Internal respiration refers to gas exchange between which structures?

• A. Exchange of gases between lungs and blood
• B. Gas exchange between blood and body tissues
• C. Gas transport in blood
• D. Diffusion in capillaries

Answer: (B)

Internal respiration is the exchange of gases between the blood and the body's tissues at systemic capillaries. Oxygen carried in the blood diffuses out into tissues to support cellular metabolism, while carbon dioxide produced by those tissues diffuses into the blood to be carried back to the lungs for elimination. This occurs across the capillary walls in tissue beds throughout the body. In contrast, external respiration is the exchange between air in the lungs and blood in pulmonary capillaries, and gas transport in blood refers to how gases are carried, not where the exchange happens. Diffusion in capillaries is part of the mechanism, but the specific event of internal respiration is the transfer between blood and tissue cells.