Which statement best describes the site of gas exchange in the lungs?

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

Answer: (C)

Gas exchange happens where gases move by diffusion across a very thin, moist barrier with a large surface area. In the lungs, that occurs in the alveoli. These tiny sacs are surrounded by an extensive network of capillaries, providing a huge surface area for contact between air and blood. The alveolar walls are extremely thin and lined with fluid, allowing oxygen to dissolve and diffuse into the blood, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled. Surfactant helps keep the alveoli open, maintaining that large surface area for exchange. The diffusion follows partial pressure differences, so oxygen moves into the blood and carbon dioxide moves into the alveolar space to be expelled. The trachea and bronchi merely conduct air, and the heart pumps blood but does not exchange gases—making the alveoli the site where gas exchange occurs.