Which organ is primarily responsible for gas exchange in respiration?

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

Answer: (D)

Gas exchange happens where oxygen from the air moves into the blood and carbon dioxide from the blood is expelled, primarily in the lungs. The lungs contain tiny air sacs called alveoli lined with capillaries, forming a large, very thin surface area that allows diffusion. Oxygen diffuses from the alveolar air (higher partial pressure) into the blood, while carbon dioxide diffuses from the blood (higher partial pressure) into the alveolar air to be exhaled. This diffusion is driven by these partial pressure gradients across the thin respiratory membrane. The heart circulates blood between the lungs and the rest of the body but does not perform the gas exchange itself. The liver and kidneys perform other jobs and are not the sites of gas exchange.