Which alveolar cell forms the thin gas-exchange surface of the alveoli?

• A. Type II pneumocytes
• B. Alveolar macrophages
• C. Clara cells
• D. Type I pneumocytes

Answer: (D)

Gas exchange relies on a very thin diffusion barrier, provided mainly by the squamous Type I pneumocytes that line most of the alveolar surface. These cells are extremely flat, placing the air-alveolar membrane very close to the capillary blood space and allowing efficient gas diffusion. Type II pneumocytes, by contrast, are cuboidal and secrete surfactant to reduce surface tension and can proliferate to replace Type I cells when needed. Alveolar macrophages roam the airspaces to clear debris, and Clara cells are located in the bronchioles rather than the alveoli. So the thin gas-exchange surface is formed by Type I pneumocytes.