Respiratory Zone

The relationship between the respiratory system and the cardiovascular system is ideally suited for the process of gas exchange between the alveolus and the blood. Pulmonary capillaries (diameter = 6-15 mm) are located in the walls of each alveolus (diameter = 250 mm) [42]. Many capillaries are in close association with each alveolus, leading to a large "common" area for gas or solute exchange estimated to be 50-120 m2 in an adult human. In traversing the air-blood barrier, gases in the alveolus must cross the alveolar epithelium, the capillary endothelium, and their basement membranes before reaching the blood, a distance in all of approximately 500 nm [42]. In some regions of the alveolus, an interstitial space (containing connective tissue elements) separates the basement membranes of the alveolar epithelium and the capillary endothelium, and it is in these loci that solutes and liquid exchange have been hypothesized to occur [43]. The large area for absorption, together with the short transit distances, optimizes the process of diffusion of gases between the alveolar space and blood. Adjacent epithelial cells lining the alveolus are connected by a tight junction that limits the intercellular passage of solutes [7,44]. Pulmonary endothelial cells are also joined by tight junctions, although the nature of these junctional processes differs from that in the epithelium, insofar as interruption of the junctions in the endothelium may occur and permit the intercellular passage of large solutes to and from the interstitium [44]. Lipophilic solutes readily diffuse across epithelial and endothelial cells. Other solutes pass through the alveolar epithelium and capillary by transcellular paths (e.g., pores, transcytosis) in a manner related inversely to their size and lipophobicity [44].

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