiotensin and prostaglandins – are performed in endothelial cells. Caveolae (or plasmalemmal vesicles) have been implicated in these processes.57,58 Caveolae are plasma membrane invaginations and associated vesicles with an outer diameter of about 50 to 70 nm. Depending on fixation, the shape of these invaginations appears omega- or cup-like.59 Their structural framework consists of members of the caveolin family of proteins associated with cholesterol and sphingolipids. Caveolae perform transport and signaling functions and are involved in membrane organization. All endocytic activity mediated by caveolae (thereby bypassing the clathrin-coated vesicle pathway) is pooled under the term potocytosis.60–63
Accessory structures develop in the wall in accord with the functional properties of the vessels. Thus, the walls of the major pulmonary arteries that are close to the heart, and therefore exposed to the pressure oscillations of large amplitude prevailing in the outflow tract of the right ventricle, are of the elastic type, that is, layers of elastic lamellae are interconnected with smooth muscle cells as in the aorta; the tone of the smooth muscle regulates the elastic modulus of the vessel wall, thereby controlling the shape of the pulse wave. In the pulmonary arterial tree, this pattern prevails out to branches of about 1 mm diameter.
In contrast, branches less than l mm in diameter are of the muscular type, that is, the smooth muscle fibers encircle the vessel lumen; they can modify the vessel's cross-section and can thus regulate blood flow through this vessel. Compared with systemic arteries, the thickness of the pulmonary arterial wall is reduced about in proportion to systolic pressure, that is, by about a factor of 1:5; in pulmonary hypertension, the wall becomes thicker. Although arterioles are a well-defined entity in the systemic vascular bed, where they constitute the major site of arterial resistance, pulmonary arterioles are more difficult to locate and define. A single muscle layer – the histologic definition of an arteriole – does occur in branches about 100 μm in diameter, but the arterial bed continues out to the precapillaries, which consist of vessels 20 to 40 μm in diameter that are enwrapped by an incomplete smooth muscle sheath. This poverty of smooth muscle contributes importantly to the low resistance to blood flow that is normally afforded by the pulmonary arterial tree.
The structure of pulmonary veins is similar to that of systemic veins in the upper half of the organism. Their walls are rich in connective tissue and contain irregular bundles of smooth muscle. Larger veins contain a large amount of elastic tissue. More extensive in rodents, but to a certain degree also in humans, cardiac muscle tissue from the left atrial myocardium forms sleeves in the adventitia of pulmonary veins where they overlap with the smooth muscle of the venous wall. The arrangement of the myocardial sleeves correlates with the distribution of foci of ectopic beats initiating atrial fibrillation.64–67
Nutritive Vessels and Nerves
The tissue of lung parenchyma is very well supplied with blood; the fact that it is venous is of no disadvantage, because O2 is easily obtained from the air. Thus, nutrient supply from pulmonary arteries combined with O2 supply from air appears to suffice not only for the parenchyma but also for bronchioles and the smaller pulmonary vessels, whose outer surface is almost directly exposed to air. The thicker-walled bronchi, with their glands and cartilage, require a nutrient blood supply from bronchial arteries.15,68,69 These derive in part directly from anterior branches of the aorta and partly from the upper intercostal arteries. They course alongside the esophagus and penetrate on both sides into the hilum. The bronchial arteries extend to the most peripheral bronchi but not into the walls of bronchioles. On the other hand, some branches supply large pulmonary vessels as vasa vasorum, whereas others course along larger septa to reach the pleura. Some bronchial arteries form anastomoses with peripheral branches of the pulmonary arteries. There have been long discussions about the role that such anastomoses may play. It seems that in the normal lung their importance has been overrated. However, in certain pathologic conditions, such as bronchiectasis and tumors, the bronchial arteries and perhaps the bronchopulmonary anastomoses appear to play an important role. They also enlarge to form a collateral circulation when branches of the pulmonary artery are obliterated. The peribronchovascular space around larger pulmonary artery branches and bronchi with its capillaries from the bronchial circulation has also been proposed as a unique compartment since it is a preferential site of leukocyte infiltration and edema formation under pathologic conditions.70 Furthermore, the bronchial circulation attenuates ischemia–reperfusion lung injury. Consequently, interruption of the bronchial circulation without revascularization during lung transplantation often leads to bronchial anastomotic complications.
Except for a few bronchial veins in the hilar region, the bronchial system does not have its own venous drainage into the systemic veins. Instead, the bronchial veins, which begin as a peribronchial venous plexus, drain into pulmonary veins; this drainage seems to constitute one source of normal venous admixture to arterial blood.
The lung is innervated by the autonomic nervous system. The parasympathetic fibers are derived from the vagal nerves and the sympathetic fibers from the upper thoracic and cervical ganglia; together they form the pulmonary nervous plexus in the region of the hilum before entering the lung. The fiber bundles follow the major bronchi and blood vessels, finally penetrating into the acini; some nerves also supply the pleura. In addition, motor nerves influence the smooth muscle tone of airways and blood vessels, and sensory nerves are involved in reflex functions (e.g., cough reflex, Hering–Breuer reflex). Moreover, the secretory function of glands as well as of type II alveolar epithelial cells is at least partly under control of this nervous system. Nerve fibers are easily found in the wall of bronchioles and bronchi, where they often follow the course of bronchial arteries. However, fibers in alveolar septa are small and scarce.
The Cells of the Alveolar Region
Basic Design of the Gas-Exchange Barrier
Efficient gas exchange in the lung depends on a very thin barrier of very large surface between air and blood.16,71 Actually, the barrier is so thin that it cannot be resolved into its constituents by light microscopy. Nevertheless, this barrier must be built of the three minimal tissue layers: an endothelium lining the capillaries, an epithelium lining the airspaces, and an interstitial layer to house the connective tissue fibers. The guiding principle in designing these cells must evidently be to minimize thickness and maximize extent. However, there is definitely a limit to this, set by the need to make the barrier and its constituent cells strong enough to resist the various forces that act on it: capillary blood pressure, tissue tension, and surface tension, in particular. Furthermore, the barrier must remain intact for a lifetime, and this requires continuous repair and turnover of the cells and their components. As a result, about half of the surface of the air–blood barrier is optimized for gas exchange in that the thin epithelial and endothelial cell extensions are only separated by a fused basement membrane. These areas are termed the thin parts of the air–blood barrier. Cell nuclei and connecting tissue fibers are concentrated in the so-called thick parts of the air–blood barrier.
In spite of this delicacy of tissue structure, we find that three-quarters of all the lung cells by volume or weight are contained in the lung parenchyma (Table 2-1). We also note that epithelium and endothelium make up about one-quarter each of the tissue barrier in the alveolar walls, whereas interstitial cells amount to 35%; the interstitial space with the connective tissue fibers makes up no more than 15% of the barrier.11,72
Table 2-1Estimated Cell Volumes in the Human Lung ||Download (.pdf) Table 2-1Estimated Cell Volumes in the Human Lung
|Cell or Tissue ||Volume (mL) ||Percent Septal Tissue |
|Tissue (excl. blood) ||284 ||— |
|Nonparenchyma ||99 ||— |
|Alveolar septa ||185 ||— |
|Cells ||213 ||— |
|Nonparenchyma ||50 ||— |
|Alveolar septa ||163 ||— |
|Parenchymal cells ||163 ||— |
|Alveolar epithelium type I ||23 ||12.6 |
|Alveolar epithelium type II ||18 ||9.7 |
|Capillary endothelium ||49 ||26.4 |
|Interstitial cells ||66 ||35.8 |
|Alveolar macrophages ||7 ||3.9 |
The alveolar epithelium is a mosaic of different cell types. The vast majority of the total surface is lined by a single layer of squamous cells; the remaining fraction – only about 3% (Table 2-2) – is occupied by cuboidal secretory cells; one usually calls the squamous lining cells type I and the secretory cells type II alveolar epithelial cells or pneumocytes. Type I and II cells occur with a numerical frequency of about 1:2. A very rare third cell type, the brush cell, can be found in some specific regions near the entrance of the acinus (see above).
Table 2-2Morphometric Characteristics of Cell Population in Human Pulmonary Parenchyma ||Download (.pdf) Table 2-2Morphometric Characteristics of Cell Population in Human Pulmonary Parenchyma
|Cell Population ||Percent of Total Cell Numbera ||Average Cell Volume (μm3) ||Average Apical Cell Surface (μm2) |
|Alveolar epithelium || || || |
|Type I ||8 ||1764 ||5098 |
|Type II ||16 ||889 ||183 |
|Endothelium ||30 ||632 ||1353 |
|Interstitial cells ||36 ||637 ||— |
|Alveolar macrophages ||10 ||2492 ||— |
The fine structural details of the different types of alveolar epithelial cells can only be fully visualized by electron microscopy, whereas molecular markers selective for either type I or II cells or some of their constituents can be detected and localized by light microscopy (Fig. 2-15; Table 2-3).
Table 2-3Markers for Alveolar Epithelial Cells ||Download (.pdf) Table 2-3Markers for Alveolar Epithelial Cells
|Type I Cell ||Type II Cell |
|HTI-56 (human) ||Surfactant proteins: |
|T1α/RTI-40 (rat, mouse) ||SP-A |
|Aquaporin 5 ||SP-B |
|Caveolin 1 ||SP-C |
|Receptors for advanced glycation end products ||SP-D |
|(RAGE) ||ABCA3 |
|Carboxypeptidase M ||HTII-280 (human) |
|Lectins: ||RTII-70 (rat) |
|Lycopersicon esculentum ||MMC4 (rat) |
|Bauhinia purpurea ||Alkaline phosphatase |
|Ricinus communis 1 ||CD44 |
| ||Lectins: |
| ||Maclura pomifera |
Immunofluorescent double labeling of alveolar epithelial cells. Type I cells are stained for Lycopersicon esculentum lectin (red), type II cells are stained for SP-D (green). Compare with Table 2-3. (Micrograph used with permission of H. Fehrenbach.)
Type I Alveolar Epithelial Cells
At first glance, the squamous type I cells show rather simple design features (Fig. 2-16). Their small, compact nucleus is surrounded by a slim rim of cytoplasm, where one finds a modest basic set of organelles, a few small mitochondria, and some cisternae of endoplasmic reticulum, seemingly the picture of a quiescent cell with no great metabolic activity.11,73
A type I alveolar epithelial cell (EP1) from human lung. The nucleus (N) is surrounded by very little cytoplasm, which extends as thin leaflets (arrows) to cover the capillaries (C). Note the basement membranes (BM) of the epithelium and endothelium (EN), which become fused in a minimal barrier. Interstitial space contains fibroblast processes (F).
At the edge of the perinuclear region, a very attenuated cytoplasmic leaflet emerges (Fig. 2-16) and spreads out broadly over the basal lamina. This leaflet is made essentially of the two plasma membranes forming the apical and basal cell face, respectively, with a very small amount of cytoplasmic ground substance interposed (Fig. 2-17). Here one rarely finds any organelles except for the numerous plasmalemmal vesicles implied in the transcellular transport of molecules. In fact, besides capillary endothelial cells, type I alveolar epithelial cells are among the richest in caveolae.
Thin, minimal tissue barrier between alveolar air (A) and capillary blood (C) is made of cytoplasmic leaflets of epithelium (EP) and endothelium (EN), joined by fused basement membranes (BM). Note that the epithelial and endothelial leaflets are bounded by plasma membranes (PM), as is the erythrocyte (EC). Arrows point to pinocytotic vesicles/caveolae. (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
The surface covered by one type I epithelial cell is about 4000 to 5000 μm2. In some texts one may find the type I cell called the “small alveolar cell” because of its small nucleus; clearly this is a misnomer, as the type I cell is a rather large cell indeed, with respect to both surface and cell volume (Table 2-2). Terminal bars are formed where the cytoplasmic leaflets of epithelial cells meet (Fig. 2-18). If one looks at the surface of the alveolar epithelium in scanning electron micrographs (Fig. 2-19), one notes that the patches covered by single type I cells are variable in size and that even the largest are much smaller than the 4000 to 5000 μm2 given earlier, a number derived by dividing the total alveolar surface by the total number of type I cell nuclei. Why is this? There seem to be three to four times as many type I cell domains encircled by terminal bars as there are nuclei. Indeed, this observation was already made some 130 years ago by Albert Kölliker; his interpretation was that part of the alveolar surface was lined by “nonnuclear” cytoplasmic plates rather than by complete cells. It turns out that an alternative explanation is possible. One finds that type I cells are not simple squamous cells but rather branched cells with multiple apical faces, as shown diagrammatically in Figure 2-20. Thus, what appears as nonnucleated plates are cytoplasmic domains connected to the perinuclear region by a stalk, spreading out on one side of the alveolar wall or the other; it is evident that several such domains may share a nucleus.74
Minimal barrier part showing intercellular junctions. Between type I epithelial cells, a “tight” junction (J1) is formed by close apposition of the cell membranes over a comparatively wide band; the junction between endothelial cells (J2) is “leaky” because membranes become apposed over a narrow strip only. Note trilaminar structure of plasma membranes (PM), the occurrence of pinocytotic vesicles/caveolae (V) in both epithelium and endothelium (EN), and the fused basement membranes (fBM). A, alveolus; C, capillary; EP1, type I epithelial cell.
Surface of the alveolar wall in the human lung seen by scanning electron microscopy reveals a mosaic of alveolar epithelium made of type I and type II (EP2) cells. Arrows indicate boundary of the cytoplasmic leaflet of the type I cell which extends over many capillaries. Note the two interalveolar pores of Kohn (PK). N, nucleus of type I cell.
Diagram of the alveolar wall showing the complexity of a type I epithelial cell (EP1) and its relation to a type II cell (EP2) and endothelial cell (EN). (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
Although type I cells cover about 97% of the alveolar surface area, they have long been neglected as being “silent,” providing solely a barrier function. Although their overall function in the human lung remains to be determined, recent animal and in vitro studies strongly suggest that type I cells are actively involved in alveolar ion and fluid homeostasis.75–77
Type I cells are easily damaged, particularly because of their extensive surface area and their complex branching architecture. However, there is an additional problem: one finds that type I cells are not capable of multiplying by mitosis in vivo, neither during lung growth when more cells are needed to coat the expanding alveolar surface nor upon damage in the adult lung when cells need to be replaced. In both instances new type I cells are made by mitotic division and transformation of type II cells, a process that takes about 2 to 5 days.
