Air density and how it affects breath rate?

[Dr. Lecter]

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].

It's so because while a very small volume of gas exchange might be enough to give you sufficient O2 intake, you would be leaving the remainder of your lungs full of uncycled gas…and the CO2 diffusing out of your lungs into the gas filling them is evenly spread around. So breathing very shallowly might not deprive you of O2, but it will prevent you from exhaling the bulk of the CO2 your body is trying to get rid of.

And then bad things happen.

 

[Zee Pet]

Ok, I got it now. Inhaling a short breath will bring enough oxygen to be sufficient; but that oxygen will partly turn into CO2 and then you would have to exhaust that CO2 as well as the CO2 from the previous breath, but there isn't enough volume on a short breath to get rid of that CO2 as well so it remains and builds because not all oxygen converts 100% efficiently to CO2?? Close enough?

 

[EFX]

Close enough.
The O2 is not converted at the lung-blood barrier. Whatever the body doesn't need which is about 16% of the O2 in air we breathe is exhaled. Physically, we can't exhale enough to get rid of the CO2 on a short inhale. Try it. Take a short inhale and exhale as fully as you can. The most I can do is two breaths. Don't force this. I'm not a physiologist but it might be possible if you exhale forcibly enough to exhaust the residual volume you could collapse a lung -- game over. From a flow perspective a short inhale prevents a full exhale allowing the ppCO2 to build up in the lungs. Initially, the pressure in the lungs rises causing a reduced flow of CO2 out of the blood. If you continue shallow breathing the ppCO2 in the blood rises sufficiently to cause increased flow. To maintain a low flow the ppCO2 in the blood must keep rising because the pp in the lungs is rising. This is necessary to maintain the blood-lung pressure drop I spoke about earlier. At some point the high blood ppCO2 overwhelms the system and you pass out and maybe die from lack of O2.

Besides the problem of reduced gas flow at depths due to the higher density, overall lung volume is reduced because of the higher ambient pressure. This in turn can excaberate higher CO2 retention if the diver continues shallow breathing. In "Shadow Divers" there was a diver breathing air at 200 feet as part of a team exploring the U869 submarine. If memory serves me this diver toward the end of the dive returned to the bottom breifly to look at something. When the team looked back he was motionless. If I remember they didn't know what happened to him but it's possible he passed out from a CO2 builbup and drowned.

 

[Zee Pet]

pp CO2 is just the ratio of pressure of CO2 to the air in the lungs, right?

 

[Fish in a Barrel]

pp CO2 is just the ratio of pressure of CO2 to the air in the lungs, right?

No, partial pressure is the amount of pressure generated by a specific gaseous component. Under constant temperature and volume, N moles of CO2 will exert constant pP_CO2 pressure. This is true regardless of what else is filling the vessel (your lungs). If you add the partial pressure of all of the components (pP_O2 + pP_N2 + pP_CO2 + pP_Ar, etc.) together you get total pressure.

 

[Zee Pet]

Ahhh, yes. I need to go review that A-level chemistry book.[emoji40]

 

[EFX]

As mentioned before O2 and CO2 exchange in opposite directions across the lung-blood barrier at the same time. The flow rate is partially dependent on the pressure drop (gradient) across that barrier, with flow going from the higher to lower pressure. O2 moves from the alveoli (lungs) at a partial pressure (pp) of 160 mmHg to the capillaries (blood) sitting at 46 mmHg. The pressure drop is 114 mmHg. CO2 goes the other way from the blood at 46 mmHg to 40 mmHG in the lungs. The pressure drop is only 6 mmHg.

One way of describing pp is if you took a gas, say air which is composed of 21% O2 and 79% N2 (neglecting trace gasses), and filled a container at the surface the total pressure is 1 atm (atmosphere) or 14.7 psia. If, in that same container, you removed all the O2 the N2 would exert a pressure of 0.79 atm. 0.79 atm is the pp of N2 at the surface. Likewise, extracting all the N2 would leave a total pressure of 0.21 atm -- O2's pp.

Where this is significant is when calculating the effects of each component of the gas on the diver at depth. A tank of air at 33 fsw (feet of sea water) will provide the diver with 2 atm of air in his lungs in order to equilize at the ambient pressure of the water. The pp's of each gas is then double what it is at the surface so O2 = 2 x 0.21 or 0.42 and N2 = 2 x 0.79 or 1.58 atm (0.42 + 1.58 = 2.0).

Unfortunately, both O2 and N2 give divers problems at depth. O2 is toxic above about 1.6 atm pp. N2 becomes highly narcotic below about 150 feet (the effect is highly dependent on individual makeup). I use a maximum of 1.4 atm of ppO2 when diving on EAN (Enhance Air Nitrox) which is a gas with more than 21% O2 in it. How deep can I go on air keeping the ppO2 to less than 1.4 atm's? If each 33 feet of O2 is 0.21 atm then it would be 1.4 / 0.21 = 6.6 or ~ 7. Subtracting out 1 atm which is what we have at the surface means I can go to a maximum depth of 6 x 33 ft or 198 feet. Below that I'm at high risk for oxtox which manifests itself as a grand mall seizure -- not a very good thing. It usually means drowning. Even more restrictive is the effects of N2 at this depth which would mean severe impairment. If I decide to reduce the ppN2 to a level at 132 feet I must keep the ppN2 to under 132/33 + 1 or 5 x 0.79 = 3.95 atm. However, I'm diving to 198 feet or 6 atm. I'm 2 atm pp over on N2. To keep the ppN2 to 3.95 atm at 198 feet I need only 3.95 / 7 or 0.56 atm originally at the surface. The total mix of my gas becomes 21% O2 + 56% N2 or 77%. To get to 100% we add 23% helium to the mix. Helium does not have the narcotic effects of nitrogen. This combination of O2, N2, and He is called trimix.

 

[Zee Pet]

Woah, that is super helpful, thanks! You also just got me interested in taking a look at the EANX course....