ABSTRACT
The extracellular fluid lining the respiratory tract and airspaces exists as a continuum from larynx to alveolus. It has a complex composition and a structure that varies from site-to-site that reflects its many functions (1). In the airways, it consists of sol and gel layers surmounted by a surfactant film of unknown composition at the air-liquid interface. In the alveolus, the extracellular fluid consists of a thin hypophase covered by a dipalmitoyl phosphatidylcholine (DPPC)-rich surfactant film. The direct demonstration of a surfactant film in alveoli and airways is relatively recent (2-4), although a surface-active film had been inferred from physiological (5) and electron microscopic studies (6) many years earlier. Such indirect proof for a surfactant film is a low surface tension of the interface. The surface tension in large airways has been measured directly with a bronchoscope from the spreading behavior of oil droplets placed onto the tracheal walls or bronchi of anesthetized sheep and horses (7). A surface tension of approximately 32 mN/m has been recorded at the mucus-air interface in these animals. This suggests the presence of a surfactant film because proteins, surface polymers of blood cells, polysaccharides, and other biopolymers all result in surface tensions between 45 and 60 mN/m (8) when they adsorb to an air-water interface.