This seems to work under normal circumstances. There are, however, conditions where this repair mechanism is too slow to cope with excessive damage, so that a syndrome of severe catastrophic respiratory failure, acute respiratory distress syndrome (ARDS), develops, which requires intensive care treatment. In such patients one finds large parts of the type I cell lining of the alveolar surface to be destroyed. As a consequence, the barrier has become leaky and the alveoli fill with alveolar edema, so that they can no longer take part in gas exchange.78,79
With proper medical care, this alveolar edema can often be resolved within a few days. The alveoli become again filled with air, but in spite of this, gas exchange does not improve. What has happened is that the repair of the severely damaged alveolar epithelium requires a lot of new cells to be made by division of type II cells.80 These form a rather thick cuboidal lining of the barrier surface, a phenomenon termed cuboidal metaplasia, and this thick barrier offers a high resistance to O2 flow. It takes several weeks until a thin barrier is restored by transformation of the cuboidal cell lining into delicate type I cells. During this process, the cells go through intermediate stages where they are often positive for both type II and type I cell markers.81,82
Type II Alveolar Epithelial Cells
The type II alveolar epithelial cell is a conspicuous but in fact relatively small cell whose mean volume is less than half that of the type I cell (Table 2-2), although it is often called the “large alveolar cell.”11 Its shape is cuboidal, the apical cell surface bulges toward the lumen and is provided, mostly around its periphery, with a tuft of microvilli (Figs. 2-21 and 2-22). Often, type II cells seem to be preferentially located in the corners of alveoli or in close proximity to interalveolar pores of Kohn. They are usually found as solitary cells; only in cases of alveolar epithelial damage, proliferation of type II cells leads to focal clusters during the repair process. Occasionally, a single type II cell might supply two or even three adjacent alveoli with its apical surface. The basement membrane beneath type II cells is occasionally interrupted. Through these apertures, foot processes of type II cells can extend to the interstitium and come in close proximity to interstitial cells.83
Higher magnification of a type II cell reveals a “crown” of short microvilli (MV) and a central “bald patch.” Note junction lines of type I cells (J) meeting with the type II cell.
A type II epithelial cell from the human lung forms junction (J) with type I epithelial cells (EP1). Its cytoplasm contains osmiophilic lamellar bodies (LB) and a rich complement of organelles: mitochondria (MI), endoplasmic reticulum (ER), and so on. The nucleus (N) is surrounded by a perinuclear cisterna (pNC) which is perforated by nuclear pores (NP). A, alveolus; BM, basement membrane; F, fibroblast; MV, microvilli.
Type II cells contain a wealth of cytoplasmic organelles of all kinds (Fig. 2-22): mitochondria, a lot of endoplasmic reticulum with ribosomes, and a well-developed Golgi complex surrounded by a set of small lysosomal granules among which so-called multivesicular bodies – membrane-bounded organelles containing a group of small vesicles – stand out (Fig. 2-23). In addition, one finds the characteristic lamellar bodies, larger membrane-bounded secretory organelles that contain densely packed phospholipid lamellae. There are notable species differences in the ultrastructural organization of lamellar bodies. In rodents, the lamellae are mostly arranged in parallel stacks whereas in humans, concentrically arranged lamellae are mostly found, which are attached to a projection core consisting of randomly arranged short stacks of densely packed membrane segments (Fig. 2-24).84 The periodicity of the lamellae is in the range of 4 to 6 nm. One human type II cell contains between 200 and 500 lamellar bodies, making up a total volume of about 2 cm3 in the entire lung. With a diameter of approximately 1 μm, lamellar bodies are among the largest secretory organelles of all cells in the body. Owing to their equipment with lysosomal enzymes (e.g., acid phosphatase, cathepsins) and proteins (e.g., members of the lysosomal membrane protein (LAMP) family and their acidic pH of about 5.5, lamellar bodies are regarded as secretory lysosome-related organelles.85
Cytoplasmic organelles of the type II cell implicated in the synthesis of surfactant are the endoplasmic reticulum (ER), Golgi complex (G), lysosomes (L), multivesicular bodies (MVB), and finally lamellar bodies (LB). The inset shows a large composite body with a stack of phospholipid lamellae (arrow). N, nucleus. (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
Immunogold labeling for SP-A (5-nm gold particles) and SP-B (15-nm gold particles) in the human lung. A. Within type II cells, SP-B is localized in the projection core (PC) of lamellar bodies (LB). B. In the alveolar lumen, SP-A is associated with tubular myelin figures (TM) whereas SP-B is found in the projection core (PC) of freshly secreted lamellar bodies (LB) and dense core particles (arrow) close to tubular myelin. UV, unilamellar vesicle.
Type II cells have two main functions: they serve as the cellular source of pulmonary surfactant and they contribute to the regeneration of the alveolar epithelium under physiologic and pathologic conditions. These properties form the basis of the concept of the type II cell as the “defender of the alveolus.”86–88
Surfactant prevents alveolar atelectasis by a surface area–dependent reduction of the alveolar surface tension (see below).89–92 Another function of surfactant as a result of the reduction of alveolar surface tension is to prevent the formation of intra-alveolar edema.93 In addition, certain surfactant components have important immunomodulatory functions in the innate host defense system.94,95 Taken together, the main functions of surfactant might be summarized as to keep alveoli open, dry, and clean. Surfactant is composed of around 90% lipids, mainly saturated phosphatidylcholine, and around 10% proteins, including the surfactant apoproteins termed SP-A, SP-B, SP-C, and SP-D. Besides its biochemical complexity, surfactant is also morphologically very heterogeneous, consisting of different surfactant subtypes with highly organized structure that represent different stages in metabolism (Figs. 2-24–2-26).84,96
Schematic diagram of pathways for synthesis and secretion of surfactant lipids and apoproteins by a type II cell, for their recycling by type II cells, and for their removal by macrophages. Note the arrangement of phospholipids and apoproteins in the lamellar bodies, in tubular myelin, and in the surface film. (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1985.)
Apical part of type II cell (EP2) with lamellar bodies (LB); one of these (LB*) is seen in the process of being secreted into the alveolar surface lining layer (ALL). The free surface of the lining layer is covered by a thin black film of lipids (arrows), which is connected with tubular myelin (TM) in the hypophase. (Reproduced with permission from Weibel ER, Gil J: Structure-function relationships at the alveolar level, in West JB, ed.: Bioengineering Aspects of the Lung. New York: Springer-Verlag; 1977.)
The alveolar epithelium (including interalveolar pores of Kohn) is lined by a thin but apparently continuous fluid layer inserted between the apical cell membrane and the surface film, thus forming a duplex lining layer.97,98 Surfactant functions in and on this layer. It is synthesized, stored, secreted, and to a large extent recycled by type II cells.84,87,99 Therefore, an intracellular surfactant pool present in type II cells and an intra-alveolar surfactant pool present at the surface of the fluid alveolar lining layer as well as within its hypophase can be distinguished. The intracellular storage form of surfactant is represented by lamellar bodies. Prior to storage, the synthesis of surfactant material involves endoplasmic reticulum, (at least partly) Golgi complex, and multivesicular bodies. In type II cells, multivesicular bodies participate in the posttranslational processing of surfactant proteins as well as in endocytosis and subsequent recycling and/or degradation of surfactant material; thus, most probably representing the junction point between the biosynthetic and endocytotic pathway. In addition, transitional forms between multivesicular bodies and lamellar bodies, termed composite bodies, have been described. Surfactant material present in lamellar bodies is secreted into the alveolar lumen via exocytosis (Figs. 2-25 and 2-26).
Most surfactant components are assembled in lamellar bodies prior to secretion (Figs. 2-24–2-26)—at least the lipid fraction and the hydrophilic surfactant proteins SP-B and SP-C, whereas the hydrophilic surfactant proteins SP-A and SP-D seem to be secreted independently via a constitutive pathway bypassing the regulated exocytosis of lamellar bodies. Lamellar body secretion starts with the fusion of its limiting membrane with the apical plasma membrane, followed by formation of a fusion pore, and finally the slow release of surfactant material through the pore. The diameter of the pore is considerably smaller than that of the lamellar body. Thus, surfactant seems to be squeezed through the pore.100 The mechanisms that regulate surfactant secretion in vivo are still not fully elucidated. It seems that, among the various stimuli that can act via several different signaling pathways, mechanical stretch during ventilation – either as a direct effect on type II cells or indirectly via type I cells or capillary endothelial cells which may act as “strain sensors” – is the physiologically most relevant.87,99,101,102
Intra-alveolar surfactant consists of several subtypes, namely freshly secreted lamellar body-like forms, tubular myelin, the surface film, and small unilamellar vesicles. After secretion, lamellar body-like forms in the hypophase associate with SP-A, which is separately secreted by type II cells,103 and undergo a major structural transformation into tubular myelin figures with a unique lattice-like structure.104 The precise physiologic function of tubular myelin, however, is still unclear. Tubular myelin is thought to be the immediate precursor of the surface film, although the existence of an additional multilayered surface-associated surfactant reservoir underneath the surface film has been suggested.96,105 “Spent” surfactant components are found in the hypophase as small unilamellar vesicles. The major route of surfactant clearance is reuptake by type II cells. Within type II cells, surfactant material can either be recycled or degraded. Other routes of surfactant clearance include ingestion and lysosomal degradation by alveolar macrophages and clearance via the airways.
After differential centrifugation of intra-alveolar surfactant material harvested by bronchoalveolar lavage, surface active large aggregates (LA), ultrastructurally largely corresponding to lamellar body-like forms and tubular myelin, and inactive small aggregates (SA), ultrastructurally largely corresponding to unilamellar vesicles, can be distinguished. Thus, the SA/LA ratio can be used to assess the biophysical activity of surfactant.90,91
A surfactant film, most likely mainly transported upward from the alveoli, is also present in the airways. Here, surfactant prevents collapse of smaller airways, prevents transepithelial fluid influx, enhances mucociliary transport, and interacts with inhaled pathogens and particles. At least some of the surfactant proteins are also synthesized and secreted by club cells. Club cells express SP-B, but not SP-C, which is exclusively expressed by type II cells. There is some controversy whether club cells express SP-A and SP-D. Although this is obviously the case in rodents, club cells in the normal adult human lung most likely express very low or no SP-A and SP-D.103,106,107 It seems that club cells are not involved in reuptake or recycling of surfactant components.108 However, their overall role in surfactant biology is not yet defined.
The surfactant apoproteins as the “smart molecules in the surfactant system”109 have important functions in surfactant subtype assembly, surfactant biophysics, surfactant homeostasis, and innate immunity.110–114 The hydrophilic proteins SP-A and SP-D belong to the collectin protein family involved in innate immunity. In addition, SP-A, together with SP-B, is important for tubular myelin formation, thus stabilizing active surfactant forms, whereas the hydrophobic proteins SP-B and SP-C and, in conjunction, SP-A enhance the adsorption of phospholipids into the surface film. SP-A might also inhibit surfactant secretion and stimulate surfactant reuptake by type II cells.
Differences in the ultrastructural organization of intracellular and intra-alveolar surfactant subtypes between humans and rodents are also reflected by a different distribution of surfactant proteins (Fig. 2-24).84 In the human lung, SP-A within type II cells is mainly found in small vesicles and multivesicular bodies and only rarely at the periphery of lamellar bodies. In the alveolar lumen, SP-A is associated with peripheral membranes of lamellar body-like forms in close proximity to tubular myelin, in the corners of the tubular myelin lattice structure, and partly at the surface film and unilamellar vesicles.103 SP-B in the human lung is localized in the projection core of lamellar bodies within type II cells and in dense core particles associated with tubular myelin in the alveolar lumen.115
The crucial role of the surfactant system for the maintenance of the functional integrity of the lung is clearly demonstrated by surfactant dysfunction disorders, which can be caused either at birth by developmental deficiency (owing to lung immaturity or mutations affecting surfactant synthesis or secretion) or later by acquired dysfunction (owing to damage of type II cells or inhibition/inactivation of intra-alveolar surfactant).90,91,116 A primary deficiency of surfactant in the immature lungs causes the respiratory distress syndrome of premature neonates (RDS). Surfactant dysfunction mutations causing either acute respiratory failure or chronic lung disease after birth have been identified in the genes encoding for SP-B, SP-C, and the ATP-binding cassette transporter ABCA3, which is present at the limiting membrane of lamellar bodies. Impairment of an originally intact surfactant system is involved in the pathogenesis of a variety of other lung diseases, such as acute lung injury/ARDS as well as obstructive, infectious, and interstitial lung diseases. Mechanisms leading to impaired surfactant activity include apoptotic or necrotic cell death of type II cells, damage of surfactant proteins and lipids by reactive oxygen and nitrogen species, and enzymatic damage by phospholipases or neutrophil elastase. In addition, plasma proteins entering the alveolar space during edema formation are also known to inactivate surfactant.
With a turnover time of about 4 to 10 hours and only a rather small intracellular surfactant reserve available for secretion onto the large alveolar surface, the ability to cope with a lack of active surfactant during lung injury is limited. Hence, there is a rationale to supplement the surfactant material available in cases of surfactant deficiency or damage. One of the major advances in neonatology in our time has been the development of surfactant replacement therapy for the treatment of RDS. The story of the treatment of premature babies with exogenous surfactant is indeed a paradigmatic example in which discoveries from basic research were successfully applied to an important clinical problem.91,117–119 The indications for surfactant replacement therapy have widened in recent years, with promising results in forms of respiratory failure not caused by a primary deficiency of endogenous surfactant but rather by impairment of an originally intact surfactant system. In these cases, however, the efficacy of exogenous surfactant therapy very much depends on the ability of the surfactant preparation to resist the inhibition/inactivation that caused alterations of the endogenous system.
The alveolar septa of the adult lung contain a single capillary network. The capillary endothelium is of the continuous (nonfenestrated) type. Alveolar capillaries are provided with pericytes, but they are rarer and less densely branched than pericytes of the systemic circulation.120 Pericytes are related to vascular smooth muscle cells in that they both are contractile perivascular cells. Thus, pericytes protect microvessel wall integrity by providing some mechanical support. However, in contrast to vascular smooth muscle cells, pericytes are embedded within the endothelial basement membrane, frequently forming contacts with capillary endothelial cells. They seem to contribute components to the capillary basement membrane and extracellular matrix and secrete vasoactive substances. In addition, pericytes are thought to be involved in the regulation of endothelial cell proliferation and differentiation and to act as progenitor cells for other cell types.121–124
Capillary Endothelial Cells
At first glance, capillary endothelial cells resemble type I alveolar epithelial cells, but in contrast to type I cells with their complex branching architecture, capillary endothelial cells form simple sheets (Fig. 2-27).57 Moreover, compared with the tight occluding junctions between alveolar epithelial cells that constitute a powerful seal of the intercellular cleft, the occluding junctions between capillary endothelial cells are rather leaky, allowing a nearly uninhibited exchange of water, solutes, and even some smaller macromolecules between the blood plasma and the interstitial space (Fig. 2-18). Occluding junctions between capillary endothelial cells are often located at the transition of the thin to the thick part of the air–blood barrier and are often covered by pericytes.
An endothelial cell (EN) of capillary (C) is similar in basic structure to a type I epithelial cell (EP1). The nucleus is enwrapped by little cytoplasm but thin leaflets extend as capillary lining (arrows). Note the intercellular junction (J) and a white blood cell/granulocyte (GC), in the capillary. (Modified with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
There is another notable and important difference between the two basically similar lining cells on the epithelial and endothelial side of the gas-exchange barrier: their size. Although the capillary surface is some 10% to 20% smaller than the alveolar surface, the capillary endothelial cells are about four times more numerous than type I cells72; this means that the surface covered by one type I epithelial cell must be about four times larger, namely 4000 to 5000 μm2, as compared with about 1000 μm2 in endothelial cells (Table 2-2).
