Topology and barrier function of membranes in the lung.

The alveolo-capillary air-blood barrier of the lung is made of two thin cell layers, epithelium and endothelium, that limit the flow of solutes and fluid from blood plasma into the narrow interstitial space and into the very thin alveolar fluid lining associated with surfactant. The conditions for maintaining a thin dry air-blood barrier are discussed. The effect of barrier thickness on gas exchange is discussed by means of the changes in barrier structure that lead to respiratory failure in the respiratory distress syndrome.