Ultrastructural organization of the alveolar-capillary unit.

As a result of its special position in the circulatory system the lung can screen and monitor the composition of the blood which comes from and is returned to all the tissues. This function, together with the exchange of gases, takes place at the level of the alveolar-capillary unit. The cellular components of the unit are: the epithelium, lining the air spaces, and the endothelium, facing the blood compartment. The epithelium is composed of two types of cells: type I--broad, squamous, highly branched cells occupying approximately 97% of the total alveolar surface; these cells seem to be involved mostly in exchange of gases; and type II--cuboidal cells containing characteristic osmiophilic lamellar bodies in their cytoplasm. The epithelial cells are connected to one another by tight junctions. A thin layer of a surface-active material--the surfactant--covers the epithelium toward the air space, where its role is to lower the surface tension. Surfactant is a mixture of lipids (approximately 75%) and proteins; dipalmitoyl-phosphatidylcholine is the major lipid component and is synthesized and secreted by type II epithelial cells. The endothelium is of continuous type and has a large population of plasmalemmal vesicles; the cells are linked together by tight junctions. Morphometric studies indicate that in humans the capillary surface area is approximately 120 m2 and the alveolar surface area is approximately 140 m2. Some of the metabolic functions ascribed to the lung have been localized to cellular components. Phospholipids needed for the constantly renewed surfactant are synthesized in type II epithelial cells. Angiotensin-converting enzyme is associated with the endothelial cell membrane and vesicles opening to the blood front. There are indications that pulmonary cells also intervene in the metabolism of circulating vasoactive substances which during their passage through the lung can be activated (angiotensin I), inactivated (bradykinin) or removed from the circulation (5-hydroxytryptamine). The lung is a metabolically active organ where the anabolism and catabolism of pharmacologically active substances and the synthesis of lipids take place and a proper balance for blood homeostasis is maintained.