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It is the inspired gas partial pressure vs tissue tension that matters for absorption and off-gassing (diffusion) RATES. ambient PP = inspired PP = fraction inert * ambient total pressure.
Hi inquis, thanks for your reply---this helps me to sharpen my confusion. Why is it that total ambient pressure is the relevant factor in bubble formation, rather than the partial pressure of the inspired gas? Given that the rates of gas diffusion from a given tissue are independent of each other (for different types of gasses), and that those rates depend only on the gas-specific partial pressure gradients, why should total ambient pressure make a difference to bubble formation? The total ambient pressure doesn't seem to have any impact on the rate at which a has diffuses. (For example, breathing pure O2 at 3m vs. 6m. wouldn't change the rate at which I off-gas Nitrogen, assuming that I start with the same Nitrogen tissue tension in each case.) I have no doubt that you're right, but hopefully the preceding has helped to clarify exactly why I'm confused.
Total ambient pressure does impact the rate of diffusion in general, but mathematically, it is in conjunction with the fraction of molecules as a product. ([ETA: when breathing O2] you're multiplying by 0, which trumps whatever else is in the product.) The physical picture, though, during diffusion, molecules generally move both directions, but we really only care about the average. In the case of breathing O2, there's effectively no nitrogen molecules to "move back" (i.e., from alveoli into the blood), no matter if you're at 20 ft or 10 ft. That's why ambient pressure doesn't matter in the specific case when on O2. However, if you're breathing EAN50, there are 23% more nitrogen molecules on the alveoli side if you're at 20 ft compared to 10 ft. There's a greater tendency for some N2 molecules to "move back" at 20 ft, reducing the average transfer rate in a depth-dependent (or ambient pressure-dependent) manner.
In a supersaturated solution micro bubbles can spontaneously form. Once a micro bubble forms, surface tension exerts pressure on it to collapse. Opposing this is the internal pressure of the bubble. The internal pressure in the bubble is the total of the partial pressures(PPs) of the gasses in the bubble. If the PP of a particular gas is lower in the bubble than the surrounding tissue, that gas diffuses into the bubble raising it's internal pressure to counteract the surface tension and it will start to expand. This happens for each gas, and the total pressure acts on the bubble.
Hi @L13, thanks for your reply---this is helpful. But notice that your explanation of bubble growth and collapse appeals just to the partial pressure of dissolved inert gasses in the tissue (as it should). I'm asking about the role of the total pressure of inspired gas (i.e., the role of the ambient pressure in the water column). That is, to use the example of a Helium bubble: why is it that the spontaneous formation (not growth) of a Helium bubble depends on the difference between the partial pressure of dissolved Helium in a particular tissue and the total pressure of inspired gas---even if that inspired gas contains no Helium, say, because of switching to Nitrox during decompression---rather than the difference between the partial pressure of dissolved Helium in that tissue and the partial pressure of inspired Helium?
Ah, good point! This is a very helpful way of seeing how the phenomenon of bubble formation and that of gas diffusion are really quite different. I'm surprised that books on the physiology of decompression (such as *Deco for Divers*) spend so much time on the science of gas diffusion, and so little time on the science of bubble formation (as opposed to the science of bubble change). I think what I'm really after is a clear account of the science of bubble formation---one that makes clear the critical role played by ambient pressure. If you know of a good reference, please pass it along!
Yes, of course you are right about this. I was too quick in what I wrote. What I meant was that the central factor determining the diffusion rate of a gas in a tissue is the partial pressure gradient between the gas dissolved in the tissue and its inspired partial pressure. But as you point out with your example of EAN50, ambient pressure is often clearly relevant to determining the inspired partial pressure.
