In the alveolar gas equation calculation, what is the PH2O value used?

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Multiple Choice

In the alveolar gas equation calculation, what is the PH2O value used?

The amount subtracted in the alveolar gas equation is the water vapor pressure of humidified alveolar gas at body temperature, which is 47 mmHg. At 37°C, inspired air in the lungs becomes fully saturated with water vapor, so the dry portion of the gas available for oxygen calculation is reduced by this 47 mmHg. That’s why 47 mmHg is used in these calculations. The other numbers would reflect different temperatures or humidity levels and aren’t the standard value for alveolar gas at physiological conditions; using 47 mmHg keeps the equation accurate for normal body conditions and explains why PAO2 is computed with PB minus PH2O.

The amount subtracted in the alveolar gas equation is the water vapor pressure of humidified alveolar gas at body temperature, which is 47 mmHg. At 37°C, inspired air in the lungs becomes fully saturated with water vapor, so the dry portion of the gas available for oxygen calculation is reduced by this 47 mmHg. That’s why 47 mmHg is used in these calculations. The other numbers would reflect different temperatures or humidity levels and aren’t the standard value for alveolar gas at physiological conditions; using 47 mmHg keeps the equation accurate for normal body conditions and explains why PAO2 is computed with PB minus PH2O.

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