Which organs exchange carbon dioxide for oxygen to feed cells?

• A. Lungs
• B. Heart
• C. Brain
• D. Skin

Answer: (A)

Gas exchange with the environment happens in the lungs. In the tiny air sacs called alveoli, oxygen from the inhaled air diffuses into the blood, while carbon dioxide from the blood diffuses into the alveoli to be exhaled. This exchange is driven by diffusion across very thin membranes and a dense network of capillaries, allowing oxygen to reach cells everywhere and carbon dioxide to be removed as waste. The lungs are the primary site where inhaled oxygen enters the bloodstream and where carbon dioxide is expelled, enabling cellular respiration to continue. The heart moves blood around the body but does not exchange gases with the air; the brain uses oxygen but doesn’t exchange it with the environment directly; the skin may exchange some gases very slowly, but not in the way the lungs do for feeding cells.