ABSTRACT
The primary function of the lungs is gas exchange. The gas exchange surface is mainly composed of a single thin layer of epithelial cells, the alveolar type I cells. Interspersed among these are larger cuboidal alveolar type II cells, which produce the alveolar lining fluid. This fluid layer must be kept thin, to permit efficient gas exchange. The alveolar epithelium has low permeability to electrolytes and plasma proteins, and actively transports sodium ions away from the lumenal surface, while tight junctions between the epithelial cells provide a highresistance barrier to fluid movement from the interstitium to the alveolar space (1). Pulmonary surfactant also lowers the surface tension of the blood-gas interface (2). Free-ranging phagocytic alveolar macrophages (AMs) are a third cell type found in varying numbers in the extracellular lining fluid on the alveolar surface. These cells patrol the alveolar surface and ingest inspired particulates and invading pathogens (3).
