Figure 17-29 Schematic summary of the ultrastructure of a pulmonary capillary. The upper side of the capillary has the endothelial and epithelial basement membranes separated by an interstitial space, whereas the lower side contains only fused endothelial and epithelial basement membranes. The dashed arrows indicate a potential pathway for fluid to move from the intravascular space to the interstitial space (through loose junctions in the endothelium) and from the interstitial space to the alveolar space (through tight junctions in the epithelium). ALV, alveolus; BM, basement membrane; ENDO, endothelium; EPI, epithelium; IS, interstitial space; LJ, loose junction; RBC, red blood cell; TJ, tight junction. (Redrawn from Fishman AP: Pulmonary edema: The water-exchanging function of the lung. Circulation 46:390, 1972.)