Which structure is primarily used for gas exchange in respiration?

• A. Larynx
• B. Pharynx
• C. Bronchi
• D. Lungs

Answer: (D)

Gas exchange occurs where oxygen moves into the blood and carbon dioxide leaves it, which happens mainly in the lungs, specifically in the tiny air sacs called alveoli that are surrounded by capillaries. The alveolar walls are very thin and moist and present a large surface area, so diffusion driven by partial pressure differences efficiently transfers gases between air and blood. The other structures have different roles: the larynx helps with voice and protects the airway, the pharynx is a passageway for air and food, and the bronchi are airways that conduct air into the lungs but do not themselves perform gas exchange.