The term for the gas exchange between the alveolar air and the pulmonary capillary blood is:

• A. External respiration.
• B. Internal respiration.
• C. Pulmonary ventilation.
• D. Cellular respiration.

Answer: (A)

Gas exchange between the air in the alveoli and the blood in the pulmonary capillaries is external respiration. This stage occurs when oxygen diffuses from the alveolar air, where its partial pressure is higher, into the blood, and carbon dioxide diffuses from the blood into the alveolar air to be exhaled. It happens across the thin respiratory membrane in the lungs, leveraging a large surface area to optimize diffusion. This is distinct from internal respiration, which is the exchange of gases between the systemic blood and body tissues; pulmonary ventilation, the act of moving air in and out of the lungs; and cellular respiration, the cellular use of oxygen to produce energy. External respiration specifically names the gas exchange between alveolar air and pulmonary capillary blood.