The partial pressure of oxygen in the alveoli is ______ mm Hg, while the partial pressure of carbon dioxide in the alveoli is ______ mm Hg.

• A. 90, 50
• B. 104, 40
• C. 70, 28
• D. 88, 30

Answer: (B)

Gas tensions in the alveoli show oxygen at a high level and carbon dioxide at a moderate level under normal room-air conditions. The alveolar oxygen pressure is set by the amount of O2 entering the lungs minus how much CO2 is removed, summarized by the alveolar gas equation. With room air (FiO2 ~ 0.21), atmospheric pressure about 760 mmHg, water vapor pressure around 47 mmHg, the inspired O2 pressure is about 150 mmHg. If alveolar CO2 is about 40 mmHg and the respiratory quotient is ~0.8, then PAO2 ≈ 150 − (40/0.8) ≈ 100 mmHg, and in textbooks this is often given as about 104 mmHg. Alveolar CO2 is kept near 40 mmHg because ventilation removes CO2 produced by metabolism at a matching rate.

So the best match for normal alveolar gas tensions is roughly 104 mmHg for oxygen and 40 mmHg for carbon dioxide. The other values miss this typical balance of high O2 and moderate CO2.