Why dont you breath less at depth?

[dhampton82]

when you are breathing O2 rich mixes (mostly on deco) after dives in which you've maxed out your O2 load, any buildup of CO2 could trigger an ox tox hit

Wouldnt that just mean that the O2 is dangerous and not the CO2?

 

[H2Andy]

nope... you can tolerate the O2 ok until CO2 shows up ... even a hard-breathing reg can build up enough CO2 to trigger the ox tox

what you try to do is minimize the CO2 buildup... that's the enemy, not the O2

 

[dhampton82]

hm...... getting a little to technical for me

SO ox tox is really just a bad combination of high levels of co2 and high levels of o2 cantradicting together?

 

[H2Andy]

SO ox tox is really just a bad combination of high levels of co2 and high levels of o2 cantradicting together?

well... in this case, CO2 is an issue because a build-up of CO2 is one of the "risk factors" for oxygen toxicity

here's a good write up on it:

http://scuba-doc.com/oxygentox.html

 

[dhampton82]

Good link andy, thanks

 

[ShakaZulu]

This is one of those posts, that when he looks back 2 years later, realise what a dumb***** question it was

 

[Doc Harry]

I know (to some) that this may seem like a stupid question, but why dont you breath less air or the same amount of air at depth?

I know that the air that you breath in is compressed. But why, since you are breathing in more oxygen with each breath, why wouldn't your body just use that oxygen and just make you need to breath fewer times? Can anyone explain to me the physics or gas laws or what ever that causes this?

I know this may be a stupid question, but my dad asked me about it the other day, and it has been on my mind ever since......

Thanks
Dustin

There are several ways in which to answer your question. But I'll try to answer the question that I think you're asking.

Breathing compressed air doesn't cause you to breath less because the added pressure does very little to increase the amount of oxygen in your blood.

The vast majority of oxygen in the blood stream is carried by hemoglobin in the red blood cells. A typical healthy diver usually has 99% of the hemoglobin saturated with oxygen on room air at sea level. Increasing air pressure does very little to increase the amount of oxygen bound to hemoglobin. There is about 20 milliliters of oxygen carried by hemoglobin per 100 milliliters of blood.

A small amount of oxygen is dissolved in the plasma. Increasing the air pressure significantly increases the amount of oxygen dissolved in the plasma. However, in the big picture, this dissolved oxygen in the plasma is really insignificant in terms of the entire oxygen load. There is only 0.3 milliliters of dissolved oxygen per 100 milliliters of blood.

If you're interested in the math, the oxygen content in normal blood at body temperature is:

Oxygen Content = (1.34 x Hgb x Sat) + (0.0031 x ppO2)

where:
1.34 = 1.34 milliliters of O2 per gram of hemoglobin
Hbg = grams of hemoglobin (typical value = 14)
Sat = Oxygen saturation (% of hemoglobin saturated with O2, typically 99%)
0.0031 = 0.0031 milliliters of O2 dissolved in 100 milliliters of blood per mm mercury
ppO2 = partial pressure of O2 in mm mercury

So to answer your question from this equation, you can see that a high ppO2 has little effect on total oxygen content because it is multiplied by a factor of 0.0031.

Another reason that increasing the partial pressure of O2 doesn't slow your breathing rate is because, for most healthy people, respiratory drive is driven by high partial pressures of carbon dioxide not low partial pressures of oxygen.

Low oxygen parital pressure can also stimulate the drive to breath, but it is usually not an important player. Except with smokers with emphysema. They typically live with very high carbon dioxide levels so their respiratory drive becomes less sensitive to elevated carbon dioxide levels. It is, then, the low oxygen levels that stimulate their breathing drive.

 

[beejw]

The body can only use, in one standard breath at the surface between 19% and 22% of the oxygen in the air. In a 1.6 liter breath that means you use between 0.304 and 0.37 liters of oxygen. As you go deeper the amount of air used per breath increases, but the percentage of oxygen used is the same, leading to an increased o2 intake. This in turn increases cellular metbolism, creating more CO2 then at the surface, triggering you to breath earlier. The hypothalumus controls co2 tiggered breathing. excess CO2 in the blood can lead to blood acidosis (CO2 forms H2CO3 ions by dissolving in the plasma, it balances blood pH with an eqilibrium between H+ ions and CO3- ions), which is a contributing factor to DCI.

 

[ScottB]

Are there studies that prove this? Do you have a link I would like to do some reading.

H2Andy's link says pretty much the same thing, but here are my two references (no links, sorry)

ANDI International Recreational Diving with Closed Circuit Rebreathers: ISC Megalodon Rebreather is where I got my information.

Chapter 2 Page 13:

Hypercapnia - Condition in which the lungs are exposed to excessive amounts of CO₂. Carbon Dioxide excess causes the blood vessels to dilate. This, in turn, causes an increase in blood flow to the tissues which can increase the rate at which the tissues absorb the chemicals and gases dissolved tehrein. CO₂ excess is also a predisposition to decompression illness since more Nitrogen may be absorbed. The likelihood of Oxygen toxicity also increases as the brain tissue is exposed to higher levels of Oxygen due to the perfusion increase caused by the dialated blood vessels. 0.5% causes distress and is ANDI's defined threshold of "Breakthrough". 0.1 ata of CO2 causes death

U.S. Navy Diving Manual Revision 5

Chapter 3 page 17

Excess carbon dioxide also dilates the arteries of the brain. This may partially explain the headaches often associated with carbon dioxide intoxication, though these headaches are more likely to occur following the exposure than during it. The increase in blood flow through the brain, which results from dilation of the arteries, is thought to explain why carbon dioxide excess speeds the onset of CNS oxygen toxicity. Excess carbon dioxide during a dive is also believed to increase the likelihood of decompression sickness, but the reasons are less clear.

Best wishes,

 

[beautybelow]

This is one of those posts, that when he looks back 2 years later, realise what a dumb***** question it was

I dont think there is such a thing as a dumb question. I learned a little, or was refreshed a little by the thread. Keep asking if you dont know, otherwise in two years you will look back and think wow, I didnt learn anything at all.