External respiration is:

• A. the exchange of gases between the lungs (specifically the alveoli) and the blood
• B. the exchange of gases between the blood and body tissues
• C. the production of carbon dioxide in cells
• D. the diffusion of oxygen within red blood cells

Answer: (A)

External respiration is the gas exchange that happens across the respiratory membrane between the air in the alveoli and the blood in the pulmonary capillaries. Oxygen moves from the alveolar air, where its partial pressure is higher, into the blood, while carbon dioxide moves from the blood, where its partial pressure is higher, into the alveolar air to be exhaled. This diffusion-driven process depends on factors like the large surface area of alveoli, the thinness of the membrane, and proper ventilation–perfusion matching.

This differs from internal respiration, which is the exchange of gases between the blood and body tissues. It also isn’t about cellular production of CO2 or diffusion of oxygen inside red blood cells; the key idea is gas transfer between lungs and blood.