Basic morphology of the pulmonary defence system.

Bronchoalveolar clearance is based on the coordination of multiple mechanisms, including surface flux from alveolus to bronchiole (owing to a surface-pressure gradient at end-expiration), adequate composition of the mucous blanket covering the larger airways, and proper ciliary function. The alveolar macrophages, the scavenger cells of the lungs, reside in the acellular lining of the terminal airspaces and eventually leave the lungs via the conducting airways. These cells are probably also involved in the turnover and removal of worn-out fragments of the surface film. There is evidence that the alevolar macrophages utilize this material as substrate for biochemical processes involved in the intracellular killing of bacteria. Lymphatic tissue is normally absent at the alveolar level but present in the mucosa of larger and medium-sized airways, especially at sites of bifurcation--i.e., where larger particles tend to impact on the bronchial surface during inspiration.