Breathing Theory

[WQFTruckster]

The reason we breathe as often as we do is because the body must get rid of the waste products from work, and in the case it is Carbon Dioxide ( CO2 ).

The body is monitoring the level of CO2 in the blood stream and when it reaches a certain point, then a signal is sent to the lungs to inhale and exhale.

I understand that you feel the need to breathe when CO2 levels reach a certain point, but it seems like it should take longer for the levels to reach this point if the lungs are able to "accept" twice as much CO2 when filled with air of twice the density. I guess I'm just way off in left field here, lol.

I know that things don't work this way, I'm just wondering why not.

 

[MikeFerrara]

What about 5% oxygen at twice the density, thus the same total amount of oxygen per lungfull?

You have the theory right which is why when we use hypoxic gas mixtures in diving we just don't breath them too close to the surface.

I don't know where the 10% came from though. We don't count on being able to stay concious below 0.16 ATA PPO2 and generally don't breath a gas on a dive unless we're at about 0.18 ATA PPO2 or above.

 

[WQFTruckster]

We don't count on being able to stay concious below 0.16 ATA PPO2 and generally don't breath a gas on a dive unless we're at about 0.18 ATA PPO2 or above.

I'm not familiar with the numbers you mention. Is ATA PPO2 "Atmospheres per part oxygen" ? I am intrigued. :lightbulb

 

[yknot]

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').

 

[pete340]

What about 5% oxygen at twice the density, thus the same total amount of oxygen per lungfull?

Yup. I was talking sea level, but convert to partial pressure if you prefer. 5% oxygen at two atmospheres is .1 atm. partial pressure, same as 10% at one atmosphere.

 

[Xanthro]

I've thought about this as well, and I think you don't need to breath as much when you are deeper, but it is not relational to with depth and surface breathing.

You have twice as much air in your lungs at 33 feet, but much of that air is not useable. Your body can only use that can reach the blood stream.

Think of your lungs as a ballon, but only the useable air is the part that touches the ballon. Now, your lungs have many small passages to increase air contact, but the analogy is the same.

The extra air in the middle of the ballon/lungs is unusable. As your blood passes through the lungs CO2 is released and O is taken in. Soon, the air in the tiny sacs where exchange takes place is saturated with CO2 and no more can be released. CO2 builds up in the blood, and you have the urge to breath.

It seems to me that a little more air is available at depth, but that much of the extra air is unreachable by the blood supply.

It could also be, that greater pressure relaxes me so I burn less energy and hence have less O use and CO2 buildup.

My breathing rate definitely goes down as I go deeper.

Xanthro

 

[pete340]

I don't know where the 10% came from though. We don't count on being able to stay concious below 0.16 ATA PPO2 and generally don't breath a gas on a dive unless we're at about 0.18 ATA PPO2 or above.

It's roughly the minimum that will sustain life. Not comfortably, though, and not necessarily conscious. So it's not something you'd aim at when diving.

 

[northcoast_diver]

So we can perform rescue breathing. 21% oxygen inhaled, 16% exhaled. You only need about 10% to sustain life.

If you breathed 10% O2 at the surface you would pass out. That is called a hypoxic mix.

You can breathe 15% O2 at the surface and not pass out, but you need to be careful not to be exerting yourself while you are breathing a mix like this at the surface.

Hypoxic mixes like 10% or 15% O2 require special training for diving. That is normally taught in an advanced trimix course.

 

[northcoast_diver]

... We don't count on being able to stay concious below 0.16 ATA PPO2 and generally don't breath a gas on a dive unless we're at about 0.18 ATA PPO2 or above.

Good advice.

This is normally taught in an advanced trimix course.

 

[northcoast_diver]

I'm not familiar with the numbers you mention. Is ATA PPO2 "Atmospheres per part oxygen" ? I am intrigued. :lightbulb

You have asked an interesting series of questions.

It is carbon dioxide in your lungs that triggers your brain to force your chest muscles to breathe. No matter how much oxygen is in a breath of air that you breathe, even pure oxygen, by the time (a few seconds) that your blood circulates through your lungs, there will be enough CO2 in your lungs to trigger your breathing reflex.

Now the story gets more interesting from here. But then the story ventures into breathhold freediving territory, and is outside of the realm of safe scuba diving. In other words, just like Costeau said when he coined the rule, Never Hold Your Breath On It (SCUBA).