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Discussion Starter #1 (Edited)
Hello

I'm doing a DMT, and haven't been able to figure this one out. The two instructors guiding me disagree on it (although in a very civilized fashion,) and I haven't been able to find it in any of my books. I suggested I asked on YD, as I've been reading from the closet for a while and think rather highly of the qualified answers I often find here.

So here goes. Often we've got customers on our boat who are good on air comming up with 100 bar left in the tank after a rather long dive. I've often heard divemasters say, to cull the temper of customers who want to empty their tank some more, that they should be glad they're good on air, as it means they've got less nitrogen loaded from the dive.

But I rather thought nitrogen absorption was dependant on PPN, not amount of nitrogen inhaled. Put another way, the way a science teacher would, I rather imagine. Two identical glass jars are half filled with a liquid, and the remaining volume is then filled with air at three bars of pressure. Both jars are also fitted with two valves, one in, one out. The jars are left pressurized for, say, an hour, but here's the thing: During that hour one jar has 5 liters of air passing though it, in one valve, out the other. The other jar has 10 liters passing through. The pressure inside the jars remain at 3 bars at all times.

There would, I assume, be an equal amount of nitrogen in the liquid, or? This theoritical model, can it also be applied to human physiology, so that one could say, air consumption has no bearing on nitrogen absorption? (assuming of course that the two jars, or persons as it would be, actually breathe.)

I hope you can sort us out

sincerely and eagerly awaiting any help, Knudsenjunior


edit: Few spelling oops sorted
 

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Nigel Hewitt
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Knudsenjunior said:
But I rather thought nitrogen absorption was dependant on PPN, not amount of nitrogen inhaled.
I would agree.

Now something like carbon-monoxide where, unfortunately, you body soaks up virtually every trace inhaled breathing rate would matter but you take so little nitrogen the ppN2 will not drop in your lungs during the breathing cycle.

I think Bühlmann discusses this in Tauchmedizin but from the other end and discovers that you don't off-gas faster for breathing more volume.
 

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life is too short
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:confused:
i agree that dive time and PPN are factors but surely amount consumed is also a factor.
1 breath with 79% would surely have a different affect than say 200 breaths at 79% as there is more nitrogen being loaded into the system?
 

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jorawley said:
:confused:
i agree that dive time and PPN are factors but surely amount consumed is also a factor.
1 breath with 79% would surely have a different affect than say 200 breaths at 79% as there is more nitrogen being loaded into the system?
But you'd have to hold that one breath for the whole dive ...

I've really no idea. :) Is there a physicist in the house?
 

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You could argue that breathing more frequently will replenish the Nitrogen gradient, so it will be absorbed faster if you breath more.
However, the effect would be minimal.
 

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Lazlo said:
But you'd have to hold that one breath for the whole dive ...
whats wrong with a 30min dive at 30metres on just one breath? thought that was the entry exam into diving....hehe:D
 

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jorawley said:
:confused:
i agree that dive time and PPN are factors but surely amount consumed is also a factor.
1 breath with 79% would surely have a different affect than say 200 breaths at 79% as there is more nitrogen being loaded into the system?
But there isn't more nitrogen being loaded into the system, that is the key point. You take in a very, very, very tiny fraction of the nitrogen in the gas that enters your lungs. The size of that fraction is determined by the PPN, and this is not changed by the tiny fraction that disappears from the gas into your body whilst it is in your lungs. So effectively your body sees the same PPN at the start of a breath and at the end of the breath. So, I would guess that for all intents and purposes, as you don't completely empty your lungs with each breath, for the whole time you are at a particular depth your body sees the same PPN and absorbs constantly.
 

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Yes IIRC the absorbtion rate of Nitrogen is constant even when at work..
If you up the partial pressure then more will be absorbed though gas flow through the lungs does not affect the PPN therefore the absorbtion rate does not go up.
 

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This gets more interesting.... Is exposed surface area a big issue, in which case....:

Do you take on more nitrogen if you're wearing a drysuit, as you've got a larger surface area exposed to air?

If you have large lungs, is that a bad thing too (for the same reason)?

Just thinking out loud :)
 

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Its a complex one. I'll have a stab at it as best I understand it.

The consensus view of on gassing is that it is a factor of PPN2 and time as everyone has said.

