# (Respiration) how water vapor affects barometric pressure

by skaai
Tags: dalton, gas laws, medicine, physics, physiology, respiration, vapor, water
 P: 345 Is there a question in there that I might have missed? The procedure you have outlined follows from considering the system as an ideal gas mixture which is reasonable at the relatively low pressures found in biological systems. See Dalton's Law: http://en.m.wikipedia.org/wiki/Dalton%27s_law
P: 1,274

## (Respiration) how water vapor affects barometric pressure

 In short 1.why do we subtract vapor pressure from atmospheric pressure? 2.why is PO2 not added and instead multiplied/fractioned? 3.at 100°C, it seems PGASES would all equal ZERO since PW = PB
1. Why do we subtract water vapour pressure ....
The percentage of oxygen of dry air is stable and this works out to be 21%. Unless you know of an easier way to determine the partial pressure of oxygen without subtracting the water vapour pressure from the total atmospheric pressure, then this method does simplify the calculation.

2. Again, partial pressure of oxygen is 0.21 of the total pressure of dry air.

3.Depends doesn't it on whether one deals with an open or closed container.
 P: 16 If I get it, the atmosphere in the lung would be 1atm like it is outside the lung, so the water vapor pressure simply insinuates itself into the equation without increasing pressure, so whatever pressure it takes up, the rest of the gases will have a smaller share. as for why the partial pressure of oxygen is 0.21, I only imagine this is because I was given the ratio... so I can use it to solve for gases. I imagine if i was given a quantity like 159.6 mmHg, I'd have to convert it to .21 first if I was to figure out the new value of oxygen... thank you so much for the insight guys, I really appreciate it!