Numerous caveolae are found in capillary endothelial cells (Figs. 2-17 and 2-18). However, at the bulging part of the capillaries, some parts of the endothelial cell extensions are free of caveolae and are thinned down to a thickness of about 20 to 30 nm, basically consisting of the two plasma membranes with only a minute amount of cytoplasm in between. These areas, rarer in human lungs than in rodents, are termed the avesicular zone of the alveolar capillary endothelium.11,57 In contrast to the endothelium of conducting vessels, Weibel–Palade bodies are missing in capillary endothelial cells, thereby underscoring the structural and functional differences between alveolar and extra-alveolar endothelial cells.11,49,125,126
The interstitium of the alveolar septum is for the most part extremely thin. At the thick parts of the air–blood barrier where epithelial and endothelial basement membranes are separated, one finds elastic fibers and bundles of collagen fibrils in the extracellular matrix as well as interstitial cells, mainly fibroblasts, the cells responsible for production of extracellular matrix components (Figs. 2-28 and 2-29). The precise arrangement of the connective tissue fibers will be discussed later in relation to the mechanical properties of the lung.
Schematic diagram of the structural organization of the alveolar interstitium. The alveolar septum extends between a free edge (right) and a perivascular connective tissue sleeve (left), enwrapping a blood vessel (bv). Basement membranes (bm) are associated with epithelium and endothelium, and they bound the interstitial space. Fiber strands (fi) form a continuum. Interstitial cells include: fibroblasts (FB), myofibroblasts (MF), smooth muscle cells (SM), pericytes (PC), various kinds of immune competent cells (ICC), mast cells (MC), lymphatic endothelial cells (LYC), and histiocytes or interstitial macrophages (IMΦ). Alveolar macrophages (AMΦ) are submerged in the alveolar surface lining layer (sll), ly, lymphatic capillary. (Reproduced with permission from Weibel ER, Crystal RG: Structural organization of the pulmonary interstitium. In: Crystal RG, West JB, Weibel ER, Barmes PJ (eds), The Lung: Scientific Foundations, 2nd ed. New York: Lippincott-Raven; 1997:685–695.)
Alveolar septum with free edge (right) showing reinforced entrance ring with elastic fibers (el), collagen fibrils (co), and smooth muscle cell (SM). The two capillaries (C) are on different sides of the septum, as are the two type II cells (EP2). A, alveolar space; EP1, type I cell. (Reproduced with permission from Weibel ER, Gil J: Structure-function relationships at the alveolar level, in West JB, ed.: Bioengineering Aspects of the Lung. New York: Springer-Verlag; 1977.)
The resident interstitial cells of the alveolar septum comprise fibroblasts and contractile cells (myofibroblasts, lipofibroblasts, smooth muscle cells, and pericytes) (Fig. 2-28). Free interstitial cells are part of the defense system usually found in the juxta-alveolar connective tissue sleeves (see below) and include interstitial macrophages (histiocytes), mast cells, and under certain conditions, lymphocytes, plasma cells, and granulocytes.
Fibroblasts are a heterogeneous cell population. Many fibroblasts have notable contractile properties; therefore, they have been termed myofibroblasts. Myofibroblasts contain bundles of microfilaments anchored in patches beneath the plasma membrane. These filament bundles span the entire width of the cell. At the places where the microfilament bundles are connected to the plasma membrane, attachments to the epithelial and/or endothelial basement membrane exist.11,127,128 Through holes in the basement membranes, myofibroblasts directly link alveolar epithelial and capillary endothelial cells.83
Some contractile fibroblasts are equipped with nonmembrane–bound lipid bodies, thus termed lipid interstitial cells or lipofibroblasts.129,130 These cells are more common in rodent than in human lungs and occur particularly during alveolar development and growth. Lipid bodies consist of an osmiophilic rim of amphipathic phospholipids, glycolipids, sterols and specific proteins, and a hydrophobic core of neutral lipids. In many cell types, lipid bodies represent specialized domains for the synthesis of eicosanoid mediators.131 Pulmonary lipofibroblasts seem to be related to the lipid-containing perisinusoidal cell (Ito cell) in the liver in that they might serve as a storage depot for retinoids.130,132 Under certain conditions, lipofibroblasts might provide fatty acid substrates for surfactant synthesis in type II cells.130
The occurrence of smooth muscle cells in the alveolar septa is mostly restricted to the free septal edges where they contribute to the network of alveolar entrance rings (Figs. 2-28 and 2-29). Pericytes abut alveolar capillaries (see above).
Structural Aspects of the Defense System of the Lung
The large and delicate alveolar surface is constantly challenged by inhaled microorganisms and particulate matter. Thus, normal lung function critically depends on an efficient defense system.94,113,133–136 At the alveolar level, the primary defense barrier is the alveolar lining layer. Here, alveolar macrophages are the sentinel phagocytic cells of the innate immune system, as we shall discuss later. In addition, protein components of the innate immune system, including the lung collectins SP-A and SP-D as well as a variety of other antimicrobial peptides (e.g., lysozyme, lactoferrin, defensins, cathelicidins), are present in the alveolar lining layer.
Another set of macrophages forms a second defense line just beneath the alveolar epithelium; that is, in the interstitial space of the lung parenchyma. In the normal lung, these interstitial macrophages (histiocytes) are not found in alveolar septa; instead, they occur only in the connective tissue sleeves at the periphery and in the center of acini where the peripheral fiber system connects with the adventitial sheath of bronchioles and pulmonary arteries (Fig. 2-30). Thus, they are found in regions where lymphatics begin their course toward the major airways in the hilar region where lymph nodes are found. In these juxta-alveolar regions of connective tissue, we usually find the common elements of the defense system (Figs. 2-30 and 2-31). These include lymphatic vessels and several mobile cells. Interstitial macrophages are constantly being replenished by blood monocytes migrating into the interstitial space. Sometimes they become permanent residents in the form of storage cells for “indigestible” foreign matter, such as carbon particles and silicates. The relationship between interstitial macrophages and dendritic cells (see below) is under discussion.137,138 Lymphocytes are less common and are mostly present as T cells whereas B cells and natural killer cells are rare in the normal lung. Granulocytes (neutrophils, eosinophils, and basophils) are present in the human lung, but they are also very rare. Mast cells contain granules storing heparin and histamine as well as peptidases such as tryptases and chymases139 that, in the human, show a characteristic scroll-like substructure (Fig. 2-32) as well as lipid bodies. According to their anatomic location, they show site-specific characteristics, thus displaying considerable heterogeneity.140 Antigen-presenting dendritic cells possess long branched dendritic cell processes (hence, their name) and an irregular, folded nucleus. Phagolysosomes are absent. Once activated, dendritic cells migrate to lymph nodes where they induce the proliferation of antigen-specific T cells; thus, providing a link between innate and adaptive immunity. In addition to their presence within the lung parenchyma, dendritic cells are found within the tracheal and bronchial epithelium where they seem to form a network comparable to the Langerhans cells in the epidermis. Like Langerhans cells, airway dendritic cells are characterized by pentalaminar plate-like organelles (Birbeck granules).141–143 In the ciliated epithelium of bronchi and bronchioles diapedesis is seen; that is, lymphocytes and other leukocytes in the process of penetrating the epithelium to reach the mucus blanket. Plasma cells occur in relatively high numbers around the acini of the seromucous glands of bronchi (Fig. 2-7); hence, it is likely that antibodies are being secreted into the mucus blanket by these glands by a process similar to that occurring in the salivary glands or in the glands of the nasal mucosa.
Light micrograph of human lung showing connective tissue sleeve (arrows) extending from the peribronchovascular space (pbv) around branch of pulmonary artery (PA) and bronchiolus (B) to pulmonary vein branch (PV). Asterisks, lymphatic.
Perivascular connective tissue with lymphatic (Ly) containing a macrophage (MA) with heterogeneous population of “lysosomal” granules. Interstitium (IN) contains fibroblasts (F) and plasma cells (PC). EN, lymphatic endothelium.
Mast cell from human lung containing granules (arrows) with scroll-like substructure. Inset: Scroll-like substructure of mast cell granule at higher magnification. Co, collagen fibrils. (Reproduced with permission from Weibel ER: Lung cell biology, in Fishman A, Fisher AB, eds. Handbook of Physiology. Section 3: The Respiratory System. vol 1. Bethesda, MD: American Physiological Society; 1985: 47–91.)
The third defense line is constituted by the lymph nodes, which are arranged along the major bronchi and extend to subsegmental bronchi about 5 mm in diameter (Fig. 2-33). The most peripheral lymph nodes are tiny, a mere 1 to 2 mm in diameter, but closer to the hilum they become larger, reaching 5 to 10 mm in diameter in the region of the tracheal bifurcation and along the trachea. The lymph nodes from adult human lungs often appear gray or even black because of deposition in the medullary cords of large numbers of macrophages loaded with carbon pigment. This material entered the lung via the airways, primarily as smoke, soot, or coal dust; depending on the size of the particles, they were either deposited on the surface of conducting airways or reached the alveoli. The further down the deposition, the greater the likelihood that this material cannot be eliminated while in the airways, that is, within the mucus blanket. The only exit from the lung parenchyma then is via the lymphatics, but this exit ultimately leads to the blood, a circumstance that is obviously to be avoided. Filtering the lymph in lymph nodes and providing a depository in the medullary cords protects the blood and hence the entire organism from dissemination of indigestible foreign matter and also, in most instances, of infective agents.
Schematic diagram of distribution of lymph nodes and main lymphatic channels along bronchial tree.
Thus, the lymphatic “circulation” in the lung plays an important defense role.6,42 It is unidirectional. It begins as interstitial fluid that seeps from the capillaries and is efficiently drained along the connective tissue fibers toward those connective tissue sleeves in the center and at the periphery of acini where lymph capillaries begin. From there, lymphatic vessels, endowed with valves and an irregular smooth muscle wall, course in septal structures, in the pleura, and peribronchial and perivascular sheaths toward the hilar region (Fig. 2-33). Lymph nodes are intercalated in the course of the lymphatics, which lead the lymph toward the tracheal bifurcation and then along the trachea into the right and left mediastinal lymph channels. The right channel drains into the right subclavian vein; the left, together with the thoracic duct, into the left subclavian vein. Because of the many anastomoses connecting parallel lymphatics, a particular lymph node receives lymph from various lung regions, but the closest regions tend to predominate.
Lung macrophages can be differentiated into several populations according to the compartment they are found in: intravascular, interstitial, airway, and alveolar macrophages.144–146 Of these, the alveolar macrophages, the cell population of the surface lining layer, are of particular importance. They are free cells, endowed with a high phagocytic capacity, which are transiently attached to the surface of the alveolar epithelium by pseudopodia and can crawl over this surface by amoeboid movement (Fig. 2-34). Occasionally, alveolar macrophages can be observed during the passage through an interalveolar pore of Kohn. However, they are submerged beneath the surface film of phospholipids (Fig. 2-35) and, therefore, are part of the surface lining layer of alveoli, more specifically of its hypophase. Alveolar macrophages exert their phagocytic activity within the surface lining layer (Fig. 2-25). Hence, it is not surprising that their vacuoles contain large amounts of ingested surfactant material, in part even tubular myelin. The importance of alveolar macrophages for surfactant removal is underscored by the acquired form of pulmonary alveolar proteinosis, where a defect in surfactant catabolism by alveolar macrophages caused by autoantibodies against granulocyte/macrophage colony-stimulating factor (GM-CSF) leads to an accumulation of surfactant material in the alveoli.147
Alveolar macrophage (MA) seen sitting on epithelial surface of human lung. Note cytoplasmic lamella (arrows) which represents the advancing edge of the cell.
Alveolar macrophage (MA) fixed in its natural position of “flat” attachment to the alveolar epithelium. Arrow points to advancing cytoplasmic leaflet.
Alveolar macrophages are derived from monocytes – indirectly, therefore, from bone marrow cells – and probably reach the alveoli in two steps: first, by settling in the pulmonary interstitial tissue, and second, by migration from the interstitial tissue into the alveoli where they constitute a partly self-reproducing cell population. Their removal seems to involve two different pathways: (1) some of the macrophages undoubtedly move up the bronchial tree in the mucus blanket and eventually appear in the sputum; and (2) others possibly return into the interstitial space. In the normal lung, however, the second path seems to occur exclusively in those alveoli that abut the connective tissue sleeves around larger vessels and conducting airways or on interacinar septa; that is, where the lymphatic capillaries are located. A preferred location appears to be in the respiratory bronchioles at the entrance into the acinus or in the center of the acinus, where one often finds congregations of dust-laden macrophages; this may be at the origin of centroacinar damage observed in smokers, which lead to progressive emphysema. In these places, macrophages either settle as carbon pigment-loaded histiocytes, or they leave the lung parenchyma via lymphatics (Fig. 2-31) to settle in the lymph nodes. The way in which macrophages and/or their ingested material are transferred from the alveolar surface to the interstitial space is still unknown.
Functional Design of the Lung
From the preceding section it has become apparent that the lung is built of a multitude of cells and tissue elements that all serve specific functions in support of the lung's main function: the exchange of oxygen and carbon dioxide between the air and the blood. But it takes more than cells to make a good lung.148 The lung's multiple component structures must be integrated to make an efficient and stable gas exchanger, and this demands a blueprint for the integral architecture of the human lung.7 This must first ensure that the airways and blood vessels are adequately correlated topologically and quantitatively to allow well-matched ventilation and blood flow. It must also realize a complex organization that allows air ventilation, blood perfusion, and gas exchange to function in the most efficient manner. The design principles that govern the architecture of the human lung toward that goal can be characterized as Complexity, Correlativity, and Connectivity. Complexity means that the microscopic gas-exchange units are an integral part of the macroscopic airways and vessels; their architectural correlation determines the efficient approximation of air and blood in the gas exchanger; and connecting all the parts into a whole is achieved with a fiber continuum that pervades the entire lung. The implementation of these principles during development is decisive for “making a good lung.”
Design of the Branching Airway Tree
The entrance to the lung's airways is the trachea (Fig. 2-3), a single tube, the gas-exchange elements where air and blood are brought into close contact are contained in several million units. Between entrance and periphery lies a meticulously designed system of branching airways that serve to conduct the inspired air into those peripheral channels that carry alveoli in their walls and can thus contribute to the exchange of gases between air and blood (Fig. 2-5).8
In the mammalian and human lung the airways are built as dichotomous trees.149 This is the result of lung morphogenesis where the end bud of each airway tube gives rise to two daughter branches. In the human lung this goes on for 23 generations, on average, and, since the number of branches doubles with each generation, there are 223 or about 8 million end branches, generally called alveolar sacs.8 This is an average value; in reality the number of branching generations needed to reach the alveolar sacs is quite variable, ranging from about 18 to 30. This variability results from the fact that the airways form a space-filling tree (Fig. 2-3) whose endings must be homogeneously distributed in space and reach into every corner and into every gap in the available space, determined by the form of the chest cavity into which the lung develops. Some spaces are filled rapidly and the airways cannot continue to divide, whereas in other places more branches are needed to fill the space.
This branching process is accompanied by growth in length and diameter of the airway segments, the tubes between the branching nodes. The length of the tubes is adjusted to cover the distances needed to fill the space homogeneously with endings, whereas the diameter is, grossly speaking, made proportional to the volume of peripheral lung that is supplied by this branch.
Figure 2-36 shows a portion of a cast of the airway tree from a human lung. It is evident that the airways branch by dichotomy and that the length and the diameter of the tubes become gradually reduced with each generation. At first sight, the airway branching seems quite regular, but there is a certain degree of asymmetry in the sense that the two daughter branches differ in length and diameter; in animal lungs asymmetry is more pronounced than in human lungs.