Breathing rate generally reflects the metabolic rate and the "work" done on the dive. This increases blood flow and perfusion and therfore - in theory - the on gassing to slow tissue groups. IIRC the old Alladin air integrated computers changed their M values slightly depending on air consumption.

The "flow" of nitrogen (and oxygen) for that matter is sufficient for gas absorbtion and excretion to take place at "normal" breathing rates.

High gas consumption tends to show shallow breathing and this has an effect on CO2 retention.

In practical terms there is more difference between individual people that the same individual wiht different breathing rates and therefore deco models work for nearly all of us. I think you would find that the mean standard deviation between individuals is larger than any variance caused by SAC.

For commercial divers that are really working there may be some slight difference I guess but I have never had access to the Comex tables!!

Chris
 

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Lazlo said:
This gets more interesting.... Is exposed surface area a big issue, in which case....:

Do you take on more nitrogen if you're wearing a drysuit, as you've got a larger surface area exposed to air?

If you have large lungs, is that a bad thing too (for the same reason)?

Just thinking out loud :)
I would think that the answer is "no" on the first point, and you don't have enough (any?) diffusion through your skin, if is mostly (all?) within your lungs. Soooo, the second point may well be a "yes". However, you may also gain by having an increased rate of off-gassing due to the larger surface area!

I am totally guessing at all this, but it is very interesting and certainly useful to think about the mechanisms like this.
 

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Lou said:
I would think that the answer is "no" on the first point, and you don't have enough (any?) diffusion through your skin, if is mostly (all?) within your lungs. Soooo, the second point may well be a "yes". However, you may also gain by having an increased rate of off-gassing due to the larger surface area!

I am totally guessing at all this, but it is very interesting and certainly useful to think about the mechanisms like this.
Whoo hooo... another isobaric counterdiffusion thread starting..... weeeee.....

Chris
 

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Lou said:
... but it is very interesting and certainly useful to think about the mechanisms like this.
Agreed. I suppose if the science was that well understood, then there would be less "accidents". Compartment models and so forth - it's all just educated guesswork, with some (a lot of) test data thrown in for good measure.

There are so many variables... people have different body tissue proportions, different dive profiles, microbubble theory, secondary factors, etc, etc...

It doesn't help when my least favourite DM subject is physiology either :(


Now give me lift bag questions and I'm in my element :)
 

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Pressure gradient

I always understood Nitrogen to be absorbed by tissues because of the increased pressure, not the breathing rate.

However, the breathing rate may have an effect on the severity of Narcosis. I say that because I went much more loopy than usual when I had to swim hard against a current. :eek:mg:
 

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Scubee said:
However, the breathing rate may have an effect on the severity of Narcosis. I say that because I went much more loopy than usual when I had to swim hard against a current. :eek:mg:
That could be CO2 retention exacerbating the effect of the N2.

I'd say in answer to the OP's post, that the rate of N2 takeup would be very marginally faster if you breathe quicker due to the fact that it will be replenished in the lungs as it diffuses into the tissues, however given enough time to reach saturation (say 5-6x the halftimes of the tissue group in question (if you beleive in tissue types :) )) then the amount of N2 will reach 80%

Just my 0.02.

r
P
 

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nigelH said:
.... but you take so little nitrogen the ppN2 will not drop in your lungs during the breathing cycle.
Don't know the facts about this, but the basic principle of on-gassing and off-gassing is not so complicated. It is based on Le Chatelier's Principle (from school physics). Systems out of equilibrium will always try to move into equilibrium, so as the partial pressure of the nitrogen in the inhaled air increases, the amount of nitrogen absorbed into the body tissues increases until it is in equilibrium again. The crucial thing is the rate at which this equilibrium is reached. The ‘size of the lungs’ (i.e. surface area) is only a factor is that the equilibrium will be reached slightly more quickly; breaths per minute has the same effect. Either way, equilibrium will be reached pretty quickly, and the effect of these two factors will only be important for that short time while equilibrium is been achieved.

On-gassing happens quickly because the air has a very high relative nitrogen concentration compared with the blood, and so the blood ‘sucks’ the nitrogen in. Off-gassing is slower because the relative concentration of nitrogen in the blood as the partial pressure in the lungs reduces is much lower, and so there is not such a high driving force for diffusion of the nitrogen back into the lungs (to be exhaled). Reduced nitrogen mixes for deco reduces the deco time because of the reduced ppN2 at the deco depth (absolute pressure) relative to air, rather than any ‘flushing’ effect.

