Why no N2 bubbles?

[H2Andy]

Fick's Law:

Rate of diffusion is proportional to the gradient of partial pressures between mediums. When breathing pure O2, the partial pressure gradient is maximized between blood and the lungs, and diffusion out of the blood into the lungs of N2 is maximized.

you're talking about diffusion, he's talking about gases coming out of solution because of different pressures in the tissue and blood

in other words, you are talking about N2 that is already out there. yes, as i said, that N2 will have an easier time heading out of the body if 100% O2 is breathed.

he is talking about the dangers of MORE N2 coming out of solution in the presence of 100% O2 and thus more damage being done to the system.

and he is using Dalton's law to understand the process, which doesn't really apply (other than to understand the difference in partial pressure in gases across a membrane).

 

[dsteding]

To fisherdvm - I think there is potential for 'tons more bubbles' after the dive. Hence the requirement for surface interval.

Bubble formation (in the blood) is going to be related to the rate that pressure decreases. If you decrease the pressure quickly, there is going to be bubbles coming out of tissue and blood, both dissolve gas and will move towards equillibrium with ambient. The body's ability to deal with this is limited by two rates, the rate at which N2 is scrubbed by the tissue, and the rate of bubble growth, at some point bubbles will get big enough to start to mess around with stuff and cause problems.

So, there could be more potential for bubbles after the dive, but that is totally dependant on your dive profile . . . the goal is to keep those bubbles small and let the rate which limits your ascent be the lungs and not the bends, so to speak.

At least, this is how I think about it, but I am a chemist and a lawyer, and not a phyisician. I'm sure someone else can explain it better.

 

[1_T_Submariner]

Steve/Andy, Great posts. Learn some more all the time, thanks.

 

[dsteding]

you're talking about diffusion, he's talking about gases coming out of solution because of different pressures in the tissue and blood

in other words, you are talking about N2 that is already out there. yes, as i said, that N2 will have an easier time heading out of the body if 100% O2 is breathed.

he is talking about the dangers of MORE N2 coming out of solution in the presence of 100% O2 and thus more damage being done to the system.

and he is using Dalton's law to understand the process, which doesn't really apply (other than to understand the difference in partial pressure in gases across a membrane).

Good point Andy, I'm seeing it now. Yes, that doesn't make much sense. Dalton's Law certainly doesn't apply. In one sense, Fick's Law could apply.

Here is how (I'm getting pinheaded here, I don't think this is actually the case):

Blood is a really fast tissue. It will equillibrate with the lungs quickly. If it does, the PN2 in blood will decrease, this will increase diffusion from other tissues to the blood. But, in reality, I don't think--at least from a rate limiting standpoint--this is the case, because my intuition is that blood isn't THAT "fast." So, the drop in ambient pressure will be the driving force in bubble formation . . . not the breathing gas.

[EDIT-I'll step aside now that Gene Hobbs is in the room-as I said earlier, I'm a chemist and a lawyer, not a doctor (well, at least that kind)]

 

[Gene_Hobbs]

I have never had a proper explanation for this.

The purpose of using tables and observing ascent rate and stops is to control the outgassing of nitrogen. That is, we are controlling the partial pressures differential of nitrogen. When using pure oxygen (transporting victim to med facilities; final 10' deco stop, say), this pressure differential is suddenly up to maximum. Why then are nitrogen bubbles not formed? Or are they?

Done this yet? http://www.diversalertnetwork.org/training/seminars/bubbles/index.asp

 

[H2Andy]

So, the drop in ambient pressuse will be the driving force in bubble formation . . . not the breathing gas.

that's what i am thinking too

 

[ReefHugger]

I think dsteding has an understanding of what I am saying. The rate of outgassing is directly related to the pressure gradient. The outgassing of N2 is much slower than ascent rates. Consequently, after a dive outgassing is still continuing. That's why we have surface intervals, no fly times, no heavy exertion after dive, etc etc. The continuing outgassing is due to the partial pressure gradient which is a result of the slower outgassing rate, relative to ascent rate.

H2andy :"why do EMS.... put bent divers on 100% O2". I accept this as the correct line of action, just as I accept the sun rises in the east.

I still would like to know how it works and why N2 does not bubble out.

 

[H2Andy]

I still would like to know how it works and why N2 does not bubble out.

because the O2 does not affect the pressure of nitrogen in solution, thus having no effect whatsoever on dissolved nitrogen or how fast it comes out of solution

 

[dsteding]

I still would like to know how it works and why N2 does not bubble out.

I guess the question is where does the N2 bubble out from. Think of the body as a bunch of boxes, the individual boxes load and unload nitrogen at different rates.

Blood goes fast, all the other boxes have to go through the blood, so they are (in half life terms) slower. So, the way I think about it, you can first have N2 coming out of solution from everywhere (blood and tissues). The blood picks up N2 coming out of tissues, and if that N2 comes out fast enough from the tissues and blood, you get bubbles.

Later on, however, say on the surface, the N2 in your blood is probably driven by diffusion from tissues [edit- it most certainly is if you are on pure O2] Breathing 02 isn't going to change that too much . . . (it won't get the PN2 down in blood enough to increase diffusion from tissues to blood). What the O2 WILL do is increase the rate of diffusion between blood and your lungs, increasing the rate of elimination of N2 from your blood, and potentially lessening the impacts of bubble formation in the blood.

But, I haven't thought this out too much, this is all kinda-sorta off the top of my head and based on conversation with people who know more than me. So take it for what it is worth.

[EDIT-what Andy said, the trick here is that there are multiple "solutions," blood and various tissues]

 

[ReefHugger]

Ambient pressures and partial pressures are directly related. Ambient pressures are used in the calculation of partial pressures. A drop in ambient does not directly cause bubble formation, but through the resulting change in the partial pressure. Hence the pressure gradient.

If ambient was the factor, then there would never be any bubble formations. That is, the diver's total body (most of the time) is always at the same ambient, therefore no pressure gradient.