Peripheral portion of cast of human airway tree reaching out to the transitional bronchioles and some respiratory bronchioles (arrows).
Despite asymmetric branching some general rules govern the progression of dimensions along the tree. The diameter of daughter branches is smaller than that of the parent in the sense that the diameter reflects the volume of peripheral lung it supplies with air: larger airways serve larger lung units, smaller airways smaller units. The progression of airway diameters follows the law of Hess (1917)150 and Murray (1926)151 that, in a dichotomous tree, the diameters of the daughter branches, d1 and d2, are related to the parent branch d0 as:
a law that predicts optimization of the airway diameters for convective air flow, providing lowest resistance for lowest dead space.
For a symmetric tree in which d1 = d2 this becomes:
which means that the airway diameter becomes reduced by a factor of cube root of 1/2 or about 0.79 with each generation. Considering the progression of airway dimensions along the tree this law should apply to all successive generations so that we predict the average diameter in generation z to be:
Figure 2-37 shows that this is approximately the case for the first 14 generations of conducting airways.
Average diameter of airways in human lung plotted by generations of regularized dichotomous branching. (Reproduced with permission from Haefeli-Bleuer B, Weibel ER: Morphometry of the human pulmonary acinus. Anat Rec. 1988;220(4):401–414.)
However, a closer look at the airways of the human lung shows that this is only approximately correct.152 It appears that the smaller bronchioles (beyond generation 10) are provided with some safety factor in that the diameter is reduced by a factor of 0.83 rather than the physically optimal 0.79. This allows regulation of airway cross-section by contraction of the bronchiolar muscle sleeve without unduly increasing flow resistance which is very low in small airways (Fig. 2-38).153 Design optimization is limited in favor of physiologic robustness.
Airway resistance to mass air flow is located mostly in the conducting airways and falls rapidly toward the periphery. (Redrawn with permission from Pedley TJ et al. The prediction of pressure drop and variation of resistance within the human bronchial airways. Respir Physiol. 1970;9(3):387–405.)
This symmetric airway model reflects the typical pathway along the airway tree. It has been very useful in modeling the basic rules governing the distribution of air flow as well as the deposition of particles entering the lung. However, it disregards the effects of asymmetric branching. It is possible to construct models that take into account irregularities in branching, for example by considering the number of airways of a given diameter, du, that exist in each generation, and the length of the bronchial pathway that intervenes between the larynx and particular airways (Fig. 2-39).8,154
Distribution of airways of diameter du = 2 mm with respect to (A), generations of branching and (B), bronchial pathway lengths. (Reproduced with permission from Weibel ER: Morphometry of the Human Lung. Heidelberg: Springer-Verlag; 1963.)
An alternative approach is to regard the airways as a system of tubes converging from the periphery, the acinus, toward the center, the trachea.155 By using an ascending ordering system that is employed in analyzing rivers (Strahler system), branches are grouped into orders according to the sequence of convergence, beginning with the smallest most peripheral branches, designated as order l. This ordering pattern is particularly well adapted to a system of irregular dichotomy because the size of branches in one order varies less than with the generations-down model. This approach does not really account for the asymmetry of branching, however; it rather represents an attempt at extracting average data with less variability in each order. The degree of asymmetric branching is reflected in the branching ratio determined as the ratio of the number of branches in order μ to that in order μ + 1. Remarkably, the progression of diameters through the various orders is again roughly proportional to the cube root of the branching ratio. Hence, from a functional point of view both models yield comparable results.
The general conclusion drawn from this type of analysis is that the diameters of the conducting airways are such as to assure optimal conditions for airflow but relaxing physical optimality conditions in the interest of physiologic robustness; the airways of the lung are thus well designed. The total volume of the conducting airways down to generation 14 (the anatomic dead space) is about 150 mL; it is rapidly flushed by simple gas flow in the course of inhaling 500 mL of fresh air during quiet inspiration. Therefore, for the larger airways, optimization for flow and its distribution to peripheral units are essential for good design.
These are the characteristics of the proximal airways built as smooth-walled tubes to distribute convective air flow into the lung. This design ends more or less abruptly when the airways reach lung parenchyma, the complex of alveoli that are arranged around peripheral airways (Fig. 2-40). The airway tree is thus subdivided into two major functional zones (Fig. 2-5): the first about 14 to 16 generations, on average, are designed as conducting airways where air flow is by convection; this is followed by about 8 generations of acinar airways where an axial channel, called alveolar duct, is enwrapped by a sleeve of alveoli with gas-exchange tissue on their surface.
Scanning electron micrograph of lung shows branching of small peripheral bronchiole (B) into transitional bronchioles (T), from where the airways continue into respiratory bronchioles and alveolar ducts (arrows). Note the location of the pulmonary artery (a) and vein (v) as well as visceral pleura (bottom).
In the human lung the transition is not abrupt. At some point the smooth bronchiolar wall becomes interrupted by one or two alveoli (Fig. 2-41). This so-called transitional bronchiole (Fig. 2-5) marks the entrance into an acinus.9 It is followed by some three generations of respiratory bronchioles where an increasing fraction of the wall surface is occupied by alveoli, until the alveolar ducts are reached where the central air duct is completely surrounded by alveoli (Fig. 2-42). These acinar airways continue to branch by dichotomy. Their length and diameter decrease with each generation, but the slope does not follow the law of reduction by the cube root of 1/2; the diameters of respiratory bronchioles and alveolar ducts change very little with each generation.9 Does this arrangement imply less than an optimal design? On the contrary, the cube-root-of-1/2 law relates to optimizing mass flow of a liquid or air. In the most peripheral airways, mass airflow is only part of the means of transporting O2 toward the air–blood barrier. Since the airways are blind-ending tubes and since a sizable amount of residual air remains in the lung periphery after expiration, O2 molecules must move into the residual air by diffusion (Fig. 2-43). However, diffusion of O2 in the gas phase is best served by establishing as large an interface as possible between residual air and the fresh air that flows in from the trachea.16 In fact, since the airway diameter remains nearly unchanged, the total airway cross section nearly doubles with each generation beyond generation 14.
Respiratory bronchiole (RB) from human lung cut along its axis toward the transition to alveolar ducts (AD). Note lining by cuboidal airway epithelium (asterisks) and the occurrence of respiratory patches (arrows) before alveoli proper (arrowheads) appear. PA marks branches of pulmonary artery. Inset: Higher magnification of one of the respiratory patches in the wall of the respiratory bronchiole with capillaries (arrow) and alveolar macrophage (M). The cuboidal epithelium (E) with cilia is replaced by thin squamous epithelium of alveolar type 1 cell. Note thick fibrous layer (F) with smooth muscle cells.
Scanning electron micrograph of a complete acinus from a silicon rubber cast of a human lung partly dissected to show transitional (T) and respiratory (R) bronchioles as well as alveolar ducts (AD) and alveolar sacs (AS). Lines mark approximate boundary of 1/8 subacinus. (Reproduced with permission from Haefeli-Bleuer B, Weibel ER: Morphometry of the human pulmonary acinus. Anat Rec. 1988;220(4):401–414.)
Oxygen molecules reach alveoli by combined mass airflow and molecular diffusion, the importance of diffusion increasing toward the periphery.
The dimensions of the airway tree influence the ventilatory flow of air in a number of ways. First of all, airflow velocity falls along the airway tree because the total cross-sectional area of the airways increases with every generation (Fig. 2-44); whereas the cross-sectional area of the trachea is about 2.5 cm2, that of the 1024 airways in the 10th generation taken together is 13 cm2, and as we approach the acinar airways, the total cross section reaches 300 cm2. However, since the same air volume flows through all generations, the flow velocity falls by more than 100-fold from the trachea to the acini: at rest, the mean flow velocity on inspiration is about 1 m s−1 in the trachea and less than 1 cm s−1 in the first-order respiratory bronchioles. In exercise, the flow velocities are up to 10 times greater, in proportion to the increased ventilation. This is discussed further when considering the relative importance of convection and diffusion in bringing O2 to the alveolar surface for gas exchange.
As total airway cross-section increases with the generations of airway branching, the mass flow velocity of inspired air decreases rapidly, falling below the molecular velocity of O2 diffusion in air as we enter the acinus (see Fig. 2-66). (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
The size of airways also determines the resistance to airflow. However, the overall resistance is rather small; it is given by the reciprocal of the ratio of ventilatory airflow to the pressure difference between the mouth and alveoli, which is normally no greater than about 1 cmH2O (mbar) or less than 1 mm Hg. It is large enough, however, to potentially affect the distribution of ventilation to the many gas-exchange units. Because, in laminar flow, the resistance is inversely proportional to d4 the distribution of air flow depends on a delicate balance of the size of parallel airway tracts. Even a slight narrowing of one of the two daughter branches at a branch point will cause disproportionate air flow to the other branch and thus result in ventilation inhomogeneity.
Since the diameter of airways decreases as they branch (Fig. 2-37), one would suspect that their resistance increases toward the periphery. Apparently this is not the case, as the major pressure drop along the airways occurs in medium-sized bronchi; because the airway diameter decreases with a factor larger than the optimal 0.79 resistance becomes very low in the small bronchioles (Fig. 2-38).153 This is further accentuated by the fact that the thin-walled bronchioles become widened as the lung expands on inspiration because they are subject to the tissue tensions in the coarse fiber system of the lung. Therefore, airway resistance is seen to fall as lung volume increases. When this effect of tissue tension is disturbed, as in emphysema, some small bronchioles may collapse. This causes ventilation of the peripheral lung units to become highly uneven.
This biophysical way of looking at the significance of the progression of airway dimensions has recently been complemented by the alternative notion that the airway and vascular trees could be determined by the laws of fractal geometry.156 Fractal trees are formed by repeating the branching pattern from one generation to the next. If the proportion between parent and daughter branches remain the same this is called self-similar branching. In a dichotomous tree the diameter is ideally reduced by a factor of 2−1/Df where Df is the fractal dimension. Since the airway tree is nearly space-filling Df ã 3, which means that the Hess–Murray law also follows from fractal geometry as a rule of optimal design, but because the reduction factor is somewhat larger than 2−1/3 it follows that the actual fractal dimension of the airway tree is a bit larger than 3; this is possible because the tree is “cut off” at the entrance to the acini and the “space” becomes filled with alveoli.157,158
Design of the Vascular Tree
In many ways, the course and pattern of dimensional changes in the pulmonary blood vessels resemble those of the airways. Figure 2-3 shows that the pulmonary arteries follow the airways closely, out to the smallest branches; together they form the axis of lung parenchymal units of varying order: acinus, lobule, segment, lobe. As indicated, the veins are differently disposed, lying in the boundary between two or three adjacent units (Figs. 2-30 and 2-45).
Casts of airways and blood vessels of human lung. A. shows how the pulmonary artery (red) closely follows the airways (yellow) to the periphery, whereas the pulmonary vein branches (blue) lie between the units. Note that the diameter of the pulmonary arteries is similar to that of the accompanying airway, but becomes relatively smaller toward the periphery (arrow); small supernumerary arteries take off at right angles. B. Higher power view of group of acini (circle), corresponding about to a secondary lobule, shows how artery penetrates into center of gas-exchange unit with veins collecting the blood around the periphery. Arrowheads point to alveolar pouches on transitional and respiratory bronchioles.
The diameter of each pulmonary artery branch also approximates closely that of the accompanying bronchus (Fig. 2-45A). Therefore, it is evident that the diameter law presented earlier for airways must also hold for the first 10 to 16 generations of pulmonary arteries (Fig. 2-37). However, the pulmonary arteries divide more frequently than the airways; very often, small branches leave the artery at right angles and supply blood to the parenchymal units adjacent to the bronchus (Fig. 2-45B). From a count of precapillaries, it seems that the pulmonary arteries divide, on the average, over 28 generations, as compared with 23 for the airways. The diameter of these terminal vessels is about 20 to 50 μm; if this range is plotted onto an extension of the graph of Figure 2-37 to generation 28, it falls on the curve that is obtained by extrapolation from the major branches8,16:
This suggests that the pulmonary arteries abide to the cube-root-of-1/2 law from beginning to end. Evidently, the blood is transported to the capillary bed by mass flow only. Therefore, there is no reason to deviate from this fundamental law of design, which minimizes the loss of energy caused by blood flow.
In a thorough analysis of the pulmonary vascular trees159 conceived as fractal structures it has been shown that the fractal dimension of both arteries and veins is 2.71, thus somewhat less than 3. The diameter reduction factor is therefore slightly smaller than cube-root-of-1/2, and the diameters follow the regression:
Therefore, in contrast to the airways, the resistance to blood flow increases along the pulmonary arteries and is highest in the most peripheral branches or arterioles. The resistance profile of the pulmonary arteries is thus the same as in the systemic circulation.
The alveolar capillary network of the lung is very different from that of the systemic circulation. Whereas in muscle, for example, long capillaries are found to be joined in a loose network, the capillaries of the alveolar walls form dense meshworks made of very short segments (Fig. 2-46).8,160 The meshes are so dense that some people believe blood flows through the alveolar walls like a sheet rather than through a system of interconnected tubes. In this sheet-flow concept,161 the sheet is bounded by two flat membranes, the air–blood barrier, connected by numerous “posts.” When blood flows through this sheet, it is not channeled in a given direction but has freedom to move in a tortuous way between the posts. Although this concept oversimplifies the actual structural conditions, it does provide a useful description of the pattern of blood flow through the alveolar walls and explains why blood flow is not interrupted when some parts of the capillary bed become squashed flat at high inflation levels (see Fig. 2-58); the capillaries that remain open in the corners are simply some channels of this broad sheet. Furthermore, it is important to note that the capillary network or sheet is continuous through many alveolar walls (Fig. 2-46), probably at least throughout the entire acinus, if not for greater distances.160 Consequently, it is not possible to isolate microvascular units. One finds, rather, that arterial end branches simply feed into this broad sheet at more or less even distances and that the veins drain these sheets in a similar pattern. However, now we must remember that the arteries reach the acinus along the airways, whereas the veins are in a peripheral location (Fig. 2-45). In principle, therefore, blood flows through the acinar capillary sheet from the center to the periphery of the acinar gas-exchange unit.
Alveolar capillary network demonstrated with gold labeling of blood plasma in a physiologically perfused preparation of a rabbit lung. The dense capillary network spans between end branches of pulmonary artery (a) and vein (v) and extend through many alveolar septa around alveolar duct (AD). Inset: Plastic cast shows the dense meshes of the network. Scale bar = 20 μm. (Inset used with permission of P. Burri.)
