The primary site of gas exchange across the alveolar membrane involves Type I pneumocytes and which other component?

• A. Alveolar macrophages
• B. Surfactant-producing cells
• C. Alveolar cartilage
• D. Pulmonary capillary endothelium

Answer: (D)

Gas exchange across the alveolar membrane occurs across the thin barrier between the air-filled alveolus and the blood in the pulmonary capillaries. The barrier is formed mainly by flat Type I pneumocytes lining the alveolar surface on one side and the pulmonary capillary endothelium on the other, with their fused basement membranes in between. This extremely thin diffusion barrier allows O2 to move from air into blood and CO2 to move from blood into air by simple diffusion driven by partial pressure differences. While Type II pneumocytes produce surfactant to reduce surface tension and alveolar macrophages patrol the airspace, these elements are not the primary components of the gas-exchange barrier. Alveolar cartilage does not exist in the alveolar walls.