Large blood vessels and those of the microvasculature branch frequently and undergo gradual transitions into structures with different histologic features and functions. For didactic purposes vessels can be classified arbitrarily as the types discussed here and listed in Table 11–1.
TABLE 11–1Size ranges, major features, and important roles of major blood vessel types. ||Download (.pdf) TABLE 11–1 Size ranges, major features, and important roles of major blood vessel types.
|Type of Artery ||Outer Diameter (Approx. Range) ||Intima ||Media ||Adventitia ||Roles in Circulatory System |
|Elastic arteries ||> 10 mm ||Endothelium; connective tissue with smooth muscle ||Many elastic lamellae alternating with smooth muscle ||Connective tissue, thinner than media, with vasa vasorum ||Conduct blood from heart and with elastic recoil help move blood forward under steady pressure |
|Muscular arteries ||10-1 mm ||Endothelium; connective tissue with smooth muscle, internal elastic lamina prominent ||Many smooth muscle layers, with much less elastic material ||Connective tissue, thinner than media; vasa vasorum maybe present ||Distribute blood to all organs and maintain steady blood pressure and flow with vasodilation and constriction |
|Small arteries ||1-0.1 mm ||Endothelium; connective tissue less smooth muscle ||3-10 layers of smooth muscle ||Connective tissue, thinner than media; no vasa vasorum ||Distribute blood to arterioles, adjusting flow with vasodilation and constriction |
|Arterioles ||100-10 µm ||Endothelium; no connective tissue or smooth muscle ||1-3 layers of smooth muscle ||Very thin connective tissue layer ||Resist and control blood flow to capillaries; major determinant of systemic blood pressure |
|Capillaries ||10-4 µm ||Endothelium only ||A few pericytes only ||None ||Exchange metabolites by diffusion to and from cells |
|Venules (postcapillary, collecting, and muscular) ||10-100 µm ||Endothelium; no valves ||Pericytes and scattered smooth muscle cells ||None ||Drain capillary beds; site of leukocyte exit from vasculature |
|Small veins ||0.1-1 mm ||Endothelium; connective tissue with scattered smooth muscle fibers ||Thin, 2-3 loose layers of smooth muscle cells ||Connective tissue, thicker than media ||Collect blood from venules |
|Medium veins ||1-10 mm ||Endothelium; connective tissue, with valves ||3-5 more distinct layers of smooth muscle ||Thicker than media; longitudinal smooth muscle may be present ||Carry blood to larger veins, with no backflow |
|Large veins ||> 10 mm ||Endothelium; connective tissue, smooth muscle cells; prominent valves ||> 5 layers of smooth muscle, with much collagen ||Thickest layer, with bundled longitudinal smooth muscle ||Return blood to heart |
Elastic arteries are the aorta, the pulmonary artery, and their largest branches; these large vessels are also called conducting arteries because their major role is to carry blood to smaller arteries. As shown in Figure 11–7a, the most prominent feature of elastic arteries is the thick tunica media in which elastic lamellae alternate with layers of smooth muscle fibers. The adult aorta has about 50 elastic lamellae (more if the individual is hypertensive).
The tunica intima is well developed, with many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood pressure and contraction of the vessel at death (Figure 11–8). Between the intima and the media is the internal elastic lamina, which is more well-defined than the elastic laminae of the media (Figures 11–7a and 11–9). The adventitia is much thinner than the media.
The numerous elastic laminae of these arteries contribute to their important function of making the blood flow more uniform. During ventricular contraction (systole) blood is moved through the arteries forcefully and the elastin is stretched, distending the wall within the limit set by the wall’s collagen. When the ventricles relax (diastole) ventricular pressure drops to a low level, but the elastin rebounds passively, helping to maintain arterial pressure. The aortic and pulmonary valves prevent backflow of blood into the heart, so the rebound continues the blood flow away from the heart. Arterial blood pressure and blood velocity decrease and become less variable as the distance from the heart increases.
