In the alveolar gas equation, what does the term (Pb - PH2O) represent?

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Multiple Choice

In the alveolar gas equation, what does the term (Pb - PH2O) represent?

This term represents the pressure of the dry portion of inspired gas after it is humidified in the airways. Water vapor in humidified air takes up part of the total pressure, so the effective pressure available for the other gases is the barometric pressure minus the water vapor pressure. At body temperature, PH2O is about 47 mmHg, so the dry-gas pressure is roughly Pb minus 47. This is why the equation uses FiO2 times (Pb − PH2O): it reflects the actual pressure of the inspired oxygenable gas that reaches the alveoli. The other options don’t fit because FiO2 is the fraction of O2, not the pressure term; Pb is the total barometric pressure, not the dry-gas pressure after humidification; and alveolar ventilation rate affects CO2, not this pressure component.

This term represents the pressure of the dry portion of inspired gas after it is humidified in the airways. Water vapor in humidified air takes up part of the total pressure, so the effective pressure available for the other gases is the barometric pressure minus the water vapor pressure. At body temperature, PH2O is about 47 mmHg, so the dry-gas pressure is roughly Pb minus 47. This is why the equation uses FiO2 times (Pb − PH2O): it reflects the actual pressure of the inspired oxygenable gas that reaches the alveoli. The other options don’t fit because FiO2 is the fraction of O2, not the pressure term; Pb is the total barometric pressure, not the dry-gas pressure after humidification; and alveolar ventilation rate affects CO2, not this pressure component.

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