Which structure is the site of gas exchange in the lungs?

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

Answer: (C)

Gas exchange relies on diffusion across a very thin barrier where air meets blood. In the lungs, that barrier is formed by the walls of the alveoli and the surrounding capillaries. Alveoli are tiny sacs with one-cell-thick walls and an extensive capillary network, creating a large surface area for gas transfer. Oxygen moves from the alveolar air into the blood, and carbon dioxide moves from the blood into the alveolar air to be exhaled. This transfer is driven by differences in partial pressures of the gases. Surfactant within the alveoli helps keep them open, maintaining surface area for efficient diffusion. The trachea and bronchioles mainly serve to conduct air to the alveoli, while the diaphragm controls breathing movement rather than gas exchange.