Atherosclerosis (Gr. athero, gruel or porridge, and scleros, hardening) is a disease of elastic arteries and large muscular arteries that may play a role in nearly half of all deaths in developed parts of the world. It is initiated by damaged or dysfunctional endothelial cells oxidizing low-density lipoproteins (LDLs) in the tunica intima, which induces adhesion and intima entry of monocytes/macrophages to remove the modified LDL. Lipid-filled macrophages (called foam cells) accumulate and, along with the free LDL, produce a pathologic sign of early atherosclerosis called fatty streaks. During disease progression these develop into fibro-fatty plaques, or atheromas, consisting of a gruel-like mix of smooth muscle cells, collagen fibers, and lymphocytes with necrotic regions of lipid, debris, and foam cells. Predisposing factors include dyslipidemia (> 3:1 ratios of LDL to HDL [high-density lipoprotein]), hyperglycemia of diabetes, hypertension, and the presence of toxins introduced by smoking.
In elastic arteries atheromas produce localized destruction within the wall, weakening it and causing arterial bulges or aneurysms which can rupture. In muscular arteries such as the coronary arteries, atheromas can occlude blood flow to downstream vessels, leading to ischemic heart disease. In elastic arteries atheromas produce localized destruction within the wall, weakening it and causing arterial bulges or aneurysms that can rupture. In muscular arteries such as the coronary arteries, atheromas can occlude blood flow to downstream vessels, leading to ischemic heart disease.
Arterial Sensory Structures
Carotid sinuses are slight dilations of the bilateral internal carotid arteries where they branch from the (elastic) common carotid arteries; they act as important baroreceptors monitoring arterial blood pressure. At these sinuses the tunica media is thinner, allowing greater distension when blood pressure rises, and the adventitia contains many sensory nerve endings from cranial nerve IX, the glossopharyngeal nerve. The brain’s vasomotor centers process these afferent impulses and adjust vasoconstriction, maintaining normal blood pressure. Functionally similar baroreceptors present in the aortic arch transmit signals pertaining to blood pressure via cranial nerve X, the vagus nerve.
Histologically more complex chemoreceptors which monitor blood CO2 and O2 levels, as well as its pH, are found in the carotid bodies and in the aortic bodies, located in the walls of the carotid sinuses and aortic arch, respectively. These structures are parts of the autonomic nervous system called paraganglia with rich capillary networks. The capillaries are closely surrounded by large, neural crest-derived glomus cells filled with dense-core vesicles containing dopamine, acetylcholine, and other neurotransmitters, which are supported by smaller satellite cells (Figure 11–10). Ion channels in the glomus cell membranes respond to stimuli in the arterial blood, primarily hypoxia (low O2), hypercapnia (excess CO2), or acidosis, by activating release of neurotransmitters. Sensory fibers branching from the glossopharyngeal nerve form synapses with the glomus cells and signal brain centers to initiate cardiovascular and respiratory adjustments that correct the condition.
Cells and capillaries in a glomus body.
Specialized regions in the walls of certain elastic arteries contain tissues acting as chemoreceptors that provide information to the brain regarding blood chemistry. The glomus bodies are two small (0.5-5 mm diameter) ganglion-like structures found near the common carotid arteries. They contain many large capillaries (C) intermingled with clusters of large glomus cells (G) filled with vesicles of various neurotransmitters. Supportive satellite cells (S) with elongated nuclei ensheath each glomus cell. Glomus cells form synaptic connections with sensory fibers. Significant changes in the blood CO2, O2, or H+ concentrations are detected by the chemoreceptive glomus cells, which then release a neurotransmitter that activates the sensory nerve to relay this information to the brain. (X400; PT)
The muscular arteries, also called distributing arteries, distribute blood to the organs and help regulate blood pressure by contracting or relaxing the smooth muscle in the media. The intima has a thin subendothelial layer and a prominent internal elastic lamina (Figure 11–11). The media may contain up to 40 layers of large smooth muscle cells interspersed with a variable number of elastic lamellae (depending on the size of the vessel). An external elastic lamina is present only in the larger muscular arteries. The adventitial connective tissue contains lymphatic capillaries, vasa vasorum, and nerves, all of which may penetrate to the outer part of the media.
