Increased nitrogen off-gassing 10ft/3m VS 20ft/6m on 100% oxygen

[lowwall]

We absolutely agree on the part I bolded. What is the partial pressure of N2 in each of the capillaries?

Thanks for pointing that out and giving me something to think about. It should be "driven by the difference in the absolute pressures of the tissue and the capillaries".

Let's say you have a small tank of 100% N2 at 10atm. It is sitting in a giant hyperbaric chamber filled with 100% 02 also at 10atm. Now fully open the valve on the N2 tank. What happens in the first second?

Very little. Molecules meander here and there (Brownian motion) so eventually the whole system comes into equilibrium. But it isn't fast.

Now rerun the experiment except with the chamber at 1atm. What happens in the first second? The N2 comes screaming out of the tank. The diffusion of N2 into the chamber driven by the difference in absolute pressure completely dwarfing the diffusion due to Brownian motion.

Deco algorithms are a series of approximations, this is just another to add to the list. The deco algorithm that Mr. Baker is discussing simply ignores this scenario. It's probably reasonable to do this because most of the time the differences between absolute pressures in the various tissues are low and the inert gas percentages in the tissues don't vary greatly. Thus most of the diffusion is driven by Brownian motion and thus and thus the rate is going to largely depend on concentration which is stated as partial pressures. The big exception is exactly what we are discussing here, when the diver switches to a very high percentage of O2. But even then the fairly narrow limits on the ppO2 that a diver can handle mean this effect will not be large.

 

[goldfishtornado]

driven by the difference in absolute pressure

I think this is not exactly accurate. Like I said earlier, the diffusion is driven by the fugacity, which is the effective partial pressure. For an ideal gas fugacity is one, so the difference in absolute pressure is a difference in partial pressure.

Because the O2 in your first scenario is at such high pressure the effective partial pressure of N2 a N2 molecule "sees" from the O2 is non-zero and therefore the process is slow. In the second case the effective partial pressure of N2 is small. This is because N2 has a non-zero chemical potential in gaseous O2.

I think the point of the oxygen window model is that the chemical potential a N2 molecule "sees" from a hemoglobin bound or not-bound to oxygen is basically the same. Further, most of the inspired O2 is bound to hemoglobin at the partial pressures we're talking about. If both of those approximations are accurate, then the effective partial pressure gradient for N2 between a hypothetical compartment and the venous system is the effectively the same.

 

[inquis]

I really don't think gas transport in the body is like opening a tank at zero PSIg and wondering how long molecules escape through a pinhole via Brownian motion. Indeed, I recently learned the alveoli have a surface area of over 1000 square feet, so those scenarios seem a bit different to me.

 

[lowwall]

I really don't think gas transport in the body is like opening a tank at zero PSIg and wondering how long molecules escape through a pinhole via Brownian motion. Indeed, I recently learned the alveoli have a surface area of over 1000 square feet, so those scenarios seem a bit different to me.

The same rules of physics apply whether the pressure delta is 10atm or .5atm. it's just a lot easier to visualize the former.

Also, there's a whole lot less surface area between the blood and the tissues we are concerned with compared to blood and the freshly inspired gas in the lungs. Tissue compartment halftimes are basically a measure of how easily gas can flow between the tissue and the blood. Long halftimes mean it's possible for large absolute pressure deltas to develop.

 

[Alaskan Scuba Dude]

You forgot to mention 4.5 meters. 100% is useless and expensive if diving once a day.

 

[inquis]

it's just a lot easier to visualize the former.

I think it might be misleading you though. It's easy to arrange a case:
Tissues are saturated at 1.0 atm total pressure and 0.79 atm ppN2. Dive down to 20 ft and start breathing on an O2 bottle. Total ambient pressure is 1.6 atm, and your example opening a scuba tank no longer applies (IMO).

I *think* you would say nitrogen off gassing is extremely slow as Brownian motion has to work against the total pressure difference. (Or maybe you'd say that nitrogen doesn't leave at all?)

The deco researchers say no problem, nitrogen easily off-gasses since tissues at 0.79 atm ppN2 are higher than the inspired inert pressure at 0 arm. After 5 half-times have elapsed, nearly all the N2 is gone from that tissue.

In hindsight, I guess the example is useless, because I don't have evidence (or can't point to a study where they find) that N2 is actually exhaled.

 

[inquis]

100% is useless and expensive if diving once a day.

I don't find getting out of the water sooner useless. What did you pay the last time you dove twice in a day and used O2?

 

[lowwall]

I think it might be misleading you though. It's easy to arrange a case:
Tissues are saturated at 1.0 atm total pressure and 0.79 atm ppN2. Dive down to 20 ft and start breathing on an O2 bottle. Total ambient pressure is 1.6 atm, and your example opening a scuba tank no longer applies (IMO).

I *think* you would say nitrogen off gassing is extremely slow as Brownian motion has to work against the total pressure difference. (Or maybe you'd say that nitrogen doesn't leave at all?)

The deco researchers say no problem, nitrogen easily off-gasses since tissues at 0.79 atm ppN2 are higher than the inspired inert pressure at 0 arm. After 5 half-times have elapsed, nearly all the N2 is gone from that tissue.

In hindsight, I guess the example is useless, because I don't have evidence (or can't point to a study where they find) that N2 is actually exhaled.

I'd say that Brownian motion would be moving the N2 into the blood.

But (this is the third time I am rewriting this), I simply don't know if it would be moving at the same or a lower rate than if you were breathing 100% O2 at the surface.

OTOH, I am quite sure that O2 would be moving from the blood into the (other) tissues faster than they would be at the surface. I think we agree on this whether we say the gases in the blood are at higher absolute pressure or have a higher ppO2 relative to other tissues.

It's my understanding (contention?) that any absolute pressure differentials between tissues is simply an additional factor in offgassing speed. Just like increasing the temperature of the tissues increases the offgassing speed. Or increasing the rate of blood flow increases offgassing speed.

 

[Jacob BV]

So I am camp it should make zero-negligible difference due to what @inquisit and others have said. Some of @Iowwall's thought experiments are interested though, and potentially relevant. What I beleive it boils down to is the half life of 2 gasses reaching absolute pressure equilibrium when there is a difference (this will be a function of how the 2 gasses are allowed to mix). I suspect this is much quicker than the inert gas half lifes - and hence not a driving or significant factor, especially as we ascend continously rather than instantaneously. Furhermore, our bodies are not like the example of opening a cylinder in a room, biology and perfusion will change this. I believe there are some models out there that model the interchange from perfusing to diffusion but I don't know much about them.

 

[inquis]

@lowwall, whats your take on the Ideal Gas Law? The essence is other gasses don't make a whit of difference to any given component. They don't interact or influence each other. (It would certainly apply at these pressures.)

So why would other gasses impact anything nitrogen is doing in the tissues? Isn't your contention that the combined pressure controls things violating the foundation of the Ideal Gas Law? By what mechanism does a non-nitrogen molecule "drag along" an N2 molecule when on-gassing? Or "block" an N2 molecule when total pressure is opposing the direction of N2 partial pressure?