syncope ??

nigelh

I didn't think that was the question.
The ppO2 is 0.21 just like it is now sitting in front of your computer.
I haven't died yet so I assume 0.21 is reasonably safe.

Let's tighten the example up a bit.
I am at 10m breathing 0.21bar O2 so its 10.5%.
I pull the loop from my mouth and inflate my jacket blowing a stream of bubbles as I wizz up.
I hit the surface and start breathing all that free air.
OK I'm a bit rich on Nitrogen and that might give me problems but the O2 in my body isn't going to change.

Faction O2 is not important. It is the ppO2 that keeps you working.

If you want to do a breath hold ascent I can do 3 minutes freediving on one bar of air and it's the CO2 that hurts.
That's a 30m dive. Why should I pass out ascending from 10 unless I choose to do it slowly and you were offering me a reg of air (0.42 when we start) in the example? You are never offering me less than 0.21bar so I'm happy. My blood/brain ppO2 cannot drop.

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nigelh

AD_ward9

Sorry Nigel, but you need to do a rebreather refresher course.

You are breathing 0.21bar of O2 at 10m.

If you hit the surface instantly, the PPO2 will be 10.5%. That instant step alone might make you pass out.

If you take 30s to hit the surface it will be about 5%. That is 20m/min ascent rate, a fast swimming ascent. Even a loss of a weight belt from 10m will not be that much faster.

If the PPO2 in your lungs is 0.05 when you are in the sea, then the O2 in your body is definitely going to change, probably permanently.

A breath hold descent and ascent is completely different because they do not release gas to the environment. I would also point out that a lot of freedivers pass out just as they get to the surface. To help people avoid thinking that because holding your breath is OK, then just coming off the loop and going for the surface, I will do the sums below.

A good freediver can fill his lungs to 6l, with a pressure of 1.1bar. That is over a litre of O2, plus the O2 in his blood (several times that). It is a closed system. The freediver will be as passive as possible in the water, so will metabolise only 0.2 to 0.3 litres of O2 per minute. He can easily survive for 6 minutes and the PPO2 in his lungs is still over 0.12.

On the other hand, you are a diver breathing out. You are not a closed loop. You cannot be a closed loop because otherwise you will have an embolism. The PPO2 in your lungs drops linearly with pressure, plus your body is metabolising O2, so you are in a much worse position in 30s than the freediver is in 6 minutes. Also looking at divers generally, the majority are in much poorer physical shape than a freediver.

Alex

nigelh

Sorry. Wrong.

Read the question.
He switched the lungs to air before the ascent.
A fluid (blood) doesn't care about the fraction or the ambient.
Tension depends on the molecule count so then 0.21bar doesn't change.

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nigelh

AD_ward9

Nigel,
Please read what you wrote instead of what you thought you wrote.

You have the diver going to the surface exhaling after being on a loop with a PPO2 of 0.21 at 10m. That is not air: it has half the PPO2 of air at 10m, and keeps on having half the O2 as the loop empties gas to the environment. Your lungs fare worse, as you consume O2 on top of that same loss as the loop.

If you put the regulator in your mouth and go to O.C. that is a different situation, but is not the one you described. We all agree, bailing out onto air the diver survives, but in both other scenarios the diver dies: staying on the loop he is dead, and ascending exhaling in free water he is dead.

Alex

nigelh

<shrug>

Original Question
My nastier developement.
I hold that my blood (meant to say brain not blood) has never been exposed to gas.
Gas considerations end at the Alveoli.
If the blood is at 0.21bar I remain happily conscious.
In the first example I think the blood ppO2 will probably peak up a bit from the first breath of air.
In the second example it will drop but only as I consume O2 and some small amount defusing back into the alveoli.

If I breathed 10% O2 yes I'd expect to fall over, probably faster than if I held my breath, but all I can loose to my lungs here is one breath's worth.

I still vote no unconsciousness.

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nigelh

brinoz

I think you forgot one thing here.

When you start breathing air, you do not get air right away in your lungs ... you have a mix of air and the previous mix.

In paul's example, you have 10 % O2 mix in your lungs at 10 m.
You switch to air, after one breath you will get sometrhing between 10 and 21 % O2.

You will need at least 3 full breath to be at 21 % (it is like a calibration of oxygen cells ) and even more due to the residual volume of yours lungs.

This is also the reason why you can breath one (not more ) breath of helium and speak like Donald duck without problem.

Regards
Bruno

PS : to my opinion, Nigelh is right !

nigelh

This was an edit but has appeared as a new post

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nigelh

AD_ward9

Nigel, I am confused by your last post, so let us go back to your original post to establish the facts clearly.
You say you are breathing 0.21bar O2 at 10m. That is a PPO2 of 0.21 to start with.

You say you pull the loop from your mouth and inflate your jacket blowing a stream of bubbles as you wizz up. I assume the bubbles come from your mouth, otherwise you would embolise.

You say you hit the surface and start breathing air. I think not.

You certainly hit the surface. However you are not conscious, so you probably breathe water. The reason you are unconcious is the PPO2 in your lungs started at 0.21 and due to the expansion and loss of gas during your ascent, it is now 0.105 minus your metabolism. As your lungs would not be full, that equates to a PPO2 of 0.05. If you can have a gas in your lungs just below the surface with a PPO2 of 0.05 and not pass out, you are a better man than 99.9% of the rest of us.

So you have passed out. Once the Rebreather fills up with water from the mouthpiece, or your buoyancy gives in, you sink.

In a step by step fashion, please take me through what bit I have misunderstood in the series of actions you have done.

On Brinoz's, point, getting He to your vocal cords is a little different to getting O2 to your brain. Sorry, but so far on the bail out front, only 3 other divers on this thread survive: Brent, Dave Sutton and Dave Cooper (Decodiver). That is a worrying.

As this is such simple arithmetic, and arguing it is boring, may I be excused for doing a little promotion of a far more interesting thread? I posted probably the most comprehensive work ever published today on Hydrostatic Imbalance issues in rebreathers. If you see the most recent post on the Open Revolution forum of this site, it is there. 261 pages of nice hard core facts and figures to help the insomniacs.

Alex

nigelh

This is where you go wrong.
The ppO2 in your lungs does not matter if the O2 tension in your blood is still 0.21bar.

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nigelh

AD_ward9

You do not have an O2 tension in your blood of 0.21bar. Ever.

Nice paper: "Oxygen tension and blood oxygen content. The relationship between oxygen tension, oxyhaemoglobin saturation, haemoglobin concentration and oxygen tension of blood", C. G. Bird. I will find the journal in a sec.

Back to the point, the oxygen saturation of your blood will fall instantly the PPO2 in your lungs falls. A lot of blood goes through your lungs every second.

Alex