Alveolar wall.  Mouse lung (485310)

Type I pneumocyte (upper right) with attenuated cytoplasm lines the alveolar surface.  Distributed throughout the alveolar walls are pulmonary capillaries lined by the endothelial cell (lower right).  The thin air-blood barrier formed by the thin cytoplasm of these two cell types are illustrated.  A large alveolar macrophage (near center) sits on the surface of the type I pneumocyte.