Which body system is responsible for bringing oxygen into the body, releasing carbon dioxide, and helping regulate blood pH?

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

Answer: (C)

Gas exchange and acid-base balance are functions of the respiratory system. The lungs bring in oxygen and expel carbon dioxide through the alveolar–capillary membranes, where oxygen diffuses into the blood and carbon dioxide diffuses out to be breathed away. This process also helps regulate blood pH because carbon dioxide affects the carbonic acid–bicarbonate buffering system in the blood: higher CO2 lowers pH (more acidic), while removing CO2 raises pH toward normal. Breathing depth and rate are adjusted by chemoreceptors to maintain pH within a narrow range. The other systems don’t perform gas exchange or primary pH regulation—digestion handles nutrients, the nervous system coordinates activities, and the circulatory system transports gases but does not carry out the gas exchange and buffering itself.