Which part of the respiratory system is primarily responsible for gas exchange?

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

Answer: (C)

Gas exchange happens where the air meets the blood: in the alveoli, tiny sacs at the ends of the airways. Oxygen from the inhaled air diffuses through the very thin walls of the alveoli into the surrounding capillaries, while carbon dioxide moves from the blood into the alveolar air to be exhaled. This process relies on a huge surface area, a very thin respiratory membrane, and a rich capillary network right next to the air-filled spaces. Surfactant keeps the alveoli open, ensuring efficient diffusion. The trachea and bronchi mainly conduct air to the lungs, and the pharynx is a passageway; neither is the primary site of gas exchange.