External respiration is the exchange of oxygen and carbon dioxide between which structures?

• A. The systemic capillaries and body cells
• B. Diffusion in the airways
• C. Cellular respiration in mitochondria
• D. Exchange of oxygen and carbon dioxide between the alveoli and pulmonary capillaries

Answer: (D)

External respiration refers to gas exchange between the air in the lungs and the blood. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled, across the alveolar-capillary membrane. This diffusion is driven by partial pressure differences and requires a thin, large-surface-area barrier. It’s different from internal respiration, which is exchange between the systemic capillaries and body tissues, and from cellular respiration, the mitochondria’s use of O2 to produce ATP. It’s not the diffusion occurring in the airways, which doesn’t involve meaningful gas exchange. So the correct description is the exchange between the alveoli and pulmonary capillaries.