Transcapillary pulmonary exchange of water in the dog.

T HE CAPILLARIES in the lungs, in contrast to capillaries elsewhere in the body, are in effect bathed in air rather than water. There is little histological evidence for a substantial pulmonary interstitial space; instead, the bulk of the lung tissue appears to consist of cells and blood vessels. Perhaps because of these anatomical peculiarities little or no consideration appears to have been given to the possibility that large amounts of water might exchange across the walls of these capillaries. The data presented below, which are part of a more general study of capillary permeability, suggest that, in the normal dog, water does cross these capillaries in amounts per unit time which approach in magnitude the pulmonary blood flow.