Design of Pulmonary Parenchyma
The airspaces and blood vessels of lung parenchyma are designed to facilitate gas exchange between air and blood. To this end a very large area of contact between air and blood must be established; for the human lung it is sometimes compared with the area of a tennis court in size. Furthermore, the tissue barrier separating air and blood must be kept as thin as possible—it is found to be about 50 times thinner than a sheet of airmail stationery. This is important, because less than 1 second is available for loading O2 onto the erythrocytes as they flow through the lung's gas-exchange region.162
The first design feature to this end is the formation of alveoli in the walls of all airways within the acinus—that is, in the ventilatory gas-exchange units beginning with a transitional bronchiole (see above) (Fig. 2-40). In the human lung, one estimates that there are about 30,000 acini,9 and 400 million alveoli163 so that each of the ventilatory gas-exchange units contains some 13,000 alveoli, on average, connected to about seven to nine generations of acinar airways, respiratory bronchioles, and alveolar ducts.9
The alveoli are so densely packed that they occupy the entire surface of alveolar ducts; they are separated from each other by delicate alveolar septa that contain the capillary network (Fig. 2-47). About half the space of the septum is taken up by blood, which is thus exposed to the air in two adjacent alveoli (Fig. 2-48A). Although the barrier separating air and blood is extremely thin, we find the capillaries to be provided with a complete endothelial lining, as the alveolar surface of the septum is lined by an epithelium.11 We have seen earlier that these two cell linings are very much attenuated over the greatest part of the surface.
Scanning electron micrograph of human lung parenchyma. Alveolar ducts (AD) are surrounded by alveoli (A), which are separated by thin septa (S). K, interalveolar pore of Kohn.
In the alveolar wall, shown in (A) in a scanning electron micrograph from a human lung, the capillary blood (C) with its erythrocytes (ec) is separated from the air by a very thin tissue barrier (B). Short arrows mark intercellular junctions of alveolar epithelium that course toward interalveolar pores of Kohn (K). The model (B) shows the capillary network (red) to be interwoven with the meshwork of septal fibers (green), the course of which is marked by asterisks in (A). The epithelial lining (yellow) that crosses the septum at interalveolar pores (K) is removed on the upper surface of the septum to show the capillary. The septal fibers are anchored on the strong fiber bundle marking the free edge of the septum or the alveolar entrance ring (AE). (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
To make the barrier very thin, the interstitial structures must also be reduced to the minimum required (Fig. 2-49). The septal interstitium contains very few cells, mostly slim fibroblast with long extensions; these contain fine bundles of contractile filaments that serve an as yet unknown mechanical function. The septal interstitium usually does not contain cells of the defense system or lymphatics.
Alveolar septum from human lung lined by type I epithelium (EP1) with capillary lined by endothelial cell (EN) that is associated with processes of pericytes (P). Substantial interstitial space (IN) with collagen and elastic fibers (cf) and fibroblasts (F) occurs on one side only, whereas minimal air–blood barrier is formed on other side by fusion of basement membranes (BM) of endothelium and epithelium.
Internal Support of Parenchymal Structures: The Pulmonary Fiber Continuum
This extraordinary reduction of the tissue mass in the alveolar septa inevitably introduces a number of major problems. How is it possible to secure the mechanical integrity of the system if we consider that several forces act on the septal tissue with a tendency to disrupt it? The thin barrier must not only withstand the distending pressure of the capillary blood due to both hemodynamic forces and gravity, particularly in the lower lung zones, but must also keep the capillary bed expanded over a very large surface—a task that is made difficult because surface forces that act on the complex alveolar surface would tend to collapse alveoli and capillaries (see further below). This requires a very subtle, economical design of the fibrous support system.164,165
The problem of supporting the capillaries on connective tissue fibers with as little tissue as possible has been solved ingeniously: we find that the fiber network is interlaced with the capillary network.166 Figure 2-48B shows that when the fibers are taut, the capillaries weave from one side of the septum to the other. This arrangement has a threefold advantage: (1) it allows the capillaries to be supported unit by unit directly on the fiber strands without the need of additional “binders”; (2) it causes the capillaries to become spread out on the alveolar surface when the fibers are stretched; and (3) it optimizes gas-exchange conditions by limiting the presence of fibers, which must interfere with O2 flow, to half the capillary surface. The thin section of a capillary shown in Figure 2-49 reveals that an interstitial space with fibers and fibroblasts exists on only one side of the capillary, whereas on the other the two lining cells, endothelium and epithelium, become closely joined with only a single common basement membrane interposed. Therefore, over half the surface of the capillary blood is separated from the air merely by a minimal tissue barrier made of epithelial and endothelial cytoplasmic sheets with their basement membranes fused leaving no interstitial space that could enlarge with interstitial pulmonary edema (Fig. 2-17).
The principal structural “backbone” of the lung is a continuous system of fibers anchored at the hilum and put under tension by the negative intrapleural pressure that tugs on the visceral pleura.165 The general construction principle follows from the formation of the mesenchymal sheath of the airway units in the developing lung; as the airway tree grows, its branches remain separated by layers of mesenchyme within which blood vessels form. When fiber networks develop within this mesenchyme, they enwrap all airway units and extend from the hilum right to the visceral pleura. The pulmonary fiber system hence forms a three-dimensional fibrous continuum that is structured by the airway system and is closely related to the blood vessels. By virtue of the design of this fibrous continuum, the lung becomes, in fact, subdivided into millions of little bellows that are connected to the airway tree, as represented schematically in Figure 2-50; these structures expand with expansion of the chest because the tension exerted on the visceral pleura by the negative intrapleural pressure becomes transmitted to the bellows' walls through that fiber system.
The lung's fiber continuum: axial fibers (red) extend from airways into the alveolar ducts as a network of entrance rings into alveoli (yellow); peripheral fibers (black) extend from the pleura to interlobular septa; the septal fibers (green) in the alveolar walls are anchored in peripheral and axial fibers. Arrows indicate the traction on the pleura exerted by thorax and diaphragm. (Reproduced with permission from Weibel ER: Looking into the lung: What can it tell us? Am J Roentgenol. 1979;133(6):1021–1031.)
To try to put some order into this fiber system, we can first single out two major components that can be identified easily (Fig. 2-50). First we find that all airways – from the mainstem bronchus that enters the lung at the hilum out to the terminal bronchioles and beyond – are enwrapped by a strong sheath of fibers. These fibers constitute the axial fiber system; they form the “bark” of the tree whose roots are at the hilum and whose branches penetrate deep into lung parenchyma, following the course of the airways. A second major fiber system is related to the visceral pleura, which is made of strong fiber bags enwrapping all lobes. We then find connective tissue septa penetrating from the visceral pleura into lung parenchyma, separating units of the airway tree. We call these fibers the peripheral fiber system because they mark the boundaries between the units of respiratory lung tissue.
The peripheral fiber system subdivides the lung into a number of units that are not simple to define because they form a continuous hierarchy in accordance with the pattern of airway tree branching. However, as we have seen, two such units appear to be natural: the lobes, which are demarcated by a more or less complete lining by visceral pleura with a serosal cleft interposed (Fig. 2-1); and the acinus, the parenchymal unit in which all airways participate in gas exchange.
The acinus is the functional unit of the pulmonary parenchyma. The airway that leads into the acinus, the transitional bronchiole, continues branching within the acinus for about 6 to 10 additional generations (Figs. 2-5 and 2-40). These intra-acinar airways, called respiratory bronchioles and alveolar ducts, also carry in their wall relatively strong fibers of the axial fiber system, which extend to the end of the duct system. However, since the walls of intra-acinar air ducts are densely settled with alveoli, these fibers are reduced to a kind of delicate network that constitutes the “wall” of the alveolar ducts. The meshes of this network that encircle the alveolar mouths are generally called alveolar entrance rings; it is this fiber network that allows alveoli to be formed as open chambers with free edges of the alveolar septa (Figs. 2-47 and 2-51).148 These fiber rings are associated with some smooth muscle cells (Fig. 2-29), and they serve as a scaffold for a network of finer fibers that spread within the alveolar septa (Figs. 2-48B and 2-51). However, in a fiber system there may be no loose ends. Accordingly, the septal fiber system must be anchored at both ends—on the network of axial fibers around the alveolar ducts, and on extensions of the peripheral fibers that penetrate into the acinus from interlobular septa. Thus, the fiber system of the lung becomes a continuum that spans the entire space of the lung, from the hilus to the visceral pleura (Fig. 2-50). It is put under varying tension as the pleural bag is expanded by the chest wall and diaphragm. It thus functions as a tensegrity structure where structural integrity is maintained only if the fiber continuum is under tension and undisrupted.167,168
Connective tissue stain reveals the strong fiber rings (arrows) that demarcate the alveolar ducts (AD) and respiratory bronchioles (RB). Pleura (PL) extends as peripheral fibers into parenchyma. (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
The continuous nature of a well-ordered fiber system is an essential design feature of the lung.148 This becomes evident in emphysema. When some fibers are disrupted, they cannot be kept under tension. They retract and larger airspaces form as the fiber system is rearranged near the damage. Small foci of emphysema form in most lungs in the course of time.
The fiber system serves mainly as a mechanical support for the blood vessels, with which it is intimately associated in an orderly fashion.148 The pulmonary artery branches in parallel with the airway tree, but it is not related to the axial fiber system. Like the pulmonary veins the pulmonary arteries are associated with those parts of the peripheral fiber system that form an adventitial sheath on the larger vessels of both types and also form a boundary sheath on the outer surface of bronchi where alveolar complexes touch on the bronchial wall. Therefore, it is justified to characterize the connective tissue surrounding bronchi and pulmonary arteries as a peribronchovascular space, which houses the lymphatics as well as the systemic bronchial arteries and their branches. In fact, this space is continuous with the septal connective tissue that enwraps the pulmonary veins (Fig. 2-30) and is continuous with the visceral pleura. However, whereas the arteries penetrate into the acinus, the veins remain at the periphery and are thus located between the airway units (Fig. 2-45). In the alveolar septa, the capillary network spreads out as a broad sheet of vessels whose paths are continuous throughout the system of interconnected alveolar septa (Fig. 2-46). We have seen that these capillaries are intimately related to the septal fiber system (Fig. 2-48B).
Parenchymal Mechanics and Tissue Design
As in all connective tissue, the fibers of the lung are composed of collagen and elastic fibers.164 The collagen fibers are bundles of fibrils bound together by proteoglycans; they are practically inextensible (less than 2%) and have a very high tensile strength; they rupture at loads of 50 to 70 dyn/cm−2, which means that a collagen fiber of 1-mm diameter can support a weight of over 500 g. In contrast, elastic fibers have a much lower tensile strength but a high extensibility. They can be stretched to about 130% of their relaxed length before rupturing.
In the fiber system of lung parenchyma, collagen and elastic fibers occur in a volume ratio of about 2.5:1, whereas this ratio is 10:1 for the visceral pleura. In a relaxed state, one finds the collagen fibers to be longer than the accompanying elastic fibers, so that they appear wavy. Because of the association between “rubber-like” elastic and “twine-like” collagen fibers, the connective tissue strands behave like an elastic band. They are easy to stretch up to the point where the collagen fibers are taut, but from there on they resist stretching very strongly.
The elastic properties of the lung's fiber system can be studied by filling the airways with fluid so as to eliminate the effects of surface tension. This reveals that the lung's fiber system has a high compliance until high levels of inflation are reached, and that the retractive or recoil force generated by the fiber system amounts to no more than a few millibars at physiologic inflation levels. The actual recoil force in the air-filled lung, reflected by the negative pressure in the pleural space, is appreciably higher, but this is caused by surface tension rather than the retractive force of the fibers.
Surface tension arises at any gas–liquid interface because the cohesive forces between the molecules of the liquid are much stronger than those between the liquid and gas.169 As a result, the liquid surface tends to become as small as possible. A curved surface, such as that of a bubble, generates a pressure that is proportional to the curvature and the surface tension coefficient γ. The general formula of Gibbs relates this pressure, Ps, to the mean curvature
In a sphere, the curvature is simply the reciprocal of the radius r (Laplace's law):
The most critical effect of surface tension is that it endangers stability of the airspace, because a set of connected “bubbles,” the alveoli, is inherently unstable: The small ones should contract and the large ones expand. Since the 400 million alveoli are all connected with each other through the airways, the lung is inherently unstable: Why do the alveoli not all collapse and empty into one large bubble? There are two principal reasons.16,170
The first reason is one of tissue structure. The alveoli are not simply soap bubbles in a froth. Rather, their walls contain an intricate fiber system, as we have seen. Thus, when an alveolus tends to shrink, the fibers in the walls of adjoining alveoli are stretched, and this prevents the alveolus from collapsing altogether. It is said that alveoli are mechanically interdependent and this stabilizes them.
The second reason is related to the fact that the alveolar surface is not simply water exposed to air but is lined by surfactant171 (Figs. 2-25 and 2-52), which has peculiar properties in that its surface tension coefficient γ is variable.169,172 From a large volume of evidence, it is now established that surface tension falls as the alveolar surface becomes smaller, and that it rises when the surface expands. Because of this feature, which is due to the phospholipoprotein nature of alveolar surfactant (see above), alveoli do not behave like soap bubbles whose surface tension remains constant. When an alveolus begins to shrink, the surface tension of its lining layer falls and the retractive force generated at the surface is reduced or even abolished. Combined with interdependence, this property of surfactant allows the complex of alveoli to remain stable.170
Alveolar septum of human lung fixed by perfusion through blood vessels shows alveolar lining layer (LL) in crevices between capillaries (C) topped by surfactant film that appears as a fine black line (arrows). Note the type II cell with lamellar bodies and the fold in thin tissue barrier (bold arrows). (Used with permission of M. Bachofen and G. Wolff Basel.)
Which of the two factors for stabilizing lung structure is now the most important: interdependence or surfactant properties? It turns out that both are essential. If one depletes the lung of its surfactant lining by washing with a detergent, the pressure–volume curve changes dramatically173 (Fig. 2-53). On deflation, lung volume falls rapidly. If we look at samples from lungs fixed at the same volume (60% total lung capacity) but derived from either normal or detergent-rinsed lungs, we find that surfactant depletion causes the alveoli to collapse. However, this causes the alveolar ducts to enlarge, stretching the strong fiber nets at the mouths of the collapsed alveoli. The ducts do not collapse because of interdependence between adjacent units.
Comparison of pressure–volume curve of a normal air-filled rabbit lung (heavy line) with that of a surfactant-depleted lung (broken line). The thin line with paired arrows represents small hysteresis when breathing between 40% and 80% TLC along the deflation curve. (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
In the normal air-filled lung, surfactant properties and interdependence owing to fiber tension both contribute to stabilizing the complex of alveoli and alveolar ducts.174 To understand this, let us examine Figure 2-54, which shows a highly simplified diagram of a parenchymal unit. Interdependence is established by the continuum of axial, septal, and peripheral fibers. Surface tension exerts an inward pull in the hollow alveoli, where curvature is negative. However, over the free edge of the alveolar septa, along the outline of the duct, the surface tension must push outward because there the curvature is positive.16 The latter force must be rather strong, because the radius of curvature is very small on the septal edge; but this force is counteracted by the strong fiber strands, usually provided with some smooth muscle cells, that we find in the free edge of the alveolar septum (Figs. 2-29, 2-47, and 2-51). Thus, interdependence is an important factor in preventing the complex hollow of the lung, where negative and positive curvatures coexist, from collapsing. However, its capacity to do so is limited and requires low surface tensions, particularly on deflation when the fibers tend to slack. If surface tension becomes too high, the lung's foam-like structure will partly collapse in spite of fiber interdependence.
