What is alveolar gas exchange?

• A. The diffusion of nitrogen into blood
• B. The movement of oxygen from alveoli into the blood and carbon dioxide from blood into the alveoli for exhalation
• C. The absorption of oxygen by the chest wall
• D. The exchange of gases in bronchial tubes

Answer: (B)

Alveolar gas exchange is the process by which oxygen moves from the air-filled alveoli into the bloodstream and carbon dioxide moves from the blood into the alveoli to be expelled. This happens across the thin alveolar-capillary membrane by diffusion, driven by differences in partial pressures: oxygen diffuses from the higher partial pressure in the alveoli to the lower pressure in the blood, while carbon dioxide diffuses from the higher pressure in the blood to the lower pressure in the alveoli.

Healthy gas exchange relies on ample surface area, a very thin membrane, and good ventilation-perfusion matching to maximize diffusion.

The other descriptions don’t fit: nitrogen diffusion into blood is negligible under normal conditions because nitrogen is poorly soluble and isn’t used for metabolic exchange; the chest wall doesn’t absorb oxygen—gas uptake happens in the lungs; and the exchange of gases in bronchial tubes is incorrect because gas exchange occurs in the alveoli, not in the airways.