Which system takes in oxygen and eliminates carbon dioxide? The gaseous exchanges occur through the walls of the air sacs of the lungs.

• A. Respiratory system
• B. Circulatory system
• C. Nervous system
• D. Immune system

Answer: (A)

Gas exchange with the environment happens in the respiratory system. It brings air into the lungs and, through the thin walls of the alveoli, exchanges gases with the blood. Oxygen from the inhaled air diffuses across the alveolar wall into surrounding capillaries, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled. The alveolar walls provide a very large, moist surface and are thin to allow diffusion to occur efficiently. The circulatory system then transports the oxygen to body tissues and carries carbon dioxide back to the lungs, but it doesn’t perform the gas exchange with the outside air. The nervous system helps regulate breathing, but the actual exchange happens in the lungs’ alveolar walls. The immune system isn’t involved in this gas exchange process.