Internal respiration is defined as:

• A. the exchange of gases between the blood and the body's tissues
• B. the diffusion of gas in the alveoli
• C. the movement of air through the trachea
• D. the transport of oxygen by hemoglobin

Answer: (A)

Internal respiration is the exchange of gases between the blood and the body's tissues. Oxygen diffuses from blood into tissues to fuel cellular metabolism, while carbon dioxide diffuses from tissues into the blood to be carried back to the lungs for expiration. This occurs at the systemic capillaries and is driven by differences in partial pressures: tissue PO2 is typically lower than blood PO2, and tissue PCO2 is higher than blood PCO2. This is distinct from external respiration, which is the diffusion of gases between the alveoli and the blood; ventilation, which is air movement through the trachea; and oxygen transport, which is the carrying of oxygen by hemoglobin in the blood.