In which organ does oxygen exchange occur in the circulatory system?

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

Answer: (A)

Gas exchange for oxygen happens in the lungs, specifically at the alveoli. In these tiny air sacs, oxygen from inhaled air diffuses across the thin alveolar-capillary membrane into the blood, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled. This process occurs as part of the pulmonary circulation, with deoxygenated blood from the heart’s right side reaching the lungs, picking up oxygen, and returning to the heart to be circulated to the rest of the body. The other organs listed—liver, spleen, and kidneys—have important roles in metabolism, immunity, and waste filtration, but they are not the sites where oxygen is exchanged between air and blood.