Which organ system is primarily responsible for gas exchange?

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

Answer: (D)

Gas exchange is the process of oxygen entering the blood and carbon dioxide leaving it, and it happens where the air-filled spaces of the lungs meet the bloodstream. The respiratory system provides the structures for this exchange: the lungs with countless tiny air sacs called alveoli and a dense capillary network. Oxygen diffuses from the alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveolar air to be breathed out. This exchange relies on differences in partial pressures across the alveolar–capillary membrane, so the surface area and thin walls of the alveoli are crucial.

The circulatory system mainly transports gases to and from tissues, but the actual gas exchange surface is in the respiratory system. The nervous system helps regulate breathing, but it’s not where the exchange occurs. The digestive system handles nutrients and has no role in gas exchange.