Which statement correctly distinguishes external respiration from internal respiration?

• A. External respiration occurs in tissues; internal respiration occurs in the lungs
• B. External respiration uses carbon dioxide to fuel cells
• C. External respiration is the gas exchange between lungs and blood; internal respiration is the exchange between blood and body's cells
• D. External respiration is the cellular conversion of energy

Answer: (C)

Gas exchange location and roles differentiate external from internal respiration. External respiration is the gas exchange between the lungs and blood in the pulmonary capillaries—oxygen moves from the alveolar air into the blood, and carbon dioxide moves from blood into the alveoli to be exhaled. Internal respiration is the exchange between blood and the body's cells—oxygen moves from blood into tissues to support metabolism, and carbon dioxide moves from tissues into blood to be carried back to the lungs. This distinction matters because external respiration prepares oxygen for transport, while internal respiration delivers that oxygen to cells and removes the CO2 produced by metabolism. The other statements misplace where the exchange occurs, suggest carbon dioxide is used as fuel, or describe cellular energy production rather than gas exchange.