In the alveolar gas equation, what is PH2O value at body temperature used in the equation?

In the alveolar gas equation, what is PH2O value at body temperature used in the equation?

• A. 47 mm Hg
• B. 50 mm Hg
• C. 37 mm Hg
• D. 24 mm Hg

Answer: (A)

In the alveolar gas equation, you account for the water vapor that humidifies inspired air inside the alveoli. The presence of water vapor reduces the portion of barometric pressure available to oxygen, so you subtract PH2O from Pb. At body temperature (37°C), the partial pressure of water vapor is 47 mmHg, making PH2O equal to 47 mmHg. That’s why 47 mmHg is used in the equation. The other values don’t reflect the water vapor pressure at body temperature (for example, 37 mmHg would be a temperature value, and about 24 mmHg is water vapor pressure at room temperature), so they aren’t appropriate in this context.

In the alveolar gas equation, you account for the water vapor that humidifies inspired air inside the alveoli. The presence of water vapor reduces the portion of barometric pressure available to oxygen, so you subtract PH2O from Pb. At body temperature (37°C), the partial pressure of water vapor is 47 mmHg, making PH2O equal to 47 mmHg. That’s why 47 mmHg is used in the equation. The other values don’t reflect the water vapor pressure at body temperature (for example, 37 mmHg would be a temperature value, and about 24 mmHg is water vapor pressure at room temperature), so they aren’t appropriate in this context.