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The pulmonary surfactant: quantity and quality.

THE evolution of life from an aquatic to a gaseous environment necessitated the emergence of specialized organs for respiration characterized by large areas for gas exchange. In the mammalian lung, this surface area is provided by the repeated branching of the bronchial tree ending in respiratory units (alveoli) in which the epithelium is overlaid by a thin film of fluid. Thus, the lung is characterized by an extensive air-liquid interface, and surface forces generated at this interface provide a major retractile force. If unopposed, their surface tension would promote alveolar collapse at the end of expiration, allowing perfusion of unaerated . . .