Definition: The alveoli are the final branchings of the respiratory tree and perform gas exchange for the lung. The thin gas-blood barrier between the alveolar space and the pulmonary capillaries allows for rapid gas exchange. Oxygen and carbon dioxide diffuses through the alveolar epithelium and the capillary endothelium.
There are two types of alveolar epithelial cells. Type I cells thinly line the alveolar walls. Type II cells produce surfactant - a lubricant that reduces surface tension.
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Source: Johns Hopkins School of Medicine's Interactive Respiratory Physiology
Update: 12/08/05