The pulmonary surfactant system: Biological functions, components, physicochemical properties and alterations during lung disease

The alveolar epithelium in the lung is covered with a liquid, the alveolar lining layer. This alveolar lining layer consists of an aqueous subphase covered by a film of pulmonary surfactant. The surfactant film reduces the air-liquid interfacial tension, thereby preventing alveolar collapse and preserving the gas exchange surface. The aqueous subphase provides a storage medium for surfactant components. Into this medium surfactant components are secreted from type II cells, adsorb to and desorb from the air-liquid interface, change their morphological structures and interact with cells of the host defense system, e.g. alveolar macrophages. Alterations of the pulmonary surfactant system occur in clinical conditions including the respiratory distress syndrome of the premature newborn and the adult. This review summarizes biological functions, components and physicochemical properties of the pulmonary surfactant system and describes alterations of the system found during respiratory distress syndrome of the adult.