Which organ is primarily responsible for gas exchange in respiration?

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

Answer: (A)

Gas exchange happens where oxygen and carbon dioxide can diffuse between air and blood. The lungs are the main site because they contain countless tiny air sacs called alveoli, each with walls that are extremely thin and are surrounded by a dense network of capillaries. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. This diffusion relies on a large surface area, a thin barrier, and a moist environment at the alveolar–capillary membrane. The other structures listed—the pharynx, trachea, and larynx—are conducting airways that filter, warm, and transport air but do not participate in gas exchange. So the lungs are primarily responsible for the gas exchange in respiration.