Fluid and electrolyte transport across monolayers of alveolar type II cells in vitro

Current cell culture techniques allow investigators to separate the endothelial and the epithelial components of the 'alveolar-capillary membrane'. Alveolar type II cells form a morphologically and pharmacologic polarized monolayer in vitro. The properties of the cultured epithelial cells as well as theoretical considerations indicate that there is active fluid resorption in vivo. We propose that sodium is actively transported from the subphase into the interstitium. The traditional focus on the endothelium and passive forces for fluid movement in the distal lung is useful because it is simple but is certain to be incomplete. Discussion of fluid movement in normal and diseased lungs should consider active as well as passive forces.