Question About Partial Pressure

[Mission Medical]

I was reading from a site which said the following -

"Now - how do we figure out the PiO2? Since we're getting the air from the atmosphere, we need to use the atmospheric number - so we start with the atmospheric pressure (which is 760 mm Hg at sea level). However, as we inhale the air, water is added to the air prior to it reaching the alveoli - so we need to account for the water vapor that has been added by the airways. We account for this by subtracting the water vapor pressure (the partial pressure of the water that has been added) from the total atmospheric pressure. Luckily, you only need to know one number for this: at body temperature, the water vapor pressure is 47 mm Hg, so our calculation (which isn't complete yet) becomes: P(atm) - PH20 --> (760 mm Hg - 47 mmHg) --> 713 mm Hg"

My question is, based on Dalton's law, the only way the above can be true is if we're assuming that the pressure of the air we breathe in is 760mmHg, right? Because Dalton's law says partial pressures of different gases are additive, so if the atmospheric pressure were indeed able to increase as you go up, then we should actually be getting 760mmHg + 47mmHg (from water vapor) = 807mmHg, but instead we're subtracting because the pressure of the air we breathe in will still be roughly 760mmHg (Even though we might be above sea level), but now 47mmHg is coming from water vapor, so the rest (713mmHg) must be from remaining gases. Is my understanding of this correct?

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[sps27]

Not sure if I quite understand what you are trying to say but to point a few things here - as you go up i.e., above sea level say on top of a mountain or a hill resort, atmospheric pressure decreases, not increases, because the column of air putting pressure on you is smaller, not bigger, so atmospheric pressure decreases, however little it may be, but it does. That is why people living in higher altitudes breathe a little heavier to get all the oxygen they want. As goes for partial pressure calculation, all they are trying to do is calculate the partial pressure of O2, so they are subtracting the partial pressure of Water vpr, Nitrogen, C02 etc from the total atmospheric pressure. I hope this helps.