The process of exchanging oxygen and carbon dioxide between the alveoli and the blood of the capillaries is called:

• A. pulmonary ventilation.
• B. alveolar ventilation.
• C. external respiration.
• D. cellular metabolism.

Answer: (C)

Gas exchange between the air in the alveoli and the blood in the surrounding capillaries is external respiration. Oxygen diffuses from the alveolar air into the blood, while carbon dioxide diffuses from the blood into the alveolar air, across the thin respiratory membrane. The alveoli’s large surface area and delicate barrier make this diffusion efficient, which is why this step is named external respiration.

This is different from pulmonary ventilation, which is simply the mechanical movement of air into and out of the lungs, and from alveolar ventilation, which refers to how much fresh air reaches the alveoli. Cellular metabolism refers to how cells use oxygen to produce energy, not the gas exchange between alveoli and blood.