The process described as gas exchange between the alveoli and blood is external respiration. Which of the following statements best describes this process?

• A. Gas exchange in tissues at the cellular level.
• B. Gas exchange between alveoli and blood.
• C. Air movement into the lungs.
• D. Removal of waste products from the blood.

Answer: (B)

External respiration is the gas exchange that occurs across the respiratory membrane between the air in the alveoli and the blood in the pulmonary capillaries. Oxygen diffuses from the alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveolar air to be exhaled. This exchange happens because of diffusion down partial pressure gradients across the thin alveolar-capillary barrier.

This is distinct from internal respiration, which is the exchange of gases between the blood and body tissues at the cellular level, and from ventilation, which is the movement of air into and out of the lungs. It’s also not about removing waste products from the blood—that’s not a respiratory process.

So the statement that best describes this process is gas exchange between the alveoli and the blood.