Which cells in the alveoli are primarily responsible for gas exchange?

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

Answer: (A)

Gas exchange happens across the very thin barrier formed by the alveolar lining. Type I pneumocytes are flat, squamous cells that cover most of the alveolar surface and create the minimal diffusion distance between air and the capillary blood. This thin, expansive layer is ideal for efficient movement of oxygen into the blood and carbon dioxide out of it. Type II pneumocytes, while important, are cuboidal cells that produce surfactant and help repair the lining, not primarily for diffusion. Alveolar macrophages reside in the airspace to engulf debris and pathogens, and Clara cells are in the bronchioles, contributing to secretion and detoxification but not gas exchange.