Alveolar epithelial barrier and acute lung injury.

The central importance of the alveolar epithelial barrier in the pathogenesis and recovery from acute lung injury has only recently been appreciated. Both in vivo and in vitro studies have provided a new understanding of the mechanisms that regulate transport of sodium, water, and protein across the alveolar epithelial barrier. This new information regarding the normal function of the alveolar epithelial barrier in regulating lung fluid and protein balance has made it possible to study the function of the alveolar barrier both experimentally and clinically in the setting of acute lung injury. The alveolar epithelial barrier is much more resistant to injury than the nearby lung endothelium. The mechanisms that cause injury to the alveolar barrier are just beginning to be explored in different experimental models of acute lung injury. Some progress has been made in understanding how alveolar barrier injury occurs, especially in bacterial pneumonia. Finally, while it is recognized that alveolar epithelial type II cells play an important role in both ion transport and surfactant production, it is now possible to study in vitro the contribution of alveolar epithelial type II cells in repair of the denuded alveolar barrier.