Model of the disposition of axial, septal, and peripheral fibers in an acinus showing the effect of surface forces (arrows). (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
This is of considerable physiologic importance. It is sometimes claimed that alveoli pop open when the lung is inflated, collapsing on deflation. That is correct when starting with a deflated lung (Fig. 2-53): collapsed alveoli open up along the inflation curve. But that is not the way we breathe. The normal breathing cycle operates on the deflation slope of the pressure–volume curve (Fig. 2-53) with small hysteresis, a state that is maintained by taking a deep sigh intermittently up to TLC. In this condition the surface tension is kept low because the surfactant lining is spread out172 and alveoli do not collapse. When we breathe in and out between 80% and 40% of total lung capacity, the range of normal breathing in exercise, alveoli change their size very little. In contrast to the twofold change in air volume the alveolar surface area changes by only about a factor of 1.2.174 The reason for this is that the main change in air volume does not occur in alveoli, but predominantly in the alveolar ducts as shown in Figure 2-55, and this is very favorable for acinar ventilation. This differential volume change can be explained by the effect of surface forces: at 40% TLC surface tension γ is nearly 0 but it increases to 12 mN·m−1 at 80% TLC 2. As the lung inflates this causes the positive surface force to become strong on the free edge of alveolar septa (Fig. 2-54), thus causing the duct cross section to widen, while shrinking when the forces decrease on deflation (Fig. 2-55). In this process the alveolar septa become stretched on inflation by only a small degree, a mere 20% in area. The acinus is thus well ventilated whereas the gas-exchange surface is little affected by varying air volume.148
Light micrographs of sections of lung parenchyma in rabbit lungs perfusion fixed on deflation to 40% TLC (A), and 80% TLC (B), respectively (compare Fig. 2-53). Note that the size of the alveolar ducts (d) is markedly enlarged in 80D due to the surface forces acting on the free edges of alveolar septa (arrow heads). (Preparations used with permission of H. Bachofen, University of Bern.)
Micromechanics of the Alveolar Septum
We must finally consider the mechanical factors that shape the alveolar septum in the air-filled lung. As we have seen, the alveolar septum is made of a single capillary network that is interlaced with fibers (Fig. 2-48). When the fibers are stretched, the capillaries bulge alternatingly to one side or the other, and this causes pits and crevices to occur in the meshes of the capillary network.
This irregular surface is to some extent evened out by the presence of an extracellular layer of lining fluid, which is rather thin over the capillaries but forms little pools in the intercapillary pits (Fig. 2-52).175 This lining consists of an aqueous layer of variable thickness, called the hypophase, and surfactant, which forms a film on the surface of the hypophase. The hypophase seems to contain considerable amounts of reserve surfactant material, which occurs in a characteristic configuration called tubular myelin (Figs. 2-25 and 2-26).
In the alveolar septum, the tissue structures are extremely delicate, as we have seen. Therefore, its configuration is not exclusively determined by structural features but results from the molding effect of various forces that must be kept in balance. Figure 2-56 shows how the three principal mechanical forces – tissue tension, surface tension, and capillary distending pressure – interact in the septum.16 The fibers of the alveolar septum are under a tension whose magnitude depends on the level of lung inflation. This tends to straighten out the fibers, so that a force (pressure) normal to the fiber axis results, which is responsible for shifting the capillaries to one side of the septum or the other (Figs. 2-48B and 2-56). The walls of the capillaries are exposed to the luminal pressure, which is the result of blood pressure in pulmonary arteries and veins but also depends on gravity, for one finds wider capillaries at the bottom of the lung than at the top. If this distending pressure acts homogeneously over the circumference of the capillary, it will push against the fibers on one side but will cause the thin barrier on the opposite side to bulge outward. This effect is to some extent counteracted by surface tension, which exerts a force normal to the surface (Fig. 2-56). This force depends on two factors. Its direction depends on the orientation of curvature, acting toward the alveolar space over concave regions (negative curvature) and toward the tissue over convexities (positive curvature); and its magnitude depends on the degree of curvature and on the value of the surface tension coefficient γ.
Model showing the micromechanical forces of surface tension, tissue tension, and capillary distending pressure that shape the alveolar septum. (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
The alveolar septum achieves a stable configuration when all these interacting forces are in balance.176 Combined forces tend to squash the capillary flat; this happens at high levels of lung inflation when the fibers are under high tension and the surface tension coefficient of surfactant reaches its highest value because of expansion of the surface. On deflation, the fibers are relaxed and surface tension falls drastically. The capillary distending pressure now exceeds both the tissue and the surface forces, with the result that the slack fibers are bent, weaving through the capillary network, whereas the capillaries bulge slightly toward the airspace. Surface tension is apparently so low as to permit a considerable degree of surface “crumpling” to persist (Fig. 2-57).
Alveolar septum of air-filled rabbit lung perfusion fixed at 60% TLC shows empty capillaries (C), which bulge toward the alveolar airspace (A). Note pools of surface lining layer in the crevices between capillaries (arrows) and film spanning across alveolar pore (double arrows). (Reproduced with permission from Gil J et al. Alveolar volume-surface area relation in air- and saline-filled lungs fixed by vascular perfusion. J Appl Physiol Respir Environ Exerc Physiol. 1979;47(5):990–1001.)
The importance of the balance between the forces that act on the septum is also shown in Figure 2-58.177 The specimen of panel B was fixed under zone 3 perfusion conditions, where capillary pressure is larger than alveolar pressure, and all the capillaries are wide, partly bulging toward the airspace, as in Figure 2-57. This is different in panel A, which was fixed under zone 2 conditions where capillary pressure is close to alveolar pressure. In the flat part of the septum, the capillaries are squashed flat, because the surface and tissue forces now exceed the vascular distending pressure. However, it is interesting that the capillaries remain wide in the corners where three septa come together. The distribution of surface forces causes the internal pressure to be lower in the region of these corners, as we can see intuitively from Figure 2-54.
Scanning electron micrographs of alveolar walls of rabbit lungs fixed under (A), zone 2 and (B), zone 3 conditions of perfusion. Note that capillaries (C) are wide in zone 3 and slit-like in zone 2, except for “corner capillaries,” which are wide in either case. (Reproduced with permission from Bachofen H et al: Morphometric estimates of diffusing capacity in lungs fixed under zone II and zone III conditions. Respir Physiol. 1983;52(1):41–52.)
The Lung as Gas Exchanger
The structures discussed so far are designed to ultimately serve the lung's main function, gas exchange between air and blood, in relation to the body's varying O2 needs.178 These are set by the energetic demands of the cells and their mitochondria when these produce ATP by oxidative phosphorylation to allow the cells to do work. This process requires a flow of O2 to be maintained from the lung to the cells, as will be discussed later. It proceeds along the respiratory system through various steps: into the lung by ventilation, to the blood by diffusion, through the circulation by blood flow, from the blood capillaries by diffusion to the cells and mitochondria, where it disappears in the process of oxidative phosphorylation.16 A number of basic features characterize this system179: (1) under steady-state conditions the O2 flow rate, O2 is the same at all levels, that is, O2 uptake in the lung is equal to O2 consumption in the tissues; (2) the basic driving force for O2 flow through the system is a cascade of O2 partial pressure which falls from inspired PO2 down to near zero in the mitochondria; (3) the O2 flow rate at each step is the product of a partial pressure difference and a conductance which is related to structural and functional properties of the organs participating in O2 transfer, as will be discussed below in detail.
With respect to gas exchange in the lung (Fig. 2-59), the O2 flow rate is determined by the Bohr equation:180
Model of gas exchange showing gradual rise of capillary Po2 (PcO2) as blood flows through capillary until it approaches alveolar Po2 (PaO2). (Reproduced with permission from Weibel ER: The Pathway for Oxygen. Cambridge, MA: Harvard University Press; 1984.)
The important point is now that all parameters to the right of this equation may be significantly affected by design features. We will see that DLO2 is largely determined by the surface area and the thickness of the air–blood barrier. The O2 partial pressure difference is established by ventilation and perfusion of the gas-exchange units, and this may be affected by the design of the airway and vascular trees, particularly in the acinus.178
The Pulmonary Diffusing Capacity
In the equation mentioned earlier, DLO2 is the total conductance of the gas exchanger for O2 diffusion from the alveolar air into the capillary erythrocytes until it is bound to hemoglobin. It can be estimated physiologically if we can measure O2 uptake V̇O2 and estimate the effective PO2 difference between alveolar air and capillary blood, not a trivial undertaking as the change in capillary PO2 as O2 is being taken up must be integrated (Fig. 2-59). On the other hand the conductance is a physical characteristic. Therefore, it should be possible to calculate a theoretical value of DLO2 from the physical properties of the gas exchanger, its dimensions and material properties.181,182 To do that we must consider the geometry of the structures involved, alveoli, tissue barrier, and capillary blood, in setting up a physical model of DLO2. In the first step, we can break the process into two steps (Fig. 2-60):183 (1) O2 flow across the barrier or what has been called the membrane conductance DMO2 and (2) O2 binding to hemoglobin in the red blood cells or the conductance of capillary blood DeO2 These two conductances are in series. Accordingly their overall effect on O2 flow is obtained by adding their resistances or the reciprocal of the conductance:
Morphometric model for calculating diffusion capacity, DL. Its two components are: (1) the membrane conductance DM, which extends from the alveolar surface (SA) to the nearest erythrocyte membrane traversing the tissue barrier, the capillary surface (Sc), and the plasma layer over the distance τb; and (2) the conductance of the erythrocyte interior, De, that depends on the capillary and the erythrocyte volume, Vc and Ve. (See text.)
The two conductances DMO2 and DeO2 are of very different nature. DMO2 is the conductance of a diffusion barrier that offers “passive” resistance to diffusion and thus depends essentially on the material properties of the barrier, estimated by a diffusion coefficient K, and on the dimensions of the barrier. The larger the surface area S and the thinner the barrier thickness τ the greater DMO2, according to the formula DMO2 = K·S/τ. In contrast, DeO2 is related to a more complex process that involves, besides diffusion, the binding of O2 to hemoglobin, which is a nonlinear process.
The Membrane Conductance (DMO2)
The structural characteristics of the membrane conductor are seen in Figure 2-60. It is made of the two layers that separate air in alveoli from the erythrocytes in the capillary: the tissue barrier and the layer of blood plasma. In addition, an alveolar lining layer of varying thickness spreads over the epithelial surface. Even though these layers have distinct characteristics; in effect they act as a single diffusion barrier.182
As discussed earlier in this chapter, the tissue barrier is a complex structure. Its two bounding surfaces are formed by independent cell layers, epithelium and endothelium, and they are related to two independent functional spaces, alveoli and capillaries. The two surfaces are not perfectly matched, and the thickness of the barrier varies considerably (Fig. 2-60). Over about half the surface the tissue barrier shows minimal thickness compatible with an intact structure: The thin cytoplasmic leaflets of type I epithelial cells are joined to the thin extensions of endothelial cells by the fused basement membranes leaving no interstitial space. In this region we also find the surface lining layer to be very thin. Over the other half the barrier is thicker because of the occurrence of supporting connective tissue fibers (Fig. 2-49) and the presence of cell bodies of epithelial and endothelial cells as well as fibroblasts, and the lining layer can form deeper pools (Fig. 2-52).
The plasma layer shows even greater variation in its thickness and distribution. Since erythrocytes are of about the same dimension as the capillaries, the plasma layer that separates them from the endothelium can be vanishingly thin where the red cell nearly touches the wall. However, erythrocytes are corpuscular particles and there are “plugs” of plasma of varying size that separate them in the direction of blood flow. Also their distortable disk shape causes the plasma layer between erythrocyte and capillary surface to be quite variable.184 Furthermore, occasional leukocytes function like plasma plugs in regard to O2 diffusion to the red cells. Therefore, the diffusion distance from the capillary wall to the red cell membrane can vary from a few nm to several μm.
Strictly speaking, these two layers of the barrier offer O2 diffusion different resistances so their conductances should be calculated separately. However, this distinction does not appear to be important under normal conditions. Indeed, it is more reasonable to treat them as a single barrier. For one, the flow velocity of the plasma layer is much lower than the diffusion of O2 so that plasma is quasistatic with respect to diffusion. Furthermore, under normal conditions the surface areas of alveoli, capillaries, and erythrocytes do not differ much, and the diffusion coefficients of tissue and plasma are also quite similar. Therefore, we prefer now to estimate the membrane diffusing capacity by considering O2 diffusion from the alveolar surface to the erythrocyte membrane as:182
where Kb is Krogh's permeation coefficient estimated at 3.3 × 10−8 cm2 min−1 mm Hg−1, τhb is the harmonic mean distance from the alveolar surface to the nearest erythrocyte membrane, and S(b) is the surface area of the barrier that we estimate as the mean of the alveolar and capillary surface areas, S(A) and S(c), respectively, the two most robust measures of the area of air–blood contact. These parameters can be estimated on sections of properly sampled lung tissue by stereologic methods.185–187
We should also mention that the presence of a surface lining layer in the living lung may modify the geometry of the barrier as we see it on electron micrographs with the consequence that both the barrier thickness and the alveolar surface are reduced to a similar degree because some thicker parts of the barrier become shifted beneath the surfactant pools (Fig. 2-52).177 Therefore, the effect on the estimate of DLO2 is negligible.
Erythrocyte Conductance (DeO2)
As mentioned, the erythrocyte conductance is of a different nature in that it involves two coupled events,183 that is, diffusion of molecular oxygen and oxyhemoglobin within the red blood cell as well as the chemical reaction of O2 with hemoglobin. A way out of this is to obtain an empirical estimate of the rate at which O2 is bound to whole blood, θO2 and to express the erythrocyte conductance DeO2 as:
where Vc is the total capillary blood volume, which can again be estimated on sections by stereologic methods.
The coefficient θO2 is estimated in vitro on whole blood, but this is difficult because of the effect of variable unstirred layers around the red cells.188,189 In addition, θO2 depends on the hematocrit or hemoglobin concentration, and it is not a constant as it falls with increasing O2–hemoglobin saturation; recent studies have shown that, as blood moves through alveolar capillaries, θO2 falls gradually from about 4 to 1 mL O2 mL−1, torr−1 so that the correct value can only be found after Bohr integration of capillary PO2. For normal human lungs and a hemoglobin content of 15 g/100 mL of blood, a value θO2 = 1.8 mL O·mL−1, torr−1 is a reasonable estimate, but if the actual hemoglobin concentration [Hb] varies a corrected value can be obtained by multiplying this standard value with a factor c = [Hb]/15.
Morphometry of the Human Lung and Diffusing Capacity
With this model in hand, we can now attempt to estimate the diffusing capacity of the human lung on the basis of morphometric data, as listed in Table 2-4. These data, obtained by electron microscopic morphometry on seven young adults,190 reveal the alveolar surface area to amount to 130 m2 and the capillary surface to be about 10% smaller. These values are higher than those most commonly quoted in textbooks derived from light microscopic studies, which did not adequately resolve the alveolar surface texture. The harmonic mean thickness of the tissue barrier is 0.6 μm, whereas the total barrier, from alveolar to red cell surface (Fig. 2-60), measures 1.11 μm.182 The capillary volume is estimated at about 200 mL. With these data we calculate DLO2 for the adult human lung to be about 150 to 200 mL O2 min−1 mm Hg−1, the variation depending on the choice of θO2.
