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Atmosphere has partial pressure of oxygen as 160mmHg but it's 105mmHg in alveoli, and the partial pressure of carbon dioxide in atmosphere is 0.3mmHg but it's 40mmHg in alveoli. How is the difference maintained, and can I know the mechanism behind it?

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There are two main reasons for the pO2 in the alveolar air being lower than the atmospheric pO2. First, the diffusion of CO2 from the blood lowers the alveolar pO2. But the most important is the second one: there is a lot of water inside the lungs, and all this water lowers the alveolar pO2 as well.

This is the equation:

PaO2 = FiO2(Patm - PH2O) - PaCO2[FiO2 + (1-FiO2) / R] 

Source: https://en.wikipedia.org/wiki/Alveolar_gas_equation

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