Water and nonelectrolyte transport across alveolar epithelium.

An overall view of the barrier properties of pulmonary alveolar epithelium is beginning to emerge from the recent studies on isolated alveolar epithelial preparations reviewed here. It appears that the alveolar epithelial barrier is normally extremely tight and is probably highly effective in preventing the passive flow of water and solutes into alveoli in normal lungs. It becomes leaky when subjected to a number of insults, suggesting an important mechanism for the development of alveolar pulmonary edema. Alveolar epithelium is also an actively transporting tissue, and, as such, probably plays a major role in normal lung fluid balance. We are just beginning to gain insight into these active and passive transport processes, and a great deal of work remains to be done in order to understand their mechanisms, importance, and regulation in vivo under both normal conditions and conditions involving pulmonary edema.