What is the structure where gas exchange occurs in the lungs?

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

Answer: (B)

Gas exchange happens where the air in the lungs meets the blood: across the thin walls of the alveoli, the tiny sacs at the ends of the airways. Oxygen diffuses from the alveolar air into the surrounding capillaries, while carbon dioxide diffuses from the blood into the alveoli to be exhaled. The alveolar membranes are extremely thin and the surface area is very large, which makes diffusion efficient. Surfactant helps keep the alveoli open so gas can move across easily. The conducting airways, like the trachea and bronchi, simply transport air to the lungs and don’t participate in gas exchange, and the aorta is a major artery carrying blood away from the heart, not involved in exchanging gases.