Partial Pressure. Does he/she breaths more

[quoggle]

hey guys
I think you missunderstood the question
academic question:
you get down, you switch to trimix. but the ratio of oxygen present in the mix is lower than what your body is used to (10% as opposed to 20%).
Do you breath at a faster rate than you would normaly at that depth if you were using air (not taking into account oxygen toxicitiy).
or does the higher partial pressure that you are breathing at, compensate for the reduced ratio of oxygen in the gas that you breath????.
(for the --ologists amongst you: does the uptake and release of oxygen by the heamoglobin complex depend on the partial pressure of oxygen or the percentage of the oxygen within the gas mixture?)

[Janos]

Originally Posted by quoggle

hey guys
I think you missunderstood the question
academic question:
you get down, you switch to trimix. but the ratio of oxygen present in the mix is lower than what your body is used to (10% as opposed to 20%).
Do you breath at a faster rate than you would normaly at that depth if you were using air (not taking into account oxygen toxicitiy).
or does the higher partial pressure that you are breathing at, compensate for the reduced ratio of oxygen in the gas that you breath????.
(for the --ologists amongst you: does the uptake and release of oxygen by the heamoglobin complex depend on the partial pressure of oxygen or the percentage of the oxygen within the gas mixture?)

No. As long as it's over the minimum then breathing rate is unaffected (and generally, when diving at highish ppO2, the oxygen metabolised is taken from the plasma, not the haemoglobin )

Janos

[SilvaFish]

AFAIK, it is the level of carbon dioxide in the blood that drives the urge to breathe, nothing to do with what percentage of oxygen is in the breathing mix. Obviously, not too little oxygen that would cause a black out without warning!

HTH's
Lou

[Steve Jones]

Originally Posted by quoggle

hey guys
I think you missunderstood the question
academic question:
you get down, you switch to trimix. but the ratio of oxygen present in the mix is lower than what your body is used to (10% as opposed to 20%).

Don't think %, think PO2

So your 10% at the surface is 0.1 PO2. Then at 20 metres it is 0.3 PO2.

You need approx 0.16 PO2 to sustain life.

So you breath normaly.

Steve

[SoggyFox]

Originally Posted by Janos

1) Most divers will drop the ppO2 by 0.04 whatever mix they are breathing provided that
2) they are breathing a mix with a ppO2 of at least 0.16.

if we use ppo2 of 0.04 then why do we need 4 times as much ppo2 to be able to breath it?

[Janos]

Originally Posted by SoggyFox

if we use ppo2 of 0.04 then why do we need 4 times as much ppo2 to be able to breath it?

Something to do with the efficiency of the lungs. The lungs need a certain minimum pressure to extract enough O2. Ifyou had very, very, very, very large lungs then I guess theoretically you could breathe Nitrox 4% and breathe out Nitrogen wiht 4% CO2, but it's not possible in practice

As to why this is? Thing of your scrubber. Your scrubber doesn't work perfectly until every last bit is used up and then stop working - there is a "wavefront" of used and unused lime that moves along the scrubber. I guess that it would be something similar in the lungs.

Or another theory might be something to do with concentration gradients and the speed of Oxygen transfer.

However, generally speaking the rate of breathing is dictated by CO2 removal, not O2 absorption.

Janos

[SoggyFox]

Originally Posted by Janos

Something to do with the efficiency of the lungs. The lungs need a certain minimum pressure to extract enough O2. Ifyou had very, very, very, very large lungs then I guess theoretically you could breathe Nitrox 4% and breathe out Nitrogen wiht 4% CO2, but it's not possible in practice

do you mean Jordan can breath nitrox 4%, will she be the new Tanya Streeter after all shes got a mighty pair of lungs...............

concentration gradient is possible but what happens when we start using 80% and 100%, are we still using ppo2 0.04?

must be the minimum pressure to get it across the membrane of the lung, i suppose anything less and no o2 would be forced through (osmotic pressure?).

[nigelH]

Originally Posted by quoggle

you get down, you switch to trimix. but the ratio of oxygen present in the mix is lower than what your body is used to (10% as opposed to 20%).

As I said: you can't tell the difference.
Take a cylinder of pure helium and breath. For the first minute of so you will breath normally and then the world will slowly fade away.

Breathing is controlled by the amount of CO2 dissolved in your blood. It has worked that way for millions of years and, except for some rare illnesses, it continues that way. Provided you are getting the CO2 out you feel happy and don't puff or pant. 95% O2 and 5% CO2 and you will be choking, gasping for breath, in distress to put it mildly.

Gases in solution are controlled by partial pressures. This is what is important so this is what we talk about. Percentages are irrelevant. Provided my oxygen input is in the region of 0.16 to 2.4 bar I'm probably OK. Since probably isn't too good we tend to make the 0.18 to 1.6 but they aren't hard limits just safety factors. At the bottom end you just go to sleep and at the high end you convulse. The high end is a bit unpredicatable so we stay well away from it.

A mix for 100m needs about 1 bar of oxygen and less than 3 of nitrogen so the diference to make up 11 bar is 7 bar of helium.
1:3:7 = 9% 27% 64%. It won't taste funny or feel much different. However if you sit in your living room and breathed it you would fall over as there isn't enough oxygen.

[Okeanos]

Originally Posted by nigelH

1:3:7 = 9% 27% 64%. It won't taste funny or feel much different. However if you sit in your living room and breathed it you would fall over as there isn't enough oxygen.

You forgot one major point, helium abosrbs heat from the body really quickly, this causes the vocal cords to constrict and make you talk with a silly voice, while you're awake Or all the time on CCR

[Mark Chase]

Originally Posted by quoggle

hey guys
I think you missunderstood the question
academic question:
you get down, you switch to trimix. but the ratio of oxygen present in the mix is lower than what your body is used to (10% as opposed to 20%).
Do you breath at a faster rate than you would normaly at that depth if you were using air (not taking into account oxygen toxicitiy).
or does the higher partial pressure that you are breathing at, compensate for the reduced ratio of oxygen in the gas that you breath????.
(for the --ologists amongst you: does the uptake and release of oxygen by the heamoglobin complex depend on the partial pressure of oxygen or the percentage of the oxygen within the gas mixture?)

The percentage of 02 in the mix is totally irrelevant the ONLY thing that is important is the Partial pressure of 02. If you breath air at ambient pressure you at 0.21PPo2 if you breath Trimix with 10% 02 at the depth of 11m you on a partial pressure of 0.21 which is exactly the same.


Fortunately the body can tolerate higher partial pressures of 02 for X amount of time. As a result 10% can be dived to a partial pressure of 1.3pp02 which is a depth of 120m for a recommended maximum of three hours. Obviously this is not possible unless your saturation diving but you get the idea.

So its PP02 that counts not %02 and its time of exposure to high PP02s that hurts you not the actual PP02. Personally i have breathed 2.5 pp02 at 65m for maybe 20 seconds with no ill effect. Sadly this means nothing as the same event may cause a tox in the same diver next week.


As for the helium? its almost irrelevant. Its an inert gas replacing another inert gas (Nitrogen). With the aim of reducing narcosis.

It has + sides Less narcosis thinner gas so lower work of breathing.

and it has - sides increased on gassing and its a cold gas.


HTH

ATB

Mark Chase