Pulmonary defense mechanisms.

The airways and gas-exchanging surfaces of the lung are an important interface between the host and the external environment. Potentially injurious particulates and gases continuously impact on this extensive epithelial surface. The pulmonary defense apparatus, which uses anatomic barriers, aerodynamics, soluble factors and a multitude of cellular processes, protects the airways and the alveolar surfaces so that gas exchange between the host and the environment can occur.