Which organ is primarily responsible for gas exchange?

• A. Heart
• B. Kidney
• C. Liver
• D. Lungs

Answer: (D)

Gas exchange between air and blood takes place primarily in the lungs, at the alveolar membranes where air sacs meet a dense network of capillaries. Oxygen diffuses from the alveolar air into the blood because its partial pressure is higher there, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled because its partial pressure is higher in the blood. This diffusion is enabled by the lungs’ large surface area, extremely thin barrier, and rich blood supply. The heart pumps blood to circulate gases but does not exchange them with the air; the kidneys and liver handle filtration, metabolism, and other processes, not direct gas exchange with the environment. Therefore, the lungs are the organ responsible for gas exchange.