Air density and how it affects breath rate?

[Zee Pet]

Hi, from once they taught us that you use your air faster at depth because the density increase, I've had a little nagging question. If the density of the air is increasing, obviously the molecules are being packed tighter in our lungs there for more oxygen per given volume is increased at depth vs the surface,right? So if our lungs have higher amounts of oxygen to be utilised, why do we breath at the same rate as the surface? If I rember correctly, the medulla oblongata is responsible for autonomous respiration and senses the level of carbon dioxide in the blood

Is this what keeps the breath rate the same even though there is still more oxygen in the blood, by using a fixed level of carbon dioxide before triggering/suggesting a breath sensation?
Off topic question: what is the difference between bottom time to date and cumulative time in the log book?

 

[pdhenry]

I can answer the second (I've seen an explanation of the first but I don't remember it well enough to relate here).

Bottom Time To Date is your accumulated bottom time prior to the dive you're logging, and Cumulative Time is the accumulated bottom time including the dive you're logging.

Bottom Time To Date + Time This Dive = Cumulative Time.

Just copy your Cumulative Time from the previous dive into Bottom Time To Date for the current dive.

Use the Actual Bottom Time values in this section, not the Total Bottom Time which corrects ABT for residual nitrogen from previous dives (TBT is used in the Recreational Dive Tables).

 

[kafkaland]

Let me try to answer the first - I think you gave the answer already. Yes, the breathing rate is determined by the need to get rid of CO2. And the rare at which you produce it is independent of depth. And when the CO2 from your blood goes into the air in the lung, you'll always generate the same partial pressure of CO2, no matter the depth. And partial CO2 pressure is what your body senses, not a molar fraction. So even though you have much more O2 available at depth, it all goes to waste once a particular CO2 partial pressure is reached, in OC at least. And that's where rebreathers come in...

 

[Zee Pet]

Ahh ok thanks to both of you. Kafkaland, the partial pressures would build at a slower rate though wouldn't they? If the body is converting O2 to CO2 at say 1mol/second and at surface you have 50 mol, and ten meters 100 mol, because rate is independent of depth, I should be converting the same rate. So I have 50 seconds at surface and theoretically 100 at 10m. So what is happening is that I'm still after 50 seconds at 10m produced 50mol of CO2, which is the amount to trigger a breath? At the most I would have thought le chateliers principle would say higher concentration of oxygen-> increase exchange rate. I wonder if its possible to build resistance or train your brain to operate/tolerate with the higher CO2 pp so that you can have even slower breathing. Or is that just really good cardiovascular fitness?

 

[kafkaland]

Ah, the paradox of partial pressures. If your body metabolizes n molecules of O2 into CO2 per minute (sorry, I don't have a number handy), and that's independent of depth, it has to get rid of those n molecules per minute, independent of depth. When you inhale fresh gas, no matter the composition it pressure, the partial pressure of the CO2 is zero. And then your lungs shove out those n molecules into the gas in your lungs, generating a partial CO2 pressure there that depends only on n and your lung volume, but not on how many other gas molecules of a different species there are. The nitrogen, oxygen, or helium neither increases nor decreases the CO2 partial pressure, or the ease of CO2 transfer from the blood to the gas phase, at least in the ideal gas limit. So, as soon as a certain number of CO2 molecules have accumulated in your lungs, generating a certain partial CO2 pressure, you have to breath. And that's independent of the pressure the other gases are under, and hence the depth. So your breathing rate stays the same, you're just running through more gas at depth.

 

[boulderjohn]

Ahh ok thanks to both of you. Kafkaland, the partial pressures would build at a slower rate though wouldn't they? If the body is converting O2 to CO2 at say 1mol/second and at surface you have 50 mol, and ten meters 100 mol, because rate is independent of depth, I should be converting the same rate. So I have 50 seconds at surface and theoretically 100 at 10m. So what is happening is that I'm still after 50 seconds at 10m produced 50mol of CO2, which is the amount to trigger a breath? At the most I would have thought le chateliers principle would say higher concentration of oxygen-> increase exchange rate. I wonder if its possible to build resistance or train your brain to operate/tolerate with the higher CO2 pp so that you can have even slower breathing. Or is that just really good cardiovascular fitness?

I am not quite clear on what you are saying here. When you breathe in air, some of the oxygen is metabolized by the body. It goes through a chemical reaction, the exact opposite of photosynthesis, to create energy and carbon dioxide. Only some of the oxygen is thus converted into carbon dioxide through this chemical reaction, and the rest--most of it by far--is exhaled. Are you thinking that because you are at twice the pressure at 10 meters, then twice as much oxygen will go through this chemical reaction to create energy and carbon dioxide?

 

[Tigerman]

Out of the 21% oxygen (ppO2 .21) we breathe in, 16-17% (ppO2 .17) is exhaled. In other words, we use a mere 20% of the oxygen available to us even on the surface. Were not tuned to use all that much oxygen per breathing cycle...

 

[Dr. Lecter]

Breathing rate is largely tied to CO2 levels in the blood, and as gas density increases it gets harder to exhale and you tend to retain a bit more CO2. Up goes your breathing rate until/unless you lower the CO2 level.

O2 has more or less nothing to do with it.

 

[TSandM]

The key is that carbon dioxide levels in the blood are inversely proportional to the VOLUME of gas that passes through the lungs, so long as that gas doesn't contain CO2 (which, for all intents and purposes, what we breathe does not). What the composition of the gas is, other than being CO2-free, doesn't matter. Density doesn't matter. So, no matter what the ppO2 of the inspired gas is, you have to run the same AMOUNT of it through the lungs to get rid of the carbon dioxide.

In fact, there is some evidence that, at least in some people, there is tolerance of a slightly increased blood CO2 level if the ppO2 is high, so the straight proportionality may not completely hold on scuba, but it's close enough to consider it valid.

 

[Dr. Lecter]

The key is that carbon dioxide levels in the blood are inversely proportional to the VOLUME of gas that passes through the lungs, so long as that gas doesn't contain CO2 (which, for all intents and purposes, what we breathe does not). What the composition of the gas is, other than being CO2-free, doesn't matter. Density doesn't matter. So, no matter what the ppO2 of the inspired gas is, you have to run the same AMOUNT of it through the lungs to get rid of the carbon dioxide.

In fact, there is some evidence that, at least in some people, there is tolerance of a slightly increased blood CO2 level if the ppO2 is high, so the straight proportionality may not completely hold on scuba, but it's close enough to consider it valid.

Let's be clear: density doesn't matter for its own sake, but it matters quite a lot in so far as it affects how much gas you're able to cycle through the lungs with a given exertion/breath--which can be quite a lot as you go deeper, particularly without a helium-rich mix.

Interesting regarding PPO2 and possible tolerance to CO2 levels…is that depth-independent? I could see it being akin to the theory regarding CNS tolerance and END, too.