Gases at pressure question

[twatto]

...Sorry, my physics and physiology is a bit rusty.

When diving at depths the inspired tank air is compressed in accordance to how deep you are.
The number of each gas molecule is increased per unit space, as would be the partial pressure.

Body consumption of O2 and production of CO2 - does that change much at depth (I can't see why).
1. How does a high molecule load of oxygen / partial pressure effect respiration rate if body consumption is similar to at surface pressures,
2. If there is a high CO2 partial pressure in the lungs / blood - how is this going to affect the stimulus to breathe? I guess the other gasses dissolve in blood, but Hb would still only saturate to a max of 100% regardless of the increased partial pressures.

Cheers.

 

[wetvet]

Your question is interesting, but you forget that CO2 makes up a very small portion of air. I dont have the numbers in front of me, but I think it starts with 0.0_%. So, even if you take it to depth, the partial pressure of CO2 will still be quite low, and shouldn't affect the blood concentration noticably. Exhaled air is somewhere to the order of 5-7% CO2 (correct me if Im wrong), so you can see how much depth would be required to impact this.
AFAIK, the pressure of O2 has little to no effect on breathing, since CO2 is the stimulus to breathe, but, although Hb is fully saturated, the high pressure of O2 will cause levels of oxygen dissolved in the blood to increase, and thus the entire amount of oxygen carried in the blood increases. This is the mechanism of oxygen toxicity at depth.

Just my 2 cents.

Wetvet

 

[Epinephelus]

For simplicity, lemme look at each gas separately.
Oxygen - oxygen transport is a chemically active process - predominantly carried by the red blood cells, which have evolved to allow essentially complete saturation at .21 ATA, increases in O2 pp make a minimal contribution to the total oxygen in the body (but with highly significant physiological consequences - see reference to O2 toxicity in wetvet's post). Furthermore, once the hemoglobin in the RBC starts handing off oxygen to tissues, it is available to scavenge excess oxygen from the surrounding blood plasma and tissues. This results in a dissolved O2 pp that runs anywhere from a third to a tenth of what you'd expect if dissolved O2 pp were simply a matter of the O2 pp in the breathing gas. (which explains why Nitrox isn't even more narcotic than air, but that's a different discussion).
CO2 - the urge to breathe is indeed tied closely to the amount of disolved CO2 in the blood, apparently through minute changes in blood pH. Depth conspires in a couple ways to make CO2 retention a problem. First, there is the increase in CO2 pp in the breathing gas - not very significant, but there. Second, and more important, is the relationship between the amount of dissolved CO2 that it takes to cause CO2 problems and the ability to offgas CO2 at depth. At rest, normal exhaled gas has about a 4% CO2 concentration. In order to get this concentration of CO2 in the expired gas, the CO2 pp in the blood must be high enough over the CO2 concentration in the breathing gas that in the course of a breath the differential will cause the migration of that much CO2. But what happens at 100' (4ATM)? The body is still producing the same amount of CO2, but exhaling the same amount would only represent 1% in the exhaled gas. But as the ability of the blood to retain CO2 has also increased fourfold, the gradient between dissolved CO2 and the breathing gas has dropped to one fourth that at the surface. In other words, you have only a fourth the force available to push (or pull, depending on your point of view) the CO2 out of solution, so the process becomes less efficient. Thankfully there is some active involvement where hemoglobin dumps CO2 to pick up O2, so it's not quite as bad as the straight math makes it seem (although high O2pp hinders this process somewhat by causing premature CO2 dumping as the Hb scavenges excess O2 before it can carry the CO2 back to the lungs), but the bottom line is that it's more difficult to get the CO2 out of the body at depth than it is at the surface, and it's easy to build CO2 in the body to headache or even dangerous levels.
Solution - don't overexert yourself; monitor your breathing, make it steady and deep.
E.

 

[Pez de Diablo]

Epinephelus,

Good post. Thanks for the info.

 

[Dr Deco]

Hi twatto:

Thanks to those who responded.

The bottom line on this is that oxygen is consumed as a function of your body’s needs (heat production + muscular activity + metabolic requirements). More present in the breathing gas will not increase the need or the uptake.

Carbon dioxide is produced according to the oxygen utilization, which as we said, is not depth dependent. Generally carbon dioxide is expired if individuals take full inspirations and exhalations. Shallow breathing is to be avoided, as it does not allow the alveoli to clear themselves fully. Carbon dioxide buildup in the blood will occur and you will end up feeling “winded” and probably get a headache.


Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :grad:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[twatto]

Thank you all... I feel enlightened.

Twatto.