With distance from the heart, arteries gradually have relatively less elastin and more smooth muscle in their walls. Most arteries, large enough to have names, are of the muscular type. A transverse section through a muscular (medium-caliber) artery shows a slightly folded intima with only sparse connective tissue between the endothelial cells (E) and internal elastic lamina (IEL). Multiple layers of smooth muscle (SM) in the media are thicker than the elastic lamellae and fibers with which they intersperse. Vasa vasorum (V) are seen in the adventitia. (X100; H&E)
Muscular arteries branch repeatedly into smaller and smaller arteries, until reaching a size with three or four layers of medial smooth muscle. The smallest arteries branch as arterioles, which have only one or two smooth muscle layers; these indicate the beginning of an organ’s microvasculature (Figures 11–12 and 11–13) where exchanges between blood and tissue fluid occur. Arterioles are generally less than 0.1 mm in diameter, with lumens approximately as wide as the wall is thick (Figure 11–14). The subendothelial layer is very thin, elastic laminae are absent, and the media consists of the circularly arranged smooth muscle cells. In both small arteries and arterioles the adventitia is very thin and inconspicuous.
Arterioles (A), capillaries (C), and venules (V) comprise the microvasculature where, in almost every organ, molecular exchange takes place between blood and the interstitial fluid of the surrounding tissues. Lacking media and adventitia tunics and with diameters of only 4-10 μm, capillaries (C) in paraffin sections can be recognized by nuclei adjacent to small lumens or by highly eosinophilic red blood cells in the lumen. As described in Figure 5–20, not all interstitial fluid formed at capillary beds is drained into venules; the excess is called lymph and collects in thin-walled, irregularly shaped lymphatic vessels (L), such as those seen in connective tissue and smooth muscle here. (200X; H&E)
Microvascular bed structure and perfusion.
Arterioles supplying a capillary bed typically form smaller branches called metarterioles in which the smooth muscle cells are dispersed as bands which act as precapillary sphincters. The distal portion of the metarteriole, sometimes called a thoroughfare channel, lacks smooth muscle cells and merges with the postcapillary venule. Branching from the metarteriole and thoroughfare channel are the smallest vessels, true capillaries, which lack smooth muscle cells (although pericytes may be present). The precapillary sphincters regulate blood flow into the true capillaries.
Part a shows a well-perfused capillary bed with all the sphincters relaxed and open; part b shows a capillary bed with the blood shunted away by contracted sphincters. At any given moment, most sphincters are at least partially closed and blood enters the capillary bed in a pulsatile manner for maximally efficient exchange of nutrients, wastes, O2, and CO2 across the endothelium. Except in the pulmonary circulation (Figure 11–1), blood enters the microvasculature well oxygenated and leaves poorly oxygenated. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).
(a) Arterioles are microvessels with an intima (I) consisting only of endothelium (E), in which the cells may have rounded nuclei. They have media (M) tunics with only one or two layers of smooth muscle, and usually thin, inconspicuous adventitia (Ad). (X350; Masson trichrome) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
(b) Three arterioles (A) of various sizes and a capillary (C) are shown here. (X400; H&E) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
(c) A large mesenteric arteriole cut obliquely and longitudinally clearly shows the endothelial cells (arrow heads) and one or two layers of smooth muscle cells (M) cut transversely. Adventitia merges imperceptibly with neighboring connective tissue. (X300; PT)
Arterioles almost always branch to form anastomosing networks of capillaries that surround the parenchymal cells of the organ. At the ends of arterioles the smooth muscle fibers act as sphincters and produce periodic blood flow into capillaries (Figure 11–13). Muscle tone normally keeps arterioles partially closed, resisting blood flow, which makes these vessels the major determinants of systemic blood pressure.
