O2 off-gassing

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Messages
19
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2
Location
New York City
# of dives
25 - 49
Why does O2 expidite nitrogen off-gassing? I understand how off-gassing works from a pressure gradient perspective, but I don't understand why O2 is more effective than air at equalizing nitrogen levels.
 
It's more effective than air because there's no N2 in it, thus maximizing the off-gassing gradient for the N2 and not having any inert gases to diffuse into the tissues from the other direction.
 
You want a gas that has the highest pressure gradient against the gas you're trying to offset, while trying to maintain a depth that won't cause excessive or large bubbles to form. 20' and 100% O2 seems to work well for this. O2 is also metabolized, while other gasses are not.
 
There are two important parts to the answer. First of course if your breathing mix is pure oxygen then there is no additional nitrogen to be adsorbed. The result will be that the pressure gradient will be increased and there will be a corresponding increase in diffusion rate.

The second thing to consider is that total gas pressure will drop on the venous side of circulation since oxygen is used up by metabolism and is replaced by carbon dioxide with is is more soluble and as a result total vapor pressure drops. That changes the size of all bubbles present which respond to total pressure across the membrane. The end result is gas transport across the bubble membrane is facilitated.

Try googlng this for additional discussion: Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, Johnny E. Brian, Jr.
 
As others have said: If you are breathing are higher oxygen mixture, then you have correspondingly less nitrogen in your lungs. This creates a bigger imbalance across the lung barrier, where the nitrogen in your body is greater than the nitrogen in your lung spaces. Those nitrogen amounts will always be compelled to 'balance'. Thus, nitrogen in your body will be drawn through the lung barrier to equalize with the nitrogen levels in your lungs. As you are constantly evacuating the contents of your lungs through respiration and replacing that with more oxygen, the process of equalization won't be complete until most of the nitrogen saturated in your body has gone.
 
The nitrogen absorbtion in air vs. O2 makes a lot of sense, and I hadn't thought of it. I can see why it helps to not contribute more N2 to your system.

Try googlng this for additional discussion: Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, Johnny E. Brian, Jr.

Thanks, it is going to take me a bit to comprehend this, but I think it is exactly what I was looking for.

This creates a bigger imbalance across the lung barrier, where the nitrogen in your body is greater than the nitrogen in your lung spaces. Those nitrogen amounts will always be compelled to 'balance'. Thus, nitrogen in your body will be drawn through the lung barrier to equalize with the nitrogen levels in your lungs.

This is exactly where I was getting confused. Why is the nitrogen compelled to balance? Does the N2 and O2 bond in some way that facilitates this? How does the N2 "know" that it should equalize across the barrier?
 
It's really a pretty simple matter of diffusion. The greater the difference in concentration between the Nitrogen in one compartment (blood) and another (airspaces in the lungs), the faster the rate of diffusion. Since air has nitrogen in it, the gradient isn't as great between the blood and the lungs, so nitrogen diffusion is slower.
 
How does the N2 "know" that it should equalize across the barrier?

It doesn't "know" where it's supposed to be, it just a matter of statistics--with an imbalance of the inert gas on the two sides of the barrier, there are more molecules traveling across it in one direction than are coming back in the other. Eventually things even out to a steady state, with molecules continuing to cross in both directions, but no longer causing any change in concentration from one side to the other.
 
Nature hates a vaccuum and when you breathe pure O2 you have a vaccuum of nitrogen in the gas you breathe.
 
This is exactly where I was getting confused. Why is the nitrogen compelled to balance? Does the N2 and O2 bond in some way that facilitates this? How does the N2 "know" that it should equalize across the barrier?

It's a physical law. Generally, all gasses seek to equalize themselves. Think about opening an air-lock in a space craft. The air rushes out into space. This happens because the air seeks to 'equalise' across the vacum of space. Individual gasses work in this way also, to achieve a uniform balance across whatever medium they are contained in.
 

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