Why do we breathe more at depth?

[Phil_C]

Was this not covered in your OW class?

I'm not so sure that it matters if it was covered in the OW class or not. Very often what people do when they are learning is accept what they see or have been told because it seems logical and sensible, then later when the concept has had a chance to sink in a bit better the "what if's" and "why's" start to crop up.

It certainly does with me - I will see or hear something which at first glance seems perfectly logical, but then as I think about it a little more the doubts in my understanding start to seep in so I start to ask questions to justify and qualify my beliefs and understanding. .

I will then start to finally properly 'understand' the concept after I have asked these questions and reviewed things again.

From a newbies perspective of the concepts around air use it COULD also be argued to be logical to say that as the amount of Oxygen in the air we breathe in a full breath at surface level is present in half that volume of air at 10 metres, so we only need to take in half a breath at 10 metres to get the oxygen we need - could be logical from one point of view but doesn't take into account the ability of the lungs to extract Oxygen from the volume of air in our lungs. This is why we take in air with 21% Oxygen and breath out air that still contains 15% Oxygen - our lungs just are not that efficient at extracting the Oxygen in the air we breath.

I know I had to think through quite a lot of the concepts behind the various gas laws and so on before I felt I could answer all the questions I had.

Phil

 

[Nirvana]

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

You ask why we breath at the same rate at depth, even though more oxygen molecules are coming into pur lungs (and indeed into our bloodstream) with each breath. It happens because our breathing rate is determined by the concentration of CO2 in the blood, not oxygen. The amount of CO2 produced depends on the level of activity of the body and is independent of the gas breathed (as long as it doesn't contain CO2, of course). Therefore, for a given level of activity, we will need a certain breathing rate to move the CO2 produced out of ours bodies, no matter how saturated with O2 we are. The conclusion is that breathing rate does not change with gas mixture and ambient pressure.

 

[TSandM]

Just a couple of other comments about your original post.

The lungs sit inside the (relatively) rigid chest wall like a balloon that's been inflated inside of a bottle. In the normal chest, there is no physical attachment of the outside of the lung to the chest wall. This is why any rupture of the lung allows the lung to collapse away from the chest, as air leaves the lung to fill the developing space between the lung and the ribs.

But, as has been discussed, the wonder of the scuba regulator is that there IS no pressure difference between the inside of the lung and the outside of the chest; air is delivered at the same pressure as what surrounds you, and the regulator automatically changes this with depth. If that weren't true, you wouldn't be able to dive very deep at all -- free divers can manage it because they never empty their lungs, but if you breathe out and the pressure around you is too high, you can't breathe in again.

It is true that there are a LOT more oxygen molecules in your lungs when you are breathing compressed gas at depth, than there are when you are breathing at the surface. If delivering oxygen were the only thing breathing was for, you wouldn't have to breathe much at all while diving. It isn't, though; the primary drive to breathe is set by the level of carbon dioxide in the blood. What gets rid of CO2 is washing it out -- it's linearly related to the VOLUME of air that you move through your lungs in any given time. And your body really doesn't tolerate high levels of CO2 well at all, because it changes the acid-base status of your blood, and that needs to be kept within very close limits for all the other chemical reactions you have to work right. So, whether the gas you are breathing is relatively thin, as it is on land, or relatively dense, as it is at depth, you still have to move the same VOLUME of gas through the lungs. You move a LOT more molecules at depth, but that isn't what matters.

 

[Qasar]

Thanks to all for your comments and now I understand more the process and reasons of a known fact.

I would like to stress Phil_C answer:

I'm not so sure that it matters if it was covered in the OW class or not. Very often what people do when they are learning is accept what they see or have been told because it seems logical and sensible, then later when the concept has had a chance to sink in a bit better the "what if's" and "why's" start to crop up.

It certainly does with me - I will see or hear something which at first glance seems perfectly logical, but then as I think about it a little more the doubts in my understanding start to seep in so I start to ask questions to justify and qualify my beliefs and understanding. .

I will then start to finally properly 'understand' the concept after I have asked these questions and reviewed things again.

From a newbies perspective of the concepts around air use it COULD also be argued to be logical to say that as the amount of Oxygen in the air we breathe in a full breath at surface level is present in half that volume of air at 10 metres, so we only need to take in half a breath at 10 metres to get the oxygen we need - could be logical from one point of view but doesn't take into account the ability of the lungs to extract Oxygen from the volume of air in our lungs. This is why we take in air with 21% Oxygen and breath out air that still contains 15% Oxygen - our lungs just are not that efficient at extracting the Oxygen in the air we breath.

I know I had to think through quite a lot of the concepts behind the various gas laws and so on before I felt I could answer all the questions I had.

Phil

This is exactly what I'm doing. I know what I need to known to dive, now I have 70 dives, but I was trying to explain this breathing matter to a non diver and I realised that I was not able to have a clear logic behind it, so I was looking for answers. By the way, saw the level of answers I received verbally from different experienced divers, I'm not sure to be alone.