Blood pressure depends on cardiac output and the total peripheral resistance to blood flow, which is mostly due to the resistance of arterioles. Hypertension or elevated blood pressure may occur secondarily to renal or endocrine problems, but is more commonly essential hypertension, due to a wide variety of mechanisms that increase arteriolar constriction.
In certain tissues and organs, arterioles deviate from this simple path to accommodate various specialized functions (Figure 11–15). For example, thermoregulation by the skin involves arterioles that can bypass capillary networks and connect directly to venules. The media and adventitia are thicker in these arteriovenous shunts (or arteriovenous anastomoses) and richly innervated by sympathetic and parasympathetic nerve fibers. The autonomic fibers control the degree of vasoconstriction at the shunts, regulating blood flow through the capillary beds. High capillary blood flow in the skin allows more heat dissipation from the body, while reduced capillary blood flow conserves heat—important functions when the environmental temperature is hot or cold, respectively.
Comparison of the simple microvascular pathway with arteriovenous shunts and portal systems.
Most capillary beds are supplied by arterioles and drain into venules, but alternative pathways are found in certain organs. In skin blood flow can be varied according to external conditions by arteriovenous (AV) shunts, or anastomoses, commonly coiled, which directly connect the arterial and venous systems and temporarily bypass capillaries.
In venous portal systems one capillary bed drains into a vein that then branches again into another capillary bed. This arrangement allows molecules entering the blood in the first set of capillaries to be delivered quickly and at high concentrations to surrounding tissues at the second capillary bed, which is important in the anterior pituitary gland and liver.
Not shown are arterial portal systems (afferent arteriole → capillaries → efferent arteriole) which occur in the kidney. (Reproduced, with permission, from McKinley M, O'Loughlin VD. Human Anatomy. 2nd ed. New York, NY: McGraw-Hill; 2008; McKinley M, O'Loughlin VD. Human Anatomy. 3rd ed. New York, NY: McGraw-Hill; 2012; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. New York, NY: McGraw-Hill; 2013; McKinley MP, O'Loughlin VD, Bidle TS. Anatomy & Physiology: An Integrative Approach. 2nd ed. New York, NY: McGraw-Hill; 2016).
Another important alternative microvascular pathway is a venous portal system (Figure 11–15), in which blood flows through two successive capillary beds separated by a portal vein. This arrangement allows for hormones or nutrients picked up by the blood in the first capillary network to be delivered most efficiently to cells around the second capillary bed before the blood is returned to the heart for general distribution. The best examples are the hepatic portal system of the liver and the hypothalamic-hypophyseal portal system in the anterior pituitary gland, both of which have major physiologic importance.
Capillaries permit and regulate metabolic exchange between blood and surrounding tissues. These smallest blood vessels always function in networks called capillary beds, whose size and overall shape conforms to that of the structure supplied. The density of the capillary bed is related to the metabolic activity of the tissues. Tissues with high metabolic rates, such as the kidney, liver, and cardiac and skeletal muscle, have abundant capillaries; the opposite is true of tissues with low metabolic rates, such as smooth muscle and dense connective tissue.
Capillary beds are supplied preferentially by one or more terminal arteriole branches called metarterioles, which are continuous with thoroughfare channels connected with the postcapillary venules (Figure 11–13). Capillaries branch from the metarterioles, which are encircled by scattered smooth muscle cells, and converge into the thoroughfare channels, which lack muscle. The metarteriole muscle cells act as precapillary sphincters that control blood flow into the capillaries. These sphincters contract and relax cyclically, with 5-10 cycles per minute, causing blood to pass through capillaries in a pulsatile manner. When the sphincters are closed, blood flows directly from the metarterioles and thoroughfare channels into postcapillary venules.
