Which structures are tiny sacs at the end of the bronchi where gas exchange occurs?

• A. Bronchi
• B. Larynx
• C. Alveoli
• D. Pleura

Answer: (C)

Gas exchange happens in the alveoli—tiny sacs at the ends of the airways. Their walls are very thin and they sit in close contact with a dense network of capillaries, giving a large surface area for diffusion. Oxygen moves from the air into the blood, and carbon dioxide moves from the blood into the air to be exhaled. Other structures have different roles: bronchi are larger air passages that conduct air, the larynx is the voice box, and the pleura is the membrane surrounding the lungs.