The alveolar gas values of 104 mm Hg O2 and 40 mm Hg CO2 correspond to which of the following descriptions?

• A. Alveolar oxygen and carbon dioxide tensions.
• B. Arterial oxygen and carbon dioxide tensions.
• C. Mixed venous oxygen tension.
• D. Oxygen diffusion capacity.

Answer: (A)

This question hinges on recognizing what the numbers represent in the lungs: partial pressures in the alveolar gas. An O2 tension around 100 mmHg and a CO2 tension around 40 mmHg are the typical alveolar values you see in a person breathing room air at sea level. The given values of about 104 mmHg for oxygen and 40 mmHg for carbon dioxide correspond to alveolar gas tensions, not to blood.

Arterial oxygen tension is usually close to, but slightly less than, alveolar oxygen tension due to the A-a gradient, so it’s typically around 95–100 mmHg. Mixed venous oxygen tension is much lower (around 40 mmHg) because tissues extract O2. Oxygen diffusion capacity is a rate measure (how much O2 can cross the alveolar-capillary membrane per unit pressure), not a gas pressure in the alveoli.

So the best description is alveolar oxygen and carbon dioxide tensions.