What gas exchange occurs in the alveoli during respiration?

• A. Carbon dioxide diffuses into blood from alveolar air; oxygen diffuses into alveolar air.
• B. Oxygen diffuses into capillaries from alveolar air; carbon dioxide diffuses into alveolar air from blood.
• C. Oxygen diffuses from blood into alveolar air; carbon dioxide diffuses from alveolar air into blood.
• D. Nitrogen diffuses into blood.

Answer: (B)

Gas exchange in the alveoli is driven by differences in partial pressures. Alveolar air has a higher partial pressure of oxygen and a lower partial pressure of carbon dioxide than the blood in the surrounding pulmonary capillaries. Therefore, oxygen moves from the alveolar air into the blood, and carbon dioxide moves from the blood into the alveolar air to be exhaled. The correct description is that oxygen diffuses into the capillaries from alveolar air, while carbon dioxide diffuses into the alveolar air from the blood. The other options either reverse these directions or involve gases like nitrogen, which do not play a major role in this exchange under normal conditions.