Capillaries are composed of the simple layer of endothelial cells rolled up as a tube surrounded by basement membrane (Figure 11–16). The average diameter of capillaries varies from 4 to 10 μm, which allows transit of blood cells only one at a time, and their individual length is usually not more than 50 μm. These minute vessels make up over 90% of the body’s vasculature, with a total length of more than 100,000 km and a total surface area of approximately 5000 m2. Because of the cyclical opening and closing of the sphincters, most capillaries are essentially empty at any given time, with only about 5% (~300 mL in an adult) of the total blood volume moving through these structures. Their thin walls, extensive surface area, and slow, pulsatile blood flow optimize capillaries for the exchange of water and solutes between blood and tissues.
Capillary with pericytes.
Capillaries consist only of an endothelium rolled as a tube, across which molecular exchange occurs between blood and tissue fluid. (a) Capillaries are normally associated with perivascular contractile cells called pericytes (P) which have a variety of functions. The more flattened nuclei belong to endothelial cells. (X400; H&E of a spread mesentery preparation)
(b) TEM of a capillary cut transversely, showing the nucleus of one thin capillary endothelial cell (E). Endothelial cells form the capillary lumen (L), are covered by a basal lamina (BL), and are bound tightly together with junctional complexes (J). One pericyte (P) is shown, surrounded by its own basal lamina (BL) and with cytoplasmic extensions which surround the endothelial cells. (X13,000)
(Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
In addition to the endothelial properties mentioned earlier in this chapter, capillary cells have many features specialized for molecular transfer by mechanisms ranging from simple diffusion to transcytosis. The average thickness of the cells is only 0.25 μm and their nuclei are often distinctively curved to accommodate the very small tubular structure (Figure 11–10). The cytoplasm contains mitochondria and most other organelles, as well as a large population of membranous vesicles typically. Along with the basal lamina, junctional complexes between the cells maintain the tubular structure, with variable numbers of tight junctions having an important role in capillary permeability.
Major structural variations in capillaries occur in organs with various functions that permit very different levels of metabolic exchange. Capillaries are generally grouped into three histologic types, depending on the continuity of the endothelial cells and their basement membrane (Figures 11–17, 11–18, 11–19, 11–20).
The vessels between arterioles and venules can be any of three types. (a) Continuous capillaries, the most common type, have tight, occluding junctions sealing the intercellular clefts between all the endothelial cells to produce minimal fluid leakage. All molecules exchanged across the endothelium must cross the cells by diffusion or transcytosis.
(b) Fenestrated capillaries also have tight junctions, but perforations (fenestrations) through the endothelial cells allow greater exchange across the endothelium. The basement membrane is continuous in both these capillary types. Fenestrated capillaries are found in organs where molecular exchange with the blood is important, such as endocrine organs, intestinal walls, and choroid plexus.
(c) Sinusoids, or discontinuous capillaries, usually have a wider diameter than the other types and have discontinuities between the endothelial cells, large fenestrations through the cells, and a partial, discontinuous basement membrane. Sinusoids are found in organs where exchange of macromolecules and cells occurs readily between tissue and blood, such as in bone marrow, liver, and spleen.
Continuous capillaries exert the tightest control over what molecules leave and enter the capillary lumen (L). The TEM shows a continuous capillary in transverse section. An endothelial cell nucleus (N) is prominent, and tight or occluding junctions are abundant in the junctional complexes (JC) at overlapping folds between the endothelial cells (E). Numerous transcytotic vesicles (V) are evident. All material that crosses continuous capillary endothelium must pass through the cells, usually by diffusion or transcytosis.
Around the capillary are a basal lamina (BL) and thin cytoplasmic extensions from pericytes (P). Collagen fibers (C) and other extracellular material are present in the perivascular space (PS). (X10,000)
Fenestrated capillaries are specialized for uptake of molecules such as hormones in endocrine glands or for outflow of molecules such as in the kidney’s filtration system. TEM of a transversely sectioned fenestrated capillary in the peritubular region of the kidney shows many typical fenestrae closed by diaphragms (arrows), with a continuous basal lamina surrounding the endothelial cell (BL). In this cell the Golgi apparatus (G), nucleus (N), and centrioles (C) can also be seen. Fenestrated capillaries allow a freer exchange of molecules than continuous capillaries and are found in the intestinal wall, kidneys, and endocrine glands. (X10,000)
(Used with permission from Dr Johannes Rhodin, Department of Cell Biology, New York University School of Medicine.)
