Alveolar Surface Mechanics

The major component of the recoil forces that tend to deflate the lungs is the surface tension of a thin liquid layer that lines the alveoli. This surface tension is well below the value for a clean air/water interface, indicating the presence of a surfactant. The surface tensions. in situ specify that the surfactant films must have certain characteristics. Following large expansions of the air/water interface during deep inhalations, the surfactant must adsorb rapidly to form the interfacial film. When compressed by the shrinking surface area during exhalation, the films must be sufficiently rigid to resist the tendency to collapse from the interface. Materials washed from the lungs show that pulmonary surfactant is a mixture containing mostly lipids with some proteins that is synthesized and secreted by the type II pneumocyte. The disorder in which an abnormality of pulmonary surfactant most clearly plays a role is the respiratory distress syndrome of premature babies, although altered surfactant function may also contribute to the acute respiratory distress syndrome that occurs in patients of all ages.