NORMAL LUNG ANATOMY AND PHYSIOLOGY
Normal adult lung is a sophisticated system of conducting airways and gas exchange surfaces. The foregut develops a ventral outpouching that progressively bifurcates to create branch points ("carinas") for conducting airways (bronchi, then bronchioles, then alveolar ducts). Conducting airways form during the "pseudoglandular" phase by 15 weeks' gestational age, and proto-alveoli are evident in loose mesenchyme during the "canalicular" phase by 24 weeks' gestational age. The mesenchyme is progressively excluded or flattened between developing alveoli, such that an adult lung has back-to-back alveolar airspaces separated by elastin-rich, capillary-rich interstitial stroma.
In parallel with the development of the functional adult lung structure, the critical type II epithelial cell matures and begins to excrete surfactant, a natural detergent that breaks the water tension of the thin layer of water that covers the alveolar wall. Without type II cell surfactant, the newborn cannot generate enough mechanical force to inflate the alveoli, and will rapidly fatigue and arrest. The probability of surfactant deficiency varies inversely with fetal age, such that surfactant deficiency will be seen in >50% of fetuses <28 weeks' gestational age. The fetal age at which 95% of fetuses can ventilate normally after delivery is 36 weeks. Fetuses delivered before that age are monitored aggressively and treated promptly with synthetic surfactant if they have persistent atelectasis (collapse of lung tissue) and begin to tire.
The anatomy of the adult lung is best described starting at the larynx and proceeding toward the alveoli. The trachea and bronchi are encircled by near-circumferential cartilage rings that prevent collapse of these large-caliber airways during the expiratory phase of the ventilatory cycle. These airways show lush surface cilia and submucosal glands that excrete mucin. Moving distally in the conducting airways, the cartilage rings and submucosal glands are lost; these small-caliber airways are called bronchioles. Like the trachea and bronchi, bronchioles have lush surface cilia. The bronchi and bronchioles run adjacent to pulmonary artery branches of similar caliber. Alveolar ducts lose the cilia, as one enters the grapelike clusters of alveoli, the critically important sites for countercurrent exchange of inhaled O2 and RBC-bound CO2. Incomplete ridges subdivide this surface area into intersecting spheres that in tissue sections look like incomplete circles separated by thin, delicate, capillary-rich alveolar septae. The alveoli are lined by a watertight monolayer of type I cells. Efficient gas exchange requires minimal diffusion distance, and evolution has left us with a fused basement membrane in common between the alveolar surface type I epithelial cells and the underlying capillary endothelial cells. The capillaries within the alveolar septae are surrounded by abundant elastin, and together comprise the interstitium (Figure 8-1).
Normal alveolar parenchyma. Thin, delicate alveolar septae facilitate bidirectional gas exchange between alveolar gas and capillary red blood cell hemoglobin.