Table 2-4Morphometric Estimate of DLO2 for Young, Healthy Adult Humans of 70-kg Body Weight, Measuring 175 cm in Height ||Download (.pdf) Table 2-4Morphometric Estimate of DLO2 for Young, Healthy Adult Humans of 70-kg Body Weight, Measuring 175 cm in Height
|Morphometric data (mean ± 1 SE) || || || |
|Total lung volume (60% TLC) ||4340 ||±285 ||mL |
|Alveolar surface area ||130 ||±12 ||m2 |
|Capillary surface area ||115 ||±12 ||m2 |
|Capillary volume ||194 ||±30 ||mL |
|Air–blood tissue barrier thickness || || || |
|Arithmetic mean ||2.2 ||± 0.2 ||μm |
|Harmonic mean ||0.62 ||±0.04 ||μm |
|Total barrier harmonic mean thickness ||1.11 ||± 0.1 ||μm |
|Diffusing Capacity (mL/min/mm Hg) || || || |
|Membrane ||DMO2 ||350 || |
|Total ||ML2 ||158 || |
These data also allow us to ask the question how the resistance to O2 diffusion is distributed between the diffusion barrier and the red cells. Table 2-4 shows that the diffusion conductance of the “membrane” and that of the red cells are very similar, which means that the resistance to O2 uptake is nearly equally divided between membrane and red cells.
These morphometric estimates of the diffusing capacity are based on model assumptions that are considered reasonable. The test of their validity must be to compare them with physiologic estimates. The standard physiologic value of DLO2 of a healthy adult at rest is about 30 mL O2 min−1 mm Hg−1; thus, considerably less than what we find on the basis of morphometric estimates. However, this is not a valid comparison, because, under resting conditions, we take up only one-tenth the amount of O2 that our lungs are capable of absorbing under conditions of heavy work. There have been a number of estimates of DLO2 in exercising humans,191 and these have yielded values of the order of 100 mL O2 min−1 mm Hg−1. This estimate should come closer to the “true capacity” of the lung for O2 transfer to the blood than the value obtained at rest. The fact that this is only about 50% lower than the morphometric estimate is not disturbing, for we do not know whether the “true diffusing capacity” is completely exploited even in heavy exercise. Inhomogeneities in the distribution of ventilation and perfusion would, for example, limit the degree to which “true” DLO2 can be exploited. One aspect of this type of limitation is discussed in the following when we consider the effect of the acinus design on gas exchange.
To test whether the morphometric estimate of DLO2 is reasonable we performed, some years ago, a combined physiologic and morphometric estimation of pulmonary diffusing capacity on four species of canids ranging from 4 to 30 kg in body mass.192
Because it is difficult to estimate mean capillary PO2 reliably, most physiologic measurements of the diffusing capacity use carbon monoxide (CO) as a tracer gas; CO binds to hemoglobin so avidly that, for practical purposes, the PbCO is zero, so that it suffices to measure CO uptake and alveolar CO concentration. It is also possible to revise the morphometric model of diffusing capacity to estimate the conductance for CO instead of O2 by appropriately changing the permeability coefficients and the rate of CO binding to erythrocytes, θCO whereas the morphometric parameters are not changed. In a study on dogs and on other canids, the calculated morphometric value of DLO2 was found to be larger than the physiologic estimate by less than a factor of 1.5, thus confirming the observation made with respect to human lungs.
Therefore, we conclude that the pulmonary gas exchanger is designed with a certain amount of redundancy or excess capacity, but this is by no means unreasonable from an engineering point of view. Indeed, to design the pulmonary gas exchanger with a certain degree of redundancy may make a lot of sense. The lung forms the interface to the environment and its functional performance will thus depend on environmental conditions, such as the prevailing O2 partial pressure, which falls as we go from sea level to higher altitudes. It has been shown that goats, whose DLO2 is about twice as large as seemingly required, can maintain their maximal level of exercise-induced V̇O2 even under hypoxic conditions whereas the dogs that have very small excess DLO2 cannot. It has also been suggested that human athletes exercising at high altitude may fully exploit their DLO2. This suggests that the apparent redundancy in DLO2 may be a safety factor to protect the good functioning of the pulmonary gas exchanger even when environmental conditions are not optimal. Recent studies with partial pneumonectomy in dogs have shown that the lung can achieve 85% of its maximal O2 uptake even when 40% of lung tissue is removed after left pneumonectomy, making use of some of this reserve capacity; but when right pneumonectomy removes 60% of lung tissue, adequate function can be achieved only after compensatory growth of the residual lung tissue to restore diffusing capacity.193–196
Design of the Acinus and Gas Exchange
The preceding section considered the overall size of the gas exchanger of the entire lung to compare it with the global performance of this organ. In reality, the surface the size of a tennis court is subdivided into some 400 million gas-exchange units. These are individually perfused with blood because they correspond to the unit capillary network that spans between pulmonary arteriole and venule (Fig. 2-46). The diameter of such a roughly disk-shaped unit is about 500 μm and has a surface area that corresponds approximately to that of an alveolus, even though alveoli and the capillary unit are not congruent as the latter spans over several alveoli and each alveolus is in contact with several capillary units.162
These gas-exchange units are arranged along the terminal generations of the airway tree that form the pulmonary acinus (Fig. 2-61B). Note that this arrangement of gas-exchange units to the airway system differs from the common representation of the alveolar–capillary unit as a terminal “bubble” (Fig. 2-61A). This has potential functional consequences because ventilation of alveoli occurs in two steps:197 (1) upon inspiration oxygen-rich air flows through the airways into the acinus carrying along O2; (2) in the peripheral airways flow velocity slows down because the airway cross-section increases, and O2 now moves toward the periphery by diffusion in the air phase, driven by the PO2 gradient that becomes established as O2 is absorbed at the alveolar surface (Fig. 2-62). Thus, in the peripheral airways diffusion along the airways is combined with diffusive permeation of O2 into the alveoli and across the tissue barrier to the blood, the actual process of gas exchange. Whereas all capillary network units are individually perfused with venous blood the alveoli are not independent in terms of their O2 supply, which depends on their location along the airway tree. Therefore, the design of the acinus has significant effects on the gas-exchange conditions.
Models of ventilation–perfusion relationship in the mammalian pulmonary gas exchanger. A. Parallel ventilation/parallel perfusion. B. Serial ventilation/parallel perfusion. (Reproduced with permission from Sapoval B, Filoche M, Weibel ER. Smaller is better, but not too small: A physical scale for the design of the mammalian pulmonary acinus. Proc Natl Acad Sci USA. 2002;99(16):10411–10416. Copyright (2002) National Academy of Sciences, USA.)
Central part of the acinar airways beginning with transitional bronchiole (T) and leading into the branched alveolar ducts. On inspiration air flows in by convection (straight arrows), but as flow velocity falls diffusion of O2 (wiggly arrows) becomes the dominant mechanism for bringing O2 to the gas-exchange surface. All along acinar airways O2 is absorbed by the capillary blood in the septa (inset, arrowheads).
The Acinar Airway System Connected to the Gas Exchanger
In a systematic study of human lungs9 the mean volume of acini was found to be 187 mm3 with a standard deviation of 79 mm3. The branching pattern for an average size human acinus is shown in Figure 2-63. The segment lengths have been drawn to scale and the terminal clusters of alveoli of the alveolar sacs are marked by a dot. This acinus has been subdivided into eight subacini whose substems are located in the third generation of acinar airways. The first three generations of acinar airways following on the transitional bronchiole are respiratory bronchioles, where there are only a few alveoli. In contrast, the alveolar ducts that follow are completely and densely lined with alveoli (Fig. 2-64). The 1/8 subacinus is a unit of functional significance, as we shall see. The intra-acinar airways branch by irregular dichotomy; terminal sacs are located in generations 6 to 11 so that the intra-acinar airways branch over an average of 8 generations (Fig. 2-5).
Graphic representation of branching pattern of acinar airways in one human acinus of 183 mm3 volume with the segment lengths drawn to scale. The airways are separated at the third generation thus displaying the branching pattern within each 1/8 subacinus. (Reproduced with permission from Haefeli-Bleuer B, Weibel ER: Morphometry of the human pulmonary acinus. Anat Rec. 1988;220(4):401–414.)
Airways of 1/8 subacinus of human lung beginning with generation 18 alveolar duct (circle). The silicon rubber cast has been spread out to show the course of the subsequent branchings. The curved line marks the approximate boundary to the last generation to show that this generation of alveolar sacs (see Fig. 2-5) comprises over half the gas-exchange area of the acinus.
The morphometry of the intra-acinar airways of the human lung shows a number of characteristic traits. The inner diameter (din) that characterizes the cross-section of the duct tube decreases from about 490 μm at the transitional bronchiole to 270 μm in the last generations.9 When this is plotted onto the graph relating airway diameter to generations of branching (Fig. 2-37), we note that this diameter falls less steeply than the cube-root-of-1/2 law we have observed for conducting airways. This is a significant finding in terms of the ventilation of alveoli by O2 diffusion.
An important morphometric characteristic of acinar airways is the total path length for O2 diffusion from the entrance at the transitional bronchiole to the terminal cluster of alveoli at the alveolar sac (Fig. 2-5). This path length is determined by two factors: the number of generations and the segment length. The length of alveolar ducts gradually decreases from 1330 to 640 μm in the peripheral generations, the alveolar sacs being a little bit longer. Since the number of branching generations varies somewhat, we can expect the path length to vary even within one acinus. In the human lung, the average longitudinal path length measures 8.3 ± 1.4 mm (Fig. 2-65).10 Because of the decreasing length of acinar ducts 3.4 mm of this total path length are for the first three generations of respiratory bronchioles, whereas the path length of alveolar ducts and sacs comprised in the 1/8 subacinus (Fig. 2-64) averages 4.7 ± 0.88 mm.
Frequency distribution of longitudinal path length from the transitional bronchiole to the alveolar sacs in the human lung. (Reproduced with permission from Haefeli-Bleuer B, Weibel ER: Morphometry of the human pulmonary acinus. Anat Rec. 1988;220(4):401–414.)
Typical Path Model of Human Acinus
In view of assessing the effect of these structural features on the functional performance of the pulmonary gas exchanger we can attempt to develop what we may call a typical path model for an average human acinus9,10; the relevant morphometric data are given in Table 2-5. Such an acinus has a volume of 187 mm3. Its airways branch over an average of eight generations to reach the terminal alveolar sacs. With each generation the number of branches doubles to end with some 256 terminal alveolar sacs in an average acinus (Fig. 2-63). Locating the transitional bronchiole (z′ = 0) in generation 14 (Fig. 2-5) the terminal air sacs are in generation 23 of the typical path airway tree. From the estimates of the lengths and inner diameters of the airway segments we can derive overall parameters of functional significance, such as the total airway cross-section per generation, Ad(z′), which is a determinant of air flow velocity (Fig. 2-44). Finally, we can also estimate the distribution of alveolar surface area to the different generations in proportion to the duct surface Sd(z′), but adjusting for the fact that only part of this surface is associated with alveoli in the respiratory bronchioles (generations z′ = 1–3). For an estimated alveolar surface of 130 m2 in the human lung (Table 2-4), there would be about 54 cm2 of gas-exchange surface per average acinus. It is seen that half this gas-exchange surface is in the last generation (see also Fig. 2-64). A final check of this model is that the path length from the entrance into the transitional bronchiole to the end of the alveolar sacs is 8.4 mm, which agrees well with the mean path length estimated in the human acini (Fig. 2-65).
Table 2-5Typical Path Model of Human Acinus ||Download (.pdf) Table 2-5Typical Path Model of Human Acinus
|Generation ||Segments ||Dimensions per Generation ||Path Length |
|Airwaysz ||Acinusz' ||N(z') ||lmm ||dinmm ||Ad(z')mm2 ||Vd(z')mm3 ||Salv(z')mm2 ||Lp(z')mm |
|15 ||0 ||1 ||1.4 ||0.50 ||0.20 ||0.32 ||7 ||1.4 |
|16 ||1 ||2 ||1.33 ||0.50 ||0.39 ||0.52 ||23 ||2.73 |
|17 ||2 ||4 ||1.12 ||0.49 ||0.75 ||0.84 ||67 ||3.85 |
|18 ||3 ||8 ||0.93 ||0.40 ||1.00 ||0.93 ||129 ||4.78 |
|19 ||4 ||16 ||0.83 ||0.38 ||1.81 ||1.50 ||219 ||5.61 |
|20 ||5 ||32 ||0.70 ||0.36 ||3.26 ||2.28 ||349 ||6.31 |
|21 ||6 ||64 ||0.70 ||0.34 ||5.81 ||4.07 ||661 ||7.01 |
|22 ||7 ||128 ||0.70 ||0.31 ||9.11 ||6.38 ||1204 ||7.71 |
|23 ||8 ||256 ||0.70 ||0.29 ||16.9 ||13.47 ||2720 ||8.41 |
Implications of Acinar Design for Gas-Exchange Function: The Phenomenon of Diffusion Screening
The gas exchange in the pulmonary acinus involves several physicochemical phenomena that occur within the complex acinar geometry described in the preceding section.197 As mentioned, in the distal regions of the lung, oxygen is transported toward the alveolar membrane both by convection and molecular diffusion. Oxygen then diffuses through the tissue membrane into the blood, where it is bound by hemoglobin. Several physical parameters govern oxygen uptake at the acinar level, such as air flow velocity, diffusion coefficient of oxygen in air, alveolar membrane permeability, blood hemoglobin content, and its reaction rate with oxygen. Conversely, carbon dioxide is discharged from the blood to the alveolar gas through diffusion across the membrane. It then diffuses backward along the airways to the zone, where convection becomes dominant, and is lastly expelled from the lung. In all these processes, the morphology of the system plays an essential role.
Since oxygen uptake into the blood is driven by the O2 partial pressure at the alveolar surface we must ask whether this driving force is the same throughout the acinus or whether there could be differences between its central and peripheral parts. Some earlier studies had shown that concentration gradients may exist as a consequence of efficient capture of oxygen by hemoglobin. More recently,197 we have come to realize that such gradients are strongly influenced by the finite permeability of the membrane that plays a dominant role in the effective properties of the acinus as the ventilatory gas-exchange unit. O2 molecules entering the unit where diffusion prevails have a larger probability to hit the surface of the alveolar membrane near the entrance than in the more distal regions. If the membrane permeability is large, O2 molecules are absorbed at the very first hits. As a consequence, O2 is absorbed into the blood in the first parts of the acinar pathway, a process called diffusional screening, so that the gas-exchange units in the deeper part of the acinus would receive less O2 (Fig. 2-61) or perhaps even not enough for gas exchange to occur. Blood perfusing these regions would not be oxygenated and would thus appear as a shunt. In contrast, if the permeability is small, molecules will be absorbed only after many collisions with the wall. They then have a fair chance to reach the deeper regions and the entire acinar surface can be effective for gas exchange.
To put this into the perspective of structure–function relations this process is related to the balance between two conductances:197 a diffusion conductance Ycross for O2 to cross the barrier from alveolar air to capillary blood, and a diffusion conductance Yreach for O2 to reach the surface through the airspaces. Both these conductances are determined by the product of: (a) a physical parameter (the permeability coefficient for O2 in tissue, and the diffusion coefficient for O2 in air, respectively); and (b) a morphometric parameter (the gas-exchange surface, and the distance along the acinar airways, respectively). The physical coefficients are given quantities, except that the tissue permeability is also affected by the thickness of the tissue barrier, a parameter that varies very little between species. On the other hand, the size and surface of the acinus can be varied during evolution and growth to adjust the two conductances. We can predict that the design of the acinus is optimized if Ycross and Yreach are about equal as this means that both the gas-exchange surface and the acinar air volume, or the diffusion distance, are matched. If Ycross were much smaller than Yreach the low permeability of the gas exchanger would need to be compensated by a larger gas-exchange surface, and this would inevitably entail a larger volume of the acinus to accommodate the surface and by that a longer diffusion distance.
