cornfed
Mindless lemming
It's the amount of oxygen you're actually concerned with.myself:
If you look at the ideal gas law (PV = nRT) you'll see that pressure and the amount of gas (n) are directly related. If you hold volume and temperature constant, more gas means more pressure and less gas means less pressure.
At sea level (1 ATA of pressure) you have some amount of gas in your lungs, of which 21% is oxygen. For the sake of argument, let's say it's 1 mole of gas. That means the amount of oxygen in your lungs is .21 moles. As the total pressure varies the concentration of oxygen stays the same, but the pressure of oxygen doesn't.
As you climb a mountain, the pressure drops not because someone has a giant regulator but because the atmosphere is getting less dense. Lower density mean fewer gas molecules and hence less pressure. This is the same reason partial pressures increases as you decend while diving. The relative concentrations are the same but the gas is denser so the partial pressure of each component goes up accordingly.
If you climbed high enough to find a pressure of 0.5 ATA you'd find that the density of the atmosphere was incredibly low. So low in fact that there wouldn't be enough oxygen molecules present to really support life. The amount of oxygen present is no longer 0.21 moles but (1/2)*0.21 moles, or 0.105 moles. My claim is that this difference (the decrease in total amount of oxygen) is what's important and not the pressure it's at.
Now let's take a look at this from a partial pressure point of view. If you claim that a P02 of 0.21 ATA is needed you have two choices, increase the pressure or increase the amount of oxygen present. The catch is that you can't change the pressure (unless you climb back down) without changing the volume or the temperature. Since your lung size is fixed you can't change the volume and you can't really change the temperature enough to effect anything without frying yourself. So you're stuck with the pressure you've got. Since you're stuck with the pressure you only have one choice... change the amount of oxygen. The part that's confusing is that by changing the amount of oxygen present you also change it's partial pressure. It seems like the oxygen partial pressure is what's important, but really the addition of oxygen is what drove the pressure up and not the other way around.
Regardless whether you're 33 ft below the ocean, standing on the beach or top of a ridiculously high mountain, two things are true about the air in your lungs. The volume of air in your lungs is constant and it's at ambient pressure. Yes, I know that the volumes change with pressure, but physical space available to your lungs doesn't change and that's the important volume to consider so think of your lungs ridgid container like a scuba tank. Since you can't change the volume of your lungs and you don't have any say as to what the ambient pressure is, the only way to change the amount of oxygen in your lungs is to swap it with something else. On top of our insanely high mountain, we only have half a mole of air, or 0.105 moles are O2. If we want to increase the amount of oxygen to 0.21 moles, we have to remove 0.105 moles of nitrogen first. When we do this we now have 0.21 moles of O2 divided by 0.5 moles of air gives an oxygen fraction of 0.42. Now, if the total pressure is 0.5, this F02 of 0.42 results in a P02 of 0.21 ATA.
It's very tempting to say that the pressure (0.5 ATA) is what's driving this. In fact I'm getting the same headache free partial pressure of oxygen I was a sea level. The problem is that pressure is a function of several variables and since some of those variables are fixed (volume and temperature) we're left with a one to one relationship between pressure and amount of oxygen. And since the amount of oxygen is the only variable we can actually control, it's the dominating factor, not pressure.
