Alveoli: Gas Exchange and Host Defense

Abstract

The alveoli as the most distal part of the respiratory tract function primarily in gas exchange. They provide a surface of approximately 75 m2 in humans for oxygen and carbon dioxide exchange between air and blood, which occurs by passive diffusion. The gas exchanging part of the alveolar septum consists of (1) a thin walled continuous capillary, (2) a dual basal lamina synthesized by endothelia and type I alveolar epithelial cells, (3) thin cytoplasmic extensions of type I alveolar epithelial cells, and (4) red blood cells (RBC) (panels A and B). The flat type I alveolar epithelial cells represent about 40% of the alveolar epithelial cell population and cover some 90% of the alveolar surface. The cuboidal type II alveolar epithelial cells (panel C), the second alveolar epithelial cell type, cover only 10% of the alveolar surface and are found predominantly in the alveolar niches. They are indirectly involved in gas exchange by producing the pulmonary surfactant, which consists of phospholipids, cholesterol, and specific surfactant proteins.