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How can alveolar gas contain water vapor and less oxygen if the breathing loop is isobaric?

I was wondering why the partial pressure of oxygen is lower in the alveoli than in the air we inspire. From this related question I found the answer that the water inside the lung saturates the air and lowers the partial pressure of oxygen. Here is the associated equation : $$p_{A}^{O2}=F_{I}^{O2}(P_{ATM}-p_{H2O})-\frac{p_{a}^{CO2}(1-F_{I}^{O2}(1-RER))}{RER}$$ On the other hand, […]