Why do we breathe more at depth?

[Qasar]

Hi all
I have a very basic question about theory.
I know the Boyle law, I learned all theory for qualification, but something still bother me: why do we really breathe more at depth?
Of course Boyle law said that air is compressed, but lungs are shrinking too, so you need less air to fill them. Someone told me that lungs are not shrinking the same way because the ribs are protecting them, so the volume is bigger, so more air.
I'm not 100% convinced with that (are the lungs really stuck to the ribs? Couldn't they shrink within the ribs volume?), but this raised another question, what we need is not a "volume" of air, it's a certain number of oxygen molecule, so when air is shrinking we still have the same number of O2 molecules, so we shouldn't need more air...
So, I know the answer is: we breathe more at depth!, but I don't fully understand the theory behind that. Could someone help me?
Thanks a lot
Qasar

 

[Rhone Man]

Your lungs don't shrink at depth unless you are free diving. On scuba they hold just as much capacity as depth, but the density of air is a multiple of what it would take to fill them at the surface.

I'll see if I can find and post a good graphic to illustrate.

 

[Rhone Man]

Here you go:



This link may also help explain further:

Phys

 

[Qasar]

Thank you Rhone man for that.
I read the document on the link, it's very interesting. I guess I miss another thing in my theory: our second stage reg will automatically deliver air at the ATA given by the depth?
I have also a concern with the article I quote here:
“An easy way to think of partial pressures of gases is that the partial pressure represents an absolute concentration of that gas, regardless of depth or pressure. If a person breathed a gas mixture containing 80% oxygen at the surface, the oxygen partial pressure would be 0.8 ATA, which is exactly the same partial pressure of oxygen when breathing air at a depth of 99 feet (30 meters). In both cases (80% oxygen at the surface and air at 99 feet/30 meters), the concentration of oxygen molecules in the lungs (i.e., the total number of oxygen molecules in the lungs on each inhaled breath) is the same.”
I think that there is a mistake here between percentage of volume (80/20) NOT same PP (e.g. 0.2 surface and 0.8 at 30m) and the total number of O2 molecules is not the same in the lungs depending of the depth?

 

[Jerry_]

Hi Qasar,

Yes, the regulators provide you with gaspressure equivalent to ambient pressure, otherwise your lungs wouldn't be able to take it in. A nice exercise that I do occasionally with my students is to give them an extended tuba (about 1.5m long), allowing them to use it at a depth of 1m. Even at such shallow depth and a pressure of 1.1 atm. your lungs will not be able to create sufficient underpressure to take air in.

As regards the example and the ratio, it is correct. The seamingly error may come from the fact that, while air is composed of approx. 21% O2 and 78% N2, the same ratio applies when breathing air at depth; however there is the coincidence of the 30m - and therefore 4 ATA - applying for the PPO2, meaning that you have 4x0.21, resulting in a PPO2 of 0.8(4), similarly the PPN2 becomes 4x0.78 (i.e. 3.12)

 

[Agility]

The drive to breath is triggered (mostly) by the CO2 in the blood, and to get rid of it you need to replace the volume in your lungs by fresh gas (air)

 

[Qasar]

Thanks Jerry

 

[RJP]

Was this not covered in your OW class?

 

[PsyWulf]

Perhaps he didnt feel to ask for clarification,or thought he understood it correctly at the time

 

[herman]

Thank you Rhone man for that.
I read the document on the link, it's very interesting. I guess I miss another thing in my theory: our second stage reg will automatically deliver air at the ATA given by the depth?
I have also a concern with the article I quote here:
“An easy way to think of partial pressures of gases is that the partial pressure represents an absolute concentration of that gas, regardless of depth or pressure. If a person breathed a gas mixture containing 80% oxygen at the surface, the oxygen partial pressure would be 0.8 ATA, which is exactly the same partial pressure of oxygen when breathing air at a depth of 99 feet (30 meters). In both cases (80% oxygen at the surface and air at 99 feet/30 meters), the concentration of oxygen molecules in the lungs (i.e., the total number of oxygen molecules in the lungs on each inhaled breath) is the same.”
I think that there is a mistake here between percentage of volume (80/20) NOT same PP (e.g. 0.2 surface and 0.8 at 30m) and the total number of O2 molecules is not the same in the lungs depending of the depth?

The statement is half right. The ratio is the same 80/20 but the total number of molucules of oxygen (and the other gasses for that matter) are increased as a function of atmopspheres....2 atm there are twice as many per breath as there are at the surface, 3 atm = 3x the molecules and so on. At depth we are breathing a compressed gas at a pressure equal to the surrounding water pressure, twice the ambient pressure means twice the molecules in each breath but the volume is the same.

Going back to your original question, regs deliver gas at the same pressure as the surrounding water.....to be more accurate, at ambient pressure reguardless of the fluid or gas around it, at altitude it would actually reduce the pressure. Because of this, the pressure inside your lungs is the same as the water on the outside so the differential pressure is the same as it is on the surface. Ambient pressure - lung pressure = 0 psi....with the exception that when you breath your lungs expand or contract to change that balance, ie lung pressure will increase or decrease SLIGHTLY which causes the gas to move. The total volume of your breath does not change as depth varies but the total number of molecules of gas that are removed from the tank with each breath changes dramatically. If you take 4x the molecules out of the tank with each breath, the tank will only last 1/4 of the time.