Gas exchange occurs primarily through the walls of which lung structures?

• A. Bronchioles
• B. Alveoli
• C. Trachea
• D. Larynx

Answer: (B)

Gas exchange occurs across the walls of the tiny air sacs in the lungs called alveoli. The alveolar walls, with their very thin epithelial layer and close contact with surrounding capillaries, form the respiratory membrane where oxygen and carbon dioxide diffuse between air and blood. Alveoli provide a huge surface area and a short diffusion distance, making them the primary site for this exchange. The conducting airways—the trachea, larynx, and bronchioles—carry air but have thicker walls and are not structured for diffusion, so gas exchange doesn’t occur there.