Air density and how it affects breath rate?

[EFX]

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?

I think Zee Pet is thinking of a direct link between the O2 metabolic and the CO2 waste process. It is not O2 -> CO2 but O2 -> energy producing process -> energy using process -> CO2. As KafKaland has already mentioned the breathing rate is mostly dependent on the rate of CO2 production. We know this because people breathing pure O2 at the surface breathe at the same rate as on air. Also, diffusion rates are independent of the partial pressures of other gasses. The flow rate is determined, in part, by the difference of pressure across a barrier only of that gas. We know this too because we can exchange CO2 at relatively low blood-lung pressure drops in the face of higher pressure drops of O2 across the same barrier. The two gasses exchange across the same barrier in opposite directions at the same time.

 

[TSandM]

Density of the breathing gas DOES affect flow through the small airways, when the density reaches a certain value. But it is all but irrelevant for people staying with recreational diving depths.

 

[EFX]

Also, diffusion rates are independent of the partial pressures of other gasses. The flow rate is determined, in part, by the difference of pressure across a barrier only of that gas. We know this too because we can exchange CO2 at relatively low blood-lung pressure drops in contrast to higher pressure drops of O2 in the lung-blood barrier. The two gasses exchange across the same barrier in opposite directions at the same time.

Ignoring the CO2 process, why we don't breathe at a lower rate at depth than on the surface can be reasoned out by working backwards from the tissues to the lungs. The higher density of air and with it more molecules of O2 implies a more efficient metabolical process to allow a lower breathing rate. We know that we don't need less energy at depth for the same energy-expending activity at the surface. If that is true than the O2 flow rate to the tissues to support the activity should be the same, and with it the breathing rate. The tissues will not require less O2 than it needs to support the activity with all other things being equal. We know that it is harder to breath a denser gas at depth than at the surface. This is because the greater number of molecules cannot flow through the same volume at the same rate as a lower density gas. The same is true for flow across the lung-blood barrier. The higher density gas cannot flow at the same rate given the same pressure drop. The lower flow rate is compensated somewhat by the higher density (O2 molecules) and the body gets satisfied. As long as the tissues get satisfied there will be a fairly constant ppO2 in the blood. If you slow down the breathing rate you reduce the average ppO2 in the lungs and since blood ppO2 is constant the pressure drop decreases. This reduces O2 flow and the body will increase respiration to make up the flow. So, breathing rates remain fairly constant for the same activity.

 

[TSandM]

EFX, that's an interesting analysis, but it isn't what actually happens. In people with normal lung function, breathing atmospheric air (or any gas with more O2 in it than that) there is plenty of oxygen to support metabolism. What determines minute ventilation is carbon dioxide, not oxygen.

When you reduce minute ventilation, eventually your alveolar ppO2 will fall, but only if you can reduce ventilation to the point where you are not providing enough oxygen for metabolic needs. This doesn't occur until pCO2 has risen to levels normal people simply won't tolerate -- they will be unbearably short of breath. CO2 will drive an increase in breathing long before oxygen does, and this is particularly true when breathing the elevated ppO2s of underwater.

 

[EFX]

I agree with you TSandM. I said in an earlier post that respiration rates were dependant on CO2 production. I wanted to show that respitory rates would not go down with increased O2 density (EVEN IF) the trigger depended not on CO2 but only on O2 levels. Using a simplified flow equation of flow rate = pressure drop / flow resistance of the lung-blood barrier shows this to be true. When I say O2 flow rate we really have to include blood flow rates. Blood is what carries O2 to the tissues. You need both diffusion (dependent on pressure drop) and perfusion (dependent on flow). And when we talk of perfusion we need to consider the vascular system and its 20-fold compliance to blood flow. Vasodilation can boost blood flow to further oxygenate tissues but the O2 cannot increase if the barrier can't supply the flow. Incidentally, a pressure drop also occurs between any two end points (ex. vein to capillary, capillary to tissue, etc.).

 

[Dr. Lecter]

Density of the breathing gas DOES affect flow through the small airways, when the density reaches a certain value. But it is all but irrelevant for people staying with recreational diving depths.

In the ~80'-130' range, whether or not increased density is irrelevant depends quite a bit on workload and diver fitness/physiology. Even outside of CCR, a lot of narc/panic issues in deeper recreational range diving are likely contributed to, if not solely caused by, CO2 retention. That in turn is coming from divers not understanding that the same breathing pattern that's sufficient for flitting about on a reef at 40' is not going to work well for them when kicking around a wreck at 120'.

Personally, I think everyone should just dive a DPV and be done with it

 

[Zee Pet]

Woah, sorry i thought i was getting mail notifications on this, guess not, oops. Ok so what i was thinking was that at 10 meters, you have the increased pressure, so more molecules of gas (but same partial pressures??) so you can take less air per breath in a way that you still only have the same molecules of oxygen at the surface, but in a smaller volume at depth? is rate dependent on the rate of co2 production, or co2 in the blood at the time? If later ill have to reconsider the way i'm looking at this, i may be on a different wavelength entirely leading to confusion.
(Hehe, get it? Wavelength... We're in water... ok back to serious now, sorry )

 

[Dr. Lecter]

You won't take less air per breath because you need to move the same lung volume to exhaust the CO2. Failure to do so (i.e., shallow breathing) won't hurt your O2 levels because you've got plenty of O2…but it'll increase your CO2 levels and that will in turn jack up your rate of breathing. Keep breathing shallow even at the increased rate and it'll continue to climb until you're in a world of hurt.

 

[Zee Pet]

Ahhhh, the "same volume to exhaust" did it, thanks. If you don't mind me asking, why is that so, is it some kind of diffusion/concentration thing; or is that what EFX was saying? I'm only up to secondary school biology[emoji15].

 

[eponym]

Ahhhh, the "same volume to exhaust" did it, thanks. If you don't mind me asking, why is that so, is it some kind of diffusion/concentration thing; or is that what EFX was saying? I'm only up to secondary school biology[emoji15].

Yes. in order to get rid of dissolved CO2, you need to exhale and then fill your lungs with CO2-free gas--in other words, inhale.