Sinusoidal capillaries or sinusoids generally have much greater diameters than most capillaries and are specialized not only for maximal molecular exchange between blood and surrounding tissue but also for easy movement of blood cells across the endothelium. The sinusoid (S) shown here is in bone marrow and is surrounded by tissue containing adipocytes (A) and masses of hematopoietic cells (H). The endothelial cells are very thin and cell nuclei are more difficult to find than in smaller capillaries. Ultrastructurally sinusoidal capillaries are seen to have large fenestrations through the cells and large discontinuities between the cells and through the basal lamina. (X200; H&E)
Continuous capillaries (Figure 11–17a) have many tight, well-developed occluding junctions between slightly overlapping endothelial cells, which provide for continuity along the endothelium and well-regulated metabolic exchange across the cells. This is the most common type of capillary and is found in muscle, connective tissue, lungs, exocrine glands, and nervous tissue. Ultrastructural studies show numerous vesicles indicating transcytosis of macromolecules in both directions across the endothelial cell cytoplasm.
Fenestrated capillaries (Figure 11–17b) have a sieve-like structure that allows more extensive molecular exchange across the endothelium. The endothelial cells are penetrated by numerous small circular openings or fenestrations (L. fenestra, perforation), approximately 80 nm in diameter. Some fenestrations are covered by very thin diaphragms of proteoglycans (Figure 11–19); others may represent membrane invaginations during transcytosis that temporarily involve both sides of the very thin cells. The basement membrane however is continuous and covers the fenestrations. Fenestrated capillaries are found in organs with rapid interchange of substances between tissues and the blood, such as the kidneys, intestine, choroid plexus, and endocrine glands.
Discontinuous capillaries, commonly called sinusoids (Figure 11–17c), permit maximal exchange of macromolecules as well as allow easier movement of cells between tissues and blood. The endothelium here has large perforations without diaphragms and irregular intercellular clefts, forming a discontinuous layer with spaces between and through the cells. Unlike other capillaries sinusoids also have highly discontinuous basement membranes and much larger diameters, often 30-40 μm, which slows blood flow. Sinusoidal capillaries of this type are found in the liver, spleen, some endocrine organs, and bone marrow (Figure 11–20).
At various locations along continuous capillaries and postcapillary venules are mesenchymal cells called pericytes (Gr. peri, around + kytos, cell), with long cytoplasmic processes partly surrounding the endothelial layer. Pericytes secrete many ECM components and form their own basal lamina, which fuses with the basement membrane of the endothelial cells (Figure 11–16). Well-developed cytoskeletal networks of myosin, actin, and tropomyosin indicate that pericytes also dilate or constrict capillaries, helping to regulate blood flow in some organs. Within the CNS pericytes are important for maintaining the endothelial blood-brain barrier. After injuries pericytes proliferate and differentiate to form smooth muscle and other cells in new vessels as the microvasculature is reestablished. In many organs the pericyte population also includes mesenchymal stem cells important for regeneration of other tissues.
The hyperglycemia or excessive blood sugar that occurs with diabetes commonly leads to diabetic microangiopathy, a diffuse thickening of capillary basal laminae and concomitant decrease in metabolic exchange at these vessels, particularly in the kidneys, retina, skeletal muscle, and skin.
The transition from capillaries to venules occurs gradually. Postcapillary venules (Figure 11–21a) are similar to capillaries with pericytes but larger, ranging in diameter from 15 to 20 μm. As described with blood in Chapter 12, postcapillary venules are the primary site at which white blood cells adhere to endothelium and leave the circulation at sites of infection or tissue damage.
Postcapillary venules converge into larger collecting venules that have more distinct contractile cells. With increasing size venules become surrounded by a recognizable tunica media with two or three smooth muscle layers and are called muscular venules. A characteristic feature of all venules is the large diameter of the lumen compared to the overall thinness of the wall (Figure 11–21).
