The alveolar epithelium consists of three cell types: Type I and Type II cells and alveolar macrophages. Type I cells are thin squamous epithelium that line the alveoli. They are the sites of gas exchange. Type II cells produce lung surfactant starting at 32 weeks of gestation. Lung surfactant coats the interior surfaces of the alveoli, reducing surface tension. This keeps the alveoli from collapsing at end-expiration. Alveolar macrophages (dust cells) engulf particles that make it into the alveoli.
The respiratory membrane is the site of gas exchange in the alveoli. Gases pass through the alveolar epithelial cell, a fused basement membrane, and the capillary endothelial cell. The respiratory membrane is very thin. Low blood pressure (CHP) in the lungs minimizes flow of fluids out of the pulmonary capillaries into the limited interstitial space of the alveoli.
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