Why does gas consumption increase with depth?

[mr.jadkowski]

Before you jump all over me about Boyle's Law, I get the relationship between pressure, volume, and density. My question is really about physiology: let's say, for example, I need to inhale 10 moles of O2 per minute and exhale 10 moles of CO2 per minute. At 2 absolute ATM of pressure those 10 moles should be contained in half the volume of gas than at 1 absolute ATM. In that example, at 2 ATA is it possible to exchange the required amount of O2 and CO2 using half the volume, either through taking half-volume breaths or by taking normal-volume breaths at half the normal rate? This is purely academic curiosity, and I haven't been able to find an answer on this board or via Google.

 

[Bowers]

Welcome to the beauty of rebreathers

We fill our lungs to normal volume which at depth means more molecules. We don’t use more O2 we just exhale more wasted O2. With that being said, when im completely relaxed at depth (on open circuit) i find i only breathe 6 to 8 breaths a minute and my pause is increased during a full lung. This makes sense because i need surface area to exchange waste co2 but need less intake of O2 do to greater partial pressures.

 

[Vicko]

Breathing underwater, and generally being underwater produces more co2 due to additional strain on your muscles, that then acts under higher partial pressure under corresponding chemical receptors causing you to feel the need to breather.

 

[lowwall]

I do long, slow exhales. I know I've got enough O2 to lower my breaths per minute, the continuous exhale keeps CO2 from building up.

 

[mr.jadkowski]

Welcome to the beauty of rebreathers

We fill our lungs to normal volume which at depth means more molecules. We don’t use more O2 we just exhale more wasted O2. With that being said, when im completely relaxed at depth (on open circuit) i find i only breathe 6 to 8 breaths a minute and my pause is increased during a full lung. This makes sense because i need surface area to exchange waste co2 but need less intake of O2 do to greater partial pressures.

So I suppose then that gas consumption doesn't necessarily have to increase at the same rate as pressure, if you are consciously (or unconsciously) slowing down your respiratory rate? Again, this is just academic, I'm not advocating for anyone diving to 4 ATA and breathing at 1/4 their normal rate!

 

[Jcp2]

Before you jump all over me about Boyle's Law, I get the relationship between pressure, volume, and density. My question is really about physiology: let's say, for example, I need to inhale 10 moles of O2 per minute and exhale 10 moles of CO2 per minute. At 2 absolute ATM of pressure those 10 moles should be contained in half the volume of gas than at 1 absolute ATM. In that example, at 2 ATA is it possible to exchange the required amount of O2 and CO2 using half the volume, either through taking half-volume breaths or by taking normal-volume breaths at half the normal rate? This is purely academic curiosity, and I haven't been able to find an answer on this board or via Google.

I suppose you could, if you had zero dead space and complete voluntary control over the size of your inhalation and exhalation volumes and were able to gauge it exactly to both depth and percentage oxygen within your open circuit scuba cylinder setup, because you are really trying to conserve oxygen. But there is dead space and breathing is only partially under voluntary control, so that it why there is gas switching on open circuit technical diving or close circuit rebreathers.

 

[Bowers]

So I suppose then that gas consumption doesn't necessarily have to increase at the same rate as pressure, if you are consciously (or unconsciously) slowing down your respiratory rate? Again, this is just academic, I'm not advocating for anyone diving to 4 ATA and breathing at 1/4 their normal rate!

Yes, you can breathe much slower but the danger with this concept is you must ensure adequate offloading of CO2. This means finding the balance between less breaths but enough exhaling.

 

[lowwall]

So I suppose then that gas consumption doesn't necessarily have to increase at the same rate as pressure, if you are consciously (or unconsciously) slowing down your respiratory rate? Again, this is just academic, I'm not advocating for anyone diving to 4 ATA and breathing at 1/4 their normal rate!

True, but there's a practical limit on how much you can slow it down, so at some point you'll be back to a linear relationship between pressure and consumption.

 

[mr.jadkowski]

Yes, you can breathe much slower but the danger with this concept is you must ensure adequate offloading of CO2. This means finding the balance between less breaths but enough exhaling.

Got it. From my understanding of respiratory physiology as your CO2 loading goes up your drive to breath will also go up, regardless of depth/pressure?

 

[Bowers]

Yes, respiratory drive is based on acidity of the blood. CO2 is an acid that will drop your ph and therefore trigger chemoreceptors to urge you to breathe. So there is a limit to how much you can slow your breathing. You can however do a few things to maintain an efficient exchange of CO2 even with slower respirations. I read a post here on SB a couple years ago in which the author of a reply described the breathing technique taught to him for a pure O2 rebreather. He described the procedure in detail and it makes a lot of sense. It also allowed him and his wife to have average rmv’s of 0.3 cf/m while on open circuit.
i have been using this practice ever since and now have an average rmv of 0.4 and on some dives see as low as 0.28.