In the distal regions of the human lung, one of the most challenging problems facing a large multicellular organism is solved-ensuring an adequate supply of oxygen for aerobic tissue metabolism while removing associated waste products. Conduits for both air and blood converge at the alveolar level to match ventilation with perfusion and thus assure the free diffusion of oxygen and carbon dioxide. Despite their thin walls and their intimate relationship to the pulmonary capillary bed, the alveolus must present a barrier function robust enough to resist alveolar flooding from the hydrostatic...