Which system is responsible for gas exchange between air and blood?

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

Answer: (B)

Gas exchange between air and blood happens in the lungs, where the respiratory system provides that interface. In the tiny air sacs called alveoli, oxygen from inhaled air diffuses through the thin alveolar and capillary walls into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. This diffusion is driven by differences in partial pressures and requires a moist, very thin barrier and a large surface area. The respiratory system handles this exchange directly. The cardiovascular system moves the gases once they're in the blood, delivering oxygen to tissues and carrying carbon dioxide back to the lungs. The nervous system controls breathing rhythm and depth but does not perform the gas exchange itself. The digestive system does not participate in this process.