Time for the kinetics to do their thing is the important thing.

I would expect higher breathing rates to appear to increase the effects of narcosis, but the real cause is CO2 build up.

Of course this may well be complete b*ll*cks. I wouldn't believe anything I said.

Cheers,

Marcus
 

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Sheff diver said:
breaths per minute has the same effect.
But why do you theorise this is the case? I just don't see it. Within a single breath cycle there isn't long enough for my body to register air in or out. A small amount of O2 is metabolised, and a small amount of nitrogen enters the bloodstream if the inspired PPN is higher than the tissue PPN. The rate of equilibirium varies according to tissue type, but I still don't see how rate of breathing comes into play, unless you are hypothesising that in one breath the rate of absorbtion into the blood stream is so rapid that the PPN in the lungs falls quick enough to be effected by upping breathing rate by, say, 20%?
 

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Nigel Hewitt
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Sheff diver said:
Of course this may well be complete b*ll*cks. I wouldn't believe anything I said.
I'm sorry old bean.
That's about the one bit that is correct.

Modelling on gassing and off gassing is not something that has changed over the last century, although we now use compartments stretching out to over 10 hours, and nobody looks at breathing rate because basic physics says it has no effect.

Consider me on the surface breathing air. The blood in my lungs has a tension of 0.79 bar matching the air in my lungs so although individual molecules are dissolving and undissolving the net flow is zero. Oxygen, naturally is flowing as the blood feeding my lungs is short on the stuff and the first bits that do dissolve are snatched up by the blood cells.

Now crash dive to 20 meters and inhale from a cylinder of air. My lungs are filled with 3 bar of air so 2.37 bar of nitrogen. Currently my blood tension is 0.79 bar. Gas will be dissolving proportionate to the 2.37 bar and undissolving proportionate to the 0.79 so I'm going to see a net flow but if I hold my breath both the oxygen and the nitrogen will continue to cross into my body and leave behind air. All that happens is that my lungs decrease in size a bit. The ppN2 will not decrease because it will still be (roughly) the same fraction hence the rate is only dependent on the increasing tension of N2 in my blood. Breath deeper or shallower and all I have in my lungs is air.

HTH
 

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Lou said:
...and a small amount of nitrogen enters the bloodstream if the inspired PPN is higher than the tissue PPN. The rate of equilibirium varies according to tissue type, but I still don't see how rate of breathing comes into play, ...
That basically what I was trying to say, maybe badly. In real terms, breathing rate has no effect.

nigelH said:
I'm sorry old bean.
That's about the one bit that is correct.

Modelling on gassing and off gassing is not something that has changed over the last century, although we now use compartments stretching out to over 10 hours, and nobody looks at breathing rate because basic physics says it has no effect.
Agree with your first comment.

I was keeping well away from tissue types for simplicity, and since the nitrogen reaches most of these tissue types via the blood. Accepted, it may not be the rate controling step, but the kinetics of gas transfer through the long walls is a significant factor for ongassing, less so, I agree, in off-gassing.


Consider me on the surface breathing air. The blood in my lungs has a tension of 0.79 bar matching the air in my lungs so although individual molecules are dissolving and undissolving the net flow is zero. Oxygen, naturally is flowing as the blood feeding my lungs is short on the stuff and the first bits that do dissolve are snatched up by the blood cells.

Now crash dive to 20 meters and inhale from a cylinder of air. My lungs are filled with 3 bar of air so 2.37 bar of nitrogen. Currently my blood tension is 0.79 bar. Gas will be dissolving proportionate to the 2.37 bar and undissolving proportionate to the 0.79 so I'm going to see a net flow but if I hold my breath both the oxygen and the nitrogen will continue to cross into my body and leave behind air (oxygen and the nitrogen?). All that happens is that my lungs decrease in size a bit. The ppN2 will not decrease because it will still be (roughly) the same fraction hence the rate is only dependent on the increasing tension of N2 in my blood. Breath deeper or shallower and all I have in my lungs is air.

HTH
The variation in PPN2 I was talking about is due to change in depth, not absorbtion of gas within the lungs during a breath cycle. With varying depth, surely the net flow cannot be zero.

Isn't over simplification wonderful. I think we are saying the same thing, though I guess I put it badly / too simply.

Oh well.....

Marcus
 
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