The alveoli are the final branchings of the respiratory tree and act as the primary gas exchange units of the lung. The gas-blood barrier between the alveolar space and the pulmonary capillaries is extremely thin, allowing for rapid gas exchange. To reach the blood, oxygen must diffuse through the alveolar epithelium, a thin interstitial space, and the capillary endothelium; CO2 follows the reverse course to reach the alveoli.
There are two types of alveolar epithelial cells. Type I cells have long cytoplasmic extensions which spread out thinly along the alveolar walls and comprise the thin alveolar epithelium. Type II cells are more compact and are responsible for producing surfactant, a phospholipid which lines the alveoli and serves to differentially reduce surface tension at different volumes, contributing to alveolar stability.