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. That surface tension is well below the value for a clean air/water interface, indicating the presence of a surfactant. The surface tensions in situ indicate that films of this pulmonary surfactant must have certain characteristics. Following large expansions of the air/water interface during deep inhalations, pulmonary surfactant must form an interfacial film quickly. 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 of 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. Altered surfactant function may also contribute to pulmonary disorders that occur in patients of all ages.