Why no N2 bubbles?

[dsteding]

A drop in ambient does not directly cause bubble formation, but through the resulting change in the partial pressure. Hence the pressure gradient.

Yes it does, but not because of a gradient. Gas solubility is proportional to pressure. At high pressure, you can get more gas into solution, be it blood or tissue. Witness the coke can with bulging outsides when closed and bubbles coming out of solution when open. What is driving the bubble formation is a drop in pressure, not a gradient. What will drive diffusion between the coke and the air above it is a pressure gradient.

But I think I see where you are going. Gradients do come into play between a tissue and the blood. If the gradient is great enough, you will have diffusion that is fast enough that the blood will get over-saturated and bubbles will form.

 

[ReefHugger]

Bingo!!

 

[Charlie99]

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?

The answer is pretty simple.

There are two different pressure differentials involved.

Bubble formation is driven by dissolved N2 partial pressure vs TOTAL AMBIENT PRESSURE. (actually it is driven by the sum of inert gas partial pressures vs. ambient pressure, but in your situation N2 is the only inert gas)

Offgassing of N2 is driven by the dissolved N2 partial pressure vs. the N2 partial pressure in the inspired gas.

Two different things.

 

[gcbryan]

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?

The rate of one gas (nitrogen) offgassing is independent of other gasses. If you vary the percentage of nitrogen/helium in a mix it doesn't affect the rate of offgassing of the other gas (correct me if I'm wrong).

Therefore using oxygen does nothing to the rate of offgassing nitrogen except that no additional nitrogen is now being inhaled and so no additional nitrogen is adding to the problem.

Exceeding saturation levels (supersaturation) is where/when bubbles are formed. Once you reach the surface offgassing is just determined by half-lives. Breathing pure O2 speeds things up only in that you aren't continuing to add to the problem by taking in more N2. Otherwise it's strictly a function of half-lives.

This is my understanding in plain english (all I understand by the way )

If I'm wrong or am misunderstanding the question please correct me.

 

[ReefHugger]

The answer is pretty simple.

There are two different pressure differentials involved.

Bubble formation is driven by dissolved N2 partial pressure vs TOTAL AMBIENT PRESSURE. (actually it is driven by the sum of inert gas partial pressures vs. ambient pressure, but in your situation N2 is the only inert gas)

Offgassing of N2 is driven by the dissolved N2 partial pressure vs. the N2 partial pressure in the inspired gas.

Two different things.

This is exactly what I am saying, which brings me back to the question.
Since the dissolved PN2 is pretty high (say, bent diver) vs. the PN2 in the inspired air (in the case of pure O2 = 0), then the pressure gradient is at max. The zero partial pressure of the inspired air is literally "sucking out" the dissolved nitrogen. With this severe pressure differential, won't the nitrogen come out in bubbles??

BTW, I am not asking about the lung interface, but referring entirely to tissue/blood interface.

 

[dsteding]

The zero partial pressure of the inspired air is literally "sucking out" the dissolved nitrogen. With this severe pressure differential, won't the nitrogen come out in bubbles??

BTW, I am not asking about the lung interface, but referring entirely to tissue/blood interface.

I don't think so, because you'd have to drive the blood PN2 down to close to zero, and since the tissues are supplying N2 to the blood, it won't get driven down to zero, despite the blood's exposure to pure O2 in the lungs.

 

[H2Andy]

Since the dissolved PN2 is pretty high (say, bent diver) vs. the PN2 in the inspired air (in the case of pure O2 = 0), then the pressure gradient is at max. The zero partial pressure of the inspired air is literally "sucking out" the dissolved nitrogen. With this severe pressure differential, won't the nitrogen come out in bubbles??

the nitrogen you are talking about is already out of solution. it will just rush out through gas exchange without pulling more nitrogen out of solution

for like the fourth time:

O2 does not have any effect on the nitrogen still in solution, thus it does not "pull" any nitrogen out of solution and it does not cause any bubbles.

 

[gangrel441]

Laymen's terms:

It is not the percentage of nitrogen you are breathing that keeps the N2 in solution. It is the pressure (regardless of what gas/liquid is creating said pressure) that keeps the N2 in solution. The only difference between O2 and N2 for the purposes of this conversation is that O2 gets metabolized while N2 is absorbed until the body reaches saturation. Bubbles are formed when pressure around the body is relieved (by ascending). This has nothing whatsoever to do with the gas mixture the diver is breathing. Two distinct and very different issues.

 

[Dr Deco]

Hello ReefHugger:

We have gotten quite a few responses to this query and they have homed in on the correct answer.

Ambient Pressure

Gases are kept in solution by the hydrostatic [or surrounding] pressure. This true, physical pressure prevents the formation of bubbles containing gaseous nitrogen. This pressure can be caused by atmospheric pressure, seawater pressure, pressure applied with a piston in a cylinder, or pressure from compressed gas that surrounds the body.

Dissolved nitrogen will always have a tendency to come out of solution and form microbubbles - even on the atomic level. The applied pressure keeps these microbubbles small, and it will essentially force then back into solution (in conjunction with the Laplace pressure caused by surface tension on the bubble surface).

The tendency for nitrogen to dissolve is dependent on the applied gas pressure. If the nitrogen makes up only a part of the applied gases, then this is the nitrogen partial pressure. The other gases will dissolve depending on their partial pressures.

Nitrogen Solubility and Bubble Formation

Nitrogen solubility depends on the partial pressure of the gas, which can be anywhere between 0 psi and the total pressure. The two pressure are independent [although maximum partial pressure can never exceed total hydrostatic pressure].

If we are talking about the movement of dissolved nitrogen, say from a tissue to the blood, then we are discussing diffusion. This is a topic conceptually independent of applied pressure. Diffusion is the result of a difference in number of [e.g., nitrogen] molecules in one location versus the number in another adjacent location. If more dissolved nitrogen molecules are present in a tissue than in blood, then a diffusion gradient will exist, and molecules will move from tissue to blood. This is a downhill concentration slope; the dissolved nitrogen will move [by random thermal motion] from a higher concentration to a lower one.

Bubbles

Bubbles form/grow because of a reduction in hydrostatic pressure , a pressure that is now less than the partial pressures of all dissolved gases (e.g. nitrogen, helium, water vapor, etc). Hydrostatic pressure has nothing to do with a concentration gradient.:shakehead

Dissolved nitrogen will move into the blood from tissue with a pressure reduction because there is less nitrogen in the blood (and breathing gas) than in the tissue. Bubbles might also form if the pressure reduction is too great.

The nitrogen gradient will be reduced faster if oxygen from the breathing gas) is substituted in the blood for nitrogen. Bubbles will not form if the hydrostatic pressure is great enough even if the nitrogen concentration gradient exists. Partial pressures of dissolved gases do not produce mechanical pressure.

Dr Deco :doctor:

 

[H2Andy]

hey, i was right!

that's like the first time since 1998...

no wait... 1997