Internal respiration takes place between which structures?

• A. The alveoli and pulmonary capillaries
• B. The airway epithelium
• C. The heart and arteries
• D. The systemic capillaries and body cells

Answer: (D)

Internal respiration is the exchange of gases between the blood and the body's tissues. Oxygen carried in the systemic capillaries diffuses into body cells to fuel metabolism, while carbon dioxide produced by those cells diffuses back into the blood to be carried away and expelled. This tissue-level gas exchange happens at the level of systemic capillaries and body cells, not in the airways or lungs.

In contrast, external respiration—the gas exchange between air in the lungs and the blood—occurs in the alveoli and pulmonary capillaries. The heart and arteries are parts of circulation, but they aren’t the sites where gas exchange with tissues happens. The airway epithelium is part of the airway lining, not the site of gas transfer between blood and cells.