Which statement best describes the primary function of the respiratory system?

• A. Digestion of nutrients
• B. Circulation of blood
• C. Regulation of body temperature
• D. Bringing in oxygen, releasing carbon dioxide, and helping regulate blood pH

Answer: (D)

Gas exchange is the essential job of the respiratory system: it brings oxygen into the body to supply tissues and removes carbon dioxide, a waste product of metabolism. This exchange happens in the lungs at the alveoli, where oxygen diffuses into the blood and carbon dioxide diffuses out to be exhaled. Beyond just moving gases, the respiratory system helps regulate blood pH because carbon dioxide influences the bicarbonate buffering system in the blood; adjusting breathing changes the level of carbon dioxide, which in turn helps keep the pH within a narrow, healthy range. Digestion, circulation, and temperature regulation involve other systems, so the statement that emphasizes bringing in oxygen, releasing carbon dioxide, and helping regulate blood pH best describes the primary function of the respiratory system.