Where does gas exchange occur in the lungs?

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

Answer: (B)

Gas exchange happens in the alveoli, the tiny air sacs at the end of the airways. These sacs have walls that are only one cell thick and are closely wrapped in a dense network of capillaries, forming a very thin respiratory membrane. This setup, along with a large surface area and a moist lining, makes diffusion of gases efficient. Oxygen in the alveolar air diffuses into the blood because its partial pressure is higher there, while carbon dioxide moves from the blood into the alveolar air to be exhaled because its partial pressure is higher in the blood. The trachea, bronchi, and bronchioles are conducting pathways and lack the thin barrier and extensive capillary network needed for gas exchange, so diffusion doesn’t occur there.