Which of the following statements regarding normal gas exchange in the lungs is correct?

• A. The actual exchange of oxygen and carbon dioxide occurs in the capillaries.
• B. Oxygen and carbon dioxide diffuse across the alveolar walls and capillaries.
• C. The oxygen content in the alveoli is highest during the exhalation phase.
• D. Blood that returns to the lungs from the body has low levels of carbon dioxide.

Answer: (B)

Gas exchange in the lungs happens by diffusion across the thin alveolar-capillary membrane, moving oxygen from the air in the alveoli into the blood and carbon dioxide from the blood into the alveolar air. This relies on partial pressure gradients: alveolar oxygen is higher than blood oxygen, so O2 diffuses into the blood; blood carbon dioxide is higher than alveolar CO2, so CO2 diffuses into the alveoli to be exhaled. Describing the exchange as occurring across the alveolar walls and the capillary walls captures both the site and the process precisely.

Saying the exchange happens “in the capillaries” is incomplete because the critical interface is the alveolar-capillary membrane, where the alveolar wall and capillary wall meet. The idea that alveolar oxygen content is highest during exhalation is incorrect; it peaks at the end of inspiration when fresh air has just filled the alveoli. Blood returning to the lungs from the body indeed has high carbon dioxide and low oxygen, not the opposite.