Breathing Theory

[WQFTruckster]

1 ATA is simply one atmosphere, or 10m/33 feet of seawater. The PPO2 is the partial pressure of oxygen. To figure the PPO2, divide the depth by 33', add 1 ATA, and multiply by the percentage of oxygen in the original mix. Therefore, a mix containing 16% oxygen (21% approx. in "air") would have a PPO2 of .16 at the surface, .32 at 33', etc. There are also limits to the PPO2. Beyond a certain point oxygen becomes toxic and a diver could black out and drown. A common suggested limit for recreational divers would be 1.4 PPO2. Under Mike's scenario, using .16 PPO2 as a minimum, you would be using a mix containing 16% oxygen. This would yield the .16 PPO2 at the surface. This mix would work to a depth of 255' if limiting the PPO2 to 1.4 ((1.4/.16) - 1 X 33').

Ah, I feel enlightened. Thanks for the explanations everyone.

 

[northcoast_diver]

Ah, I feel enlightened. Thanks for the explanations everyone.

The best thing to do is just remember the basics.

Breathe in, breathe out, breathe slighly slower than normal, and slightly deeper than normal, on scuba, and do not hold your breath. Some people call this "breathing normally."

There are exceptions. Photographers hold their breath when they snap a photo. Hunters hold their breath when they shoot their spearguns. But don't start holding your breath until you start hunting or photography. And then always remember that you are breaking a rule if you are holding your breath for any reason on scuba, and therefore you could die, by accident, worst case.

 

[roakey]

We don't count on being able to stay concious below 0.16 ATA PPO2

Of course, sitting here at home at my PC I'm breathing 0.156 PPO2. Which could explain a lot, I guess

Roak (Black Forest, Colorado)

 

[MikeFerrara]

Of course, sitting here at home at my PC I'm breathing 0.156 PPO2. Which could explain a lot, I guess

Roak (Black Forest, Colorado)

Do you use a travel gas?

 

[perpet1]

Also remember that the same principal of pressure at depth is working on your lung's volume. At surface you may have a lung capacity of 4 liters but at 1 atmo. it is cut to half or 2 liters. In math terms, twice the desity of air pressure in half the volume of space is the same volume of air at sea level.

James

Although an interesting post this is not the case.

Think of a diver at say 231fsw do you really think a deep breath would be a short gasp? Using your logic the volume of the lung would be so small that a single deep breath would be a very short gasp of air given your assumption that lung volume is 1/8 its original size.

In general terms if you were at 33' using your logic, a deep breath would take half the time to fill your lungs. I think we can all attest that this does not happen this way. Yes the air is twice a dense but the volume of your lungs do not change all that much.

If your logic holds true then the SAC would remain constant since the lung volume would be decreasing proportional to pressure just as density is increasing proportional to pressure. The net result being a SAC rate that is constant at all depths and I am sure most would agree this is not the case.

Pete

 

[Kamala]

This is a great discussion, but people seem to have gotten away from Truckster's question. The reason you still have to breathe just as much at depth is that neither the amount of surface area for gas exchange or the efficiency of that surface area in your lungs has changed just because you've submerged. The lungs can only do so much with the molecules that are presented to them. Similarly, you have to breath harder at altitude because the gas is less dense so there is less oxygen in a given volume of air.

 

[miketsp]

You managed to contradict yourself:

neither the amount of surface area for gas exchange or the efficiency of that surface area in your lungs has changed just because you've submerged.

The diffusion rate depends on the pressure gradient of the O2 and since your PP will be higher at depth the diffusion rate is faster ie more efficient.

Similarly, you have to breath harder at altitude because the gas is less dense so there is less oxygen in a given volume of air.

Since the O2PP at altitude is lower the diffusion rate is slower (less efficient) and you need to keep renewing the air to keep the O2PP average as high as possible.

 

[String]

Not seen this link posted so:

http://www.meddean.luc.edu/lumen/MedEd/medicine/pulmonar/physio/pf11.htm

good intro to breathing reflex.

As others have stated CO2 is the trigger for the reflex (or more accurately it lowers the pH of the blood triggering the receptors). Its the reason when snorkel free diving no more than 3 deep breaths are recommended. The reason being that you have removed near all of the CO2 from the lungs thereby supressing the urge to breath and free divers pass out near the surface when the amount of available O2 drops below the level needed to sustain conciousness but they havent needed to "breathe" as the CO2 concentration isnt higher enough to trigger the reflex.

As for the altitude comment there is a slight bit to add here, the increase of red blood cells to make the use of available oxygen more efficient. Hence the high altitude training for athletes competing at sea level.

 

[jonnythan]

People have missed one important thing about CO2. Your lungs don't respond to a percentage of CO2, they respond to an absolute amount. So, despite the fact that as you go deeper, you can fit many more times of CO2 molecules in your lungs, they "know" exactly how much is there and that's what forces you to breathe.

 

[jonnythan]

BTW, the same is more or less true for oxygen, I believe. I understand it like this: your body has a certain partial pressure of oxygen it tries to maintain in arterial circulation.. as you go deeper on compressed air, that partial pressure goes way up since the gas is so compressed. Therefore, you have way more oxygen than you need... but this is normally the case. CO2 still remains the limiting factor, just as it is on the surface, and you can't get away from that. Therefore, your minimum possible breathing rate is the same on the surface as it is at 200 feet, whatever your mix is (assuming your mix is reasonable for your depth ).