The morphometric study of acini in various mammalian species9,197,198 revealed that the size of the acini is such that Ycross ã Yreach so that their morphology seems to be at least partially adapted to minimize the effects of screening. Note that the problem of screening occurs in that part of the acinus where O2 moves to the surface by diffusion only (Fig. 2-62), in what is called the diffusion cell. The transition between convection and diffusion is determined by the Peclet number (Fig. 2-66), essentially the ratio between air flow and diffusion velocities197; diffusion is more effective than convection when the Peclet number is smaller than 1. In the human lung, under resting conditions, this transition occurs in generation 18 and that is the entrance to the 1/8 subacinus (Fig. 2-63); accordingly the diffusion cell corresponds to the 1/8 subacinus. In exercise, where O2 consumption as well as ventilation is increased, convective transport of O2 is effective out to generation 21 (Fig. 2-66). So in exercise there are only two to three generations of acinar airways that act as diffusion cell, but that is still highly significant because these generations accommodate 75% of the gas-exchange surface (Fig. 2-64 and Table 2-5).
In the human acinus the Peclet number, reflecting the relation between convective flow velocity and diffusion velocity of O2, falls as the airway cross-section increases. Below 1 diffusion becomes the dominant mechanism of alveolar ventilation. This transition point is about in generation 18 at rest and extends out to generation 21 in heavy exercise. (Reproduced with permission from Sapoval B, Filoche M, Weibel ER. Smaller is better, but not too small: A physical scale for the design of the mammalian pulmonary acinus. Proc Natl Acad Sci USA. 2002;99(16):10411–10416. Copyright (2002) National Academy of Sciences, USA.)
Note that what has been discussed so far relates essentially to about half the respiratory cycle, namely, inspiration when fresh O2-rich air is actively brought into the acinus. During expiration things are in a way reversed: CO2 that has diffused from the blood into the acinar air now dilutes O2 and the convection–diffusion front is moved toward the bronchi. For this reason, the effective duty cycle of the gas-exchange system is smaller than 1, particularly under the conditions of high O2 uptake rate in exercise. This must be considered when modeling gas exchange. Recent refined model studies using the same morphometric data together with reasonable assumptions on the physiologic conditions revealed that the size of the human pulmonary acinus is such as to avoid negative effects of diffusional screening.199
The Lung as Part of the Pathway for Oxygen
The lung's main function, gas exchange between air and blood, serves the body's varying O2 needs as they are set by the energetic demands of the cells and their mitochondria when these produce ATP by oxidative phosphorylation to allow the cells to do work. This process requires a flow of O2 to be maintained from the lung to the cells16 which proceeds along the respiratory system through various steps from the lung to the blood, by circulatory blood flow to the cells and mitochondria (Fig. 2-67). A number of basic features characterize this system: (1) under steady-state conditions the O2 flow rate, V̇O2, is the same at all levels; (2) the basic driving force for O2 flow through the system is a cascade of O2 partial pressures, which fall from inspired PO2 down to near zero around the mitochondria; (3) the O2 flow rate at each step is the product of an O2 partial pressure difference and a conductance G, which is related to structural and functional properties of the organs participating in O2 transfer. In the preceding section we have seen that the principal design features of the lung that determine one of the key conductances, the pulmonary diffusing capacity, are sized to just yield a conductance that allows the O2 uptake required to satisfy the demands of the whole body cell system at work, with a small margin of safety under normal conditions. Therefore, the lung appears designed to serve the body's needs efficiently and economically. The question we may now ask is whether the other parts of the respiratory system, from the heart to the mitochondria are also designed for economic functional performance.200
Model of the respiratory system from the lung to the cells. Oxygen flow is driven through the system by a cascade of PO2 ranging from inspired PlO2 to near zero at the mitochondria. At each level the flow rate is determined by a partial pressure difference and a conductance. (Modified with permission from Taylor CR, Weibel ER: Design of the mammalian respiratory system. I. Problem and strategy, Respir Physiol. 1981;44(1):1–10.)
Let us first look at the overall functional performance of the system. We first note that O2 consumption is highly variable, increasing by about a factor of 10 between resting conditions and heavy exercise when 90% of the O2 is consumed in the locomotor muscles. Figure 2-68 shows that the oxygen consumption in muscle is proportional to the energy output,201 measured for example as running speed, and that it reaches a limit V̇O2max beyond this the running speed can still be increased, but the additional energy required by the higher speed is then supplied through glycolysis or anaerobic ATP production with the result that lactic acid concentration in the blood gradually increases. It is now interesting to note that V̇O2max is a characteristic of the work capacity of an individual: well-trained athletes reach their V̇O2max at a higher running speed and a higher level of oxygen consumption, and lactic acid concentration in the blood also begins to increases at the higher performance levels corresponding to V̇O2max (Fig. 2-68).
Rate of O2 consumption (left ordinate) and lactic acid production (ordinate at right) in exercise are plotted as a function of the work intensity and, therefore, of the energy requirement (abscissa). Oxygen consumption increases linearly up to a point corresponding to an energy requirement of 220 cal/kg min−1; if work is pushed beyond that there is no further increase in O2 consumption (V̇O2max is reached) but glycolysis now generates the required energy resulting in an increase in lactic acid production. The broken lines refer to athletes (middle- and long-distance runners) whose maximum oxygen consumption is higher; the line of the lactic acid for these subjects is correspondingly shifted to the right. (Reproduced with permission from Margaria R, Cerretelli P, Diprampero PE, et al. Kinetics and mechanism of oxygen debt contraction in man. J Appl Physiol. 1963;18:371–377.)
One may now raise the question whether this variable limitation of oxidative metabolism is a result of variable functional constraints affecting the regulation of metabolic rate and circulatory transport, or whether it could be set by variations in design constraints characterizing the structural components of the pathway, one possible candidate being the pulmonary diffusing capacity. The answer to this question depends on an integrated study of structure and function of the respiratory system. For this we need a quantitative model of the oxygen pathway that identifies all the functional variables and the design parameters at the four levels of the system202: the lung, circulation of blood with the heart, capillaries, and mitochondria (Table 2-6). This model is a further development of the one shown in Figure 2-67 in the sense that, at each level, the equation describing oxygen flow rate sorts out the parameters of functional regulation and those of structural design. These are distinguished in the following sense: Functional variables are regulated according to need with short time constants (seconds), whereas structural design parameters are genetically determined static elements that can be adjusted to a certain extent, for example, by training, but with time constants of weeks to months.
Table 2-6Model of Structure–Function Relations in Pathway for Oxygen Separating Functional and Structural Parameters in the Equations Defining O2 Flow Rate Through Four Levels ||Download (.pdf) Table 2-6Model of Structure–Function Relations in Pathway for Oxygen Separating Functional and Structural Parameters in the Equations Defining O2 Flow Rate Through Four Levels
Thus, design variables set the capacity of the system because they are determined by structures whose quantitative properties cannot be adjusted at short notice. If the system were designed according to the principle of symmorphosis we would predict that the design variables are adjusted to V̇O2max at all levels from the lung to the mitochondria.
The experimental test of this hypothesis requires the integrated measurement of V̇O2max of the relevant functional parameters, and of all the design parameters, which must then be correlated on the basis of the model of Table 2-6. This cannot be easily done in the human so that is where we can learn from studies in comparative physiology. We know that V̇O2max is highly variable among mammals. Some species such as dogs, horses, or pronghorn antelopes have a much higher level of V̇O2max than “normal” species of the same size such as goats or cows; this is called adaptive variation.203 On the other hand body size matters so that small animals have a higher metabolic rate per unit body mass than large species, which is called allometric variation.200 These are genetically determined variations, the result of evolution and selection by fitness, in contrast to the changes in overall work capacity and V̇O2max induced by exercise training in human athletes, which are epigenetic variations.204,205 In all these cases we can ask how and to what extent the structural design parameters are adjusted to meet the different requirements for O2 to cover the energetic need at the limit of the aerobic work capacity. If there is a bottleneck, then there will be one and only one parameter whose variation is perfectly matched to the variation in the limit of O2 flow, V̇O2max, whereas all the parameters that are overdesigned would appear in haphazard relations to the flow limit. On the other hand, if the limiting resistances are distributed all steps would have to be matched to the varying V̇O2max. If we take the bold view that the organisms are economically designed we would predict that the structural parameters at all levels should be sized to the maximal total O2 flow requirement with no unnecessary excess capacity because that would be a waste. We have called this design principle symmorphosis, meaning that there should be no more structure built into the system than required to serve the functional needs.179
Testing the Hypothesis of Symmorphosis
To test such a hypothesis we can first compare mammals that greatly differ in terms of their maximal O2 consumption. The first type of this variation is found in comparing normal with athletic species, such as dogs with goats or horses with steers.203 It has been found that such athletic animals can achieve a V̇O2max that is about 2.5 times higher than that of normal species of the same size. This is much more than what human athletes can achieve. The relevant morphometric data on such species are shown in Table 2-7 for three species pairs.206 If we go through the respiratory system, beginning at the bottom with the mitochondria, we note that their total volume in the locomotor muscles is also 2.5 times greater in the athletic species with the result that, at V̇O2max the unit volume of mitochondria consumes the same amount of oxygen in all these six species, namely about 5 mL O2 per minute and mL mitochondria. In the next level up, the muscle capillaries, we note that the capillary volume is only 1.7 times greater in the athletic species. However we note that in the athletes the hematocrit, that is, the concentration of erythrocytes in the blood, is larger so that as a result the capillary erythrocyte volume, the product of capillary volume with hematocrit, is 2.44 times greater, thus well matched to the mitochondrial O2 demands. Note that this is what counts because oxygen is delivered exclusively from the capillary red blood cells. When we look at the determinants of total blood flow the heart is the central element. We notice that athletic species have larger hearts resulting in a larger stroke volume Vs, but that the maximal heart frequency is not different between the species pairs so that cardiac output is determined by the stroke volume. This is only 1.7 times greater in the athletic species. However, note that, here again, the hematocrit plays an important role as it determines the amount of O2 that can be transported to the capillaries. If we calculate the cardiac erythrocyte output Q̇ (ec) we find that it is again 2.4 times greater in the athletic species. Thus the design parameters of the internal steps of the O2 transport cascade are quantitatively adjusted to the needs for O2 flow under limiting conditions. Thus, it appears that the resistance to O2 flow is distributed to all levels.
Table 2-7Comparison of Morphometric and Physiologic Parameters of Muscle Mitochondria and Capillaries, and of Heart, Blood and Lung with Variation of V̇O2max in Three Pairs of Athletic and Sedentary Species. Data per Unit Body Mass ||Download (.pdf) Table 2-7Comparison of Morphometric and Physiologic Parameters of Muscle Mitochondria and Capillaries, and of Heart, Blood and Lung with Variation of V̇O2max in Three Pairs of Athletic and Sedentary Species. Data per Unit Body Mass
| ||Mitochondria ||Blood ||Capillaries ||Heart ||Lung |
|Design Function ||V̇O2max/Mb mL·min−1·kg−1 ||V(mt)/Mb mL·kg−1 ||VV(ec) ||V(c)/Mb mL·kg−1 ||V(ec)/Mb mL·kg−1 ||fH min−1 ||Vs/Mb mL·kg−1 ||Q̇ (ec)/Mb mL·min−1·kg−1 ||DlO2/Mb mL·min−1·mmHg−1 kg−1 |
|25–30 kg || || || || || || || || || |
|Dog ||137.4 ||40.6 ||0.50 ||8.2 ||4.10 ||274 ||3.17 ||434.3 ||424.8 |
|Goat ||57.0 ||13.8 ||0.30 ||4.5 ||1.35 ||268 ||2.07 ||166.4 ||288.0 |
|D/G ||2.4 ||2.9 ||1.68a ||1.8a ||3.0 ||1.02a ||1.53a ||2.61 ||1.48a |
|150 kg || || || || || || || || || |
|Pony ||88.8 ||19.5 ||0.42 ||5.1 ||2.14 ||215 ||2.50 ||225.7 ||284.4 |
|Calf ||36.6 ||9.2 ||0.31 ||3.2 ||0.99 ||213 ||1.78 ||117.5 ||180.0 |
|P/C ||2.4 ||2.13 ||1.35a ||1.6a ||2.16 ||1.02a ||1.40a ||1.92 ||1.57a |
|450 kg || || || || || || || || || |
|Horse ||133.8 ||30.0 ||0.55 ||8.3 ||4.57 ||202 ||3.11 ||345.5 ||388.9 |
|Steer ||51.0 ||11.6 ||0.40 ||5.3 ||2.12 ||216 ||1.52 ||131.3 ||194.4 |
|H/S ||2.6 ||2.6 ||1.4a ||0.94a ||2.16 ||2.1a ||2.0a ||2.63 ||2.0a |
|Ath/Sedb ||2.5 ||2.5 ||1.5a ||1.7a ||2.44 ||1.0a ||1.7a ||2.39 ||1.7a |
When we then consider the design of the pulmonary gas exchanger we note that the O2 diffusing capacity of the lung of athletic species is only 1.7 times greater than that of normal species. Considering that we found that the human lung may have some excess capacity by about a factor of 1.5, this may signify that normal sedentary species such as goats or cows have a greater excess capacity than athletic species. Indeed, this can be shown to be the case in two ways:207 (1) when one calculates the progression of O2 loading on capillary blood (Bohr integration, Fig. 2-59) one finds that dogs reach saturation just before the blood leaves the capillaries into arterial blood, whereas the goats have some 30% reserve capacity; (2) when goats are run on a treadmill while breathing hypoxic air one finds that they can maintain their V̇O2max in contrast, dogs cannot run at their established V̇O2max under such conditions. We concluded from this observation that athletic species have designed a lung to match the requirements for maximal O2 uptake with no excess capacity while normal sedentary species apparently allow for a certain safety margin which allows them to perform well also under unfavorable hypoxic conditions. If this is now applied to our observations on the human lung this may mean that the excess capacity of the normal lung may just be sufficient to allow athletes to increase their V̇O2max by training by a factor 1.5, just about what they can achieve (Fig. 2-67).
One has also found that highly trained athletes do not tolerate heavy exercise at very high altitudes as they cannot achieve O2 saturation of their arterial blood. Thus, it seems that the pulmonary gas exchanger is now the limiting factor for O2 transfer to the working muscles. The reason for this is that the lung of the adult cannot enlarge its gas-exchange surfaces to match the increased demands of trained muscles. So an athlete must make do with the lung she or he has developed during growth. This contrasts with the changes induced by exercise training in muscle with an increase in mitochondria and capillaries, and in the heart by enlargement of the ventricles, all well matched to the maximal O2 demands.202 Therefore, it is fortunate – and perhaps a sign of good design – that the lung is designed with some excess diffusing capacity to allow the lower, internal, levels of the respiratory system to exploit their capacity to adapt to increased energetic needs.
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