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. 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. These surfactant films are the first point of interaction between inhaled particles and the host and influence their deposition, clearance, and toxicity (1).
