Which system allows gas exchange?

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

Answer: (B)

Gas exchange requires a specialized surface where oxygen can move from air into blood and carbon dioxide can move from blood into air. The respiratory system provides that surface in the lungs, especially the tiny alveoli with a very thin, moist barrier and a large surface area. Diffusion drives the movement: oxygen partial pressure is higher in the alveolar air than in the blood, so oxygen diffuses into the blood; carbon dioxide is at higher pressure in the blood, so it diffuses into the alveolar air to be exhaled. Hemoglobin helps carry oxygen to tissues, while most carbon dioxide is transported as bicarbonate in plasma, but the key exchange with the external environment happens at the alveolar-capillary interface. The circulatory system moves these gases around the body, and the nervous system helps regulate breathing, but the actual gas exchange with the environment occurs in the respiratory system. The digestive system, meanwhile, is unrelated to gas exchange.