A series of increasingly larger and more organized venules lie between capillaries and veins.
(a) Compared to arterioles (A), postcapillary venules (V) have large lumens and an intima of simple endothelial cells, with occasional pericytes (P). (X400; Toluidine blue [TB])
(b) Larger collecting venules (V) have much greater diameters than arterioles (A), but the wall is still very thin, consisting of an endothelium with more numerous pericytes or smooth muscle cells. (X200; H&E) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
(c) The muscular venule cut lengthwise here has a better defined tunica media, with as many as three layers of smooth muscle (M) in some areas, a very thin intima (I) of endothelial cells (E), and a more distinct adventitia (Ad). Part of an arteriole (A) shows a thicker wall than the venule. (X200; Masson trichrome)
As discussed with white blood cells in Chapter 12, postcapillary venules are important as the site in the vasculature where these cells leave the circulation to become functional in the interstitial space of surrounding tissues when such tissues are inflamed or infected.
(d) Postcapillary venule (V) from an infected small intestine shows several leukocytes adhering to and migrating across the intima. (X200; H&E)
Veins carry blood back to the heart from microvasculature all over the body. Blood entering veins is under very low pressure and moves toward the heart by contraction of the smooth muscle fibers in the media and by external compressions from surrounding skeletal muscles and other organs. Most veins are classified as small or medium veins (Figure 11–22), with diameters of 10 mm or less (Table 11–1). These veins are usually located close and parallel to corresponding muscular arteries. The tunica intima is usually thin, the media has small bundles of smooth muscle cells mixed with a network of reticular fibers and delicate elastic fibers, and the collagenous adventitial layer is thick and well developed.
Veins usually travel as companions to arteries and are classified as small, medium, or large based on size and development of the tunics.
(a) Micrograph of small vein (V) shows a relatively large lumen compared to the small muscular artery (A) with its thick media (M) and adventitia (Ad). The wall of a small vein is very thin, containing only two or three layers of smooth muscle. (X200; H&E)
(b) Micrograph showing valve in an oblique section of a small vein (arrow). Valves are thin folds of intima projecting well into the lumen, which act to prevent backflow of blood. (X200; Aldehyde fuchsin & van Gieson) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
(c) Micrograph of a medium vein (MV) shows a thicker wall but still less prominent than that of the accompanying muscular artery (MA). Both the media and adventitia are better developed, but the wall is often folded around the relatively large lumen (X100; Aldehyde fuchsin & van Gieson). (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
(d) Micrograph of a medium vein contains blood and shows valve folds (arrows). (X200; Masson trichrome) (Reproduced, with permission, from Berman I. Color Atlas of Basic Histology. 3rd ed. New York, NY: McGraw-Hill; 2003).
The big venous trunks, paired with elastic arteries close to the heart, are the large veins (Figure 11–7b). These have well-developed intimal layers, but relatively thin media with alternating smooth muscle and connective tissue. The tunica adventitia is thicker than the media in large veins and frequently contains longitudinal bundles of smooth muscle. Both the media and adventitia contain elastic fibers, and an internal elastic lamina like those of arteries may be present.
An important feature of large and medium veins are valves, which consist of thin, paired folds of the tunica intima projecting across the lumen, rich in elastic fibers and covered on both sides by endothelium (Figures 11–22 and 11–23). The valves, which are especially numerous in veins of the legs, help keep the flow of venous blood directed toward the heart.
Wall of large vein with valve.
Large veins have a muscular media layer (M) which is very thin compared to the surrounding adventitia (A) of dense irregular connective tissue. The wall is often folded as shown here, with the intima (I) projecting into the lumen as a valve (V) composed of the subendothelial connective tissue with endothelium on both sides. (X100; PT)
Junctions between endothelial cells of postcapillary venules are the loosest of the microvasculature. This facilitates transendothelial migration of leukocytes at these locations during inflammation, as well as a characteristic loss of fluid here during the inflammatory response, leading to tissue edema.