What is internal respiration?

• A. Exchange of gases between lungs and blood
• B. Diffusion of oxygen within tissues of the body
• C. Exchange of gases between cells of the body and the blood
• D. Breathing in air

Answer: (C)

Internal respiration is the gas exchange that occurs between the blood in the systemic capillaries and the body's cells. Oxygen diffuses from the blood into tissue cells to support cellular respiration, while carbon dioxide produced by those cells diffuses back into the blood to be carried to the lungs for elimination. This bidirectional exchange happens throughout the body's tissues and is distinct from external respiration, which is the exchange of gases between the lungs and the blood, and from breathing in air, which is ventilation. The option describing exchange between cells and the blood best captures the full process involved in internal respiration, unlike descriptions that focus only on diffusion within tissues or on inhaling air.