Which cells comprise the alveolar gas exchange surface?

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Gas exchange happens across the thin lining of the alveoli, which is formed by the alveolar epithelium. The cells that line the alveoli are two types of pneumocytes. Type I pneumocytes are very thin and cover most of the alveolar surface, providing the diffusion path for oxygen and carbon dioxide. Type II pneumocytes are cuboidal cells scattered among them; they secrete surfactant to reduce surface tension and can proliferate to repair the epithelium, so they are part of the alveolar lining as well. Alveolar macrophages reside in the air spaces to clean debris, goblet cells make mucus in the larger airways, and endothelial cells line the capillaries forming the other side of the diffusion barrier. The alveolar gas exchange surface is best described as the alveolar epithelium formed by both types of pneumocytes, with the capillary endothelium on the opposite side.