Nitrogen and Oxygen

[Akimbo]

It all comes back the concept of diffusion -- movement of molecules from higher to lower concentrations. The big win for divers is decreasing the concentration of diluent in the lungs -- the partial pressure matters, not the percentage.

So now partial pressure and percentage are unrelated? Or did you somehow manage to decrease the concentration of diluent in the lungs w/o changing its percentage? I'm confused here.

The PPO2 matters in my example because it takes into account the increased concentration under pressure. Pure Oxygen is 100% at any depth. However, pure Oxygen has twice the number of molecules of Oxygen compressed into the same volume at 33' or 10 Meters or 2 ATA as on the surface -- but is still 100% Oxygen. Nitrogen molecules in the blood are also doubled at this pressure so the difference in concentration between the lungs and bloodstream is doubled.

That was the context for this statement:

... the partial pressure matters, not the percentage.

Does that make sense?

 

[tarponchik]

@Akimbo: " Nitrogen molecules in the blood are also doubled at this pressure" only if you breathe air. Where would more N2 molecules come from if you breathe 100% O2?

"Does that make sense?" No It is really quite simple, do not complicate things.

 

[Kevrumbo]

@Akimbo: "It all comes back the concept of diffusion -- movement of molecules from higher to lower concentrations. The big win for divers is decreasing the concentration of diluent in the lungs -- the partial pressure matters, not the percentage" So now partial pressure and percentage are unrelated? Or did you somehow manage to decrease the concentration of diluent in the lungs w/o changing its percentage? I'm confused here.

@Akimbo: " Nitrogen molecules in the blood are also doubled at this pressure" only if you breathe air. Where would more N2 molecules come from if you breathe 100% O2?

"Does that make sense?" No It is really quite simple, do not complicate things.

This makes better sense:

Arterial PO2 is increased to a maximum tolerated value, either by increasing depth or increasing FiO2 of the inspired gas mix, or both. Oxygen Toxicity considerations clearly limits this depth & FiO2 dynamic to the max recommended ppO2 of 1.6 bar for in-water decompression, and 3.0 bar for Hyperbaric Oxygen Therapy (HBOT) in a dry Recompression Chamber.

For example: breathing a FiO2 of 50% at 70fsw/21msw in-water and then a FiO2 of 100% at 20fsw/6msw -all equal a ppO2 of approx 1.6 bar.

Breathing a FiO2 of 50% at 165fsw/50msw for HBOT and then a FiO2 of 100% at 60fsw/18msw -all equal a ppO2 of approx 3.0 bar.

Breathing a high ppO2 of oxygen at a deeper depth has a theoretical advantage of a greater hydrostatic pressure to hold dissolved gas in solution [i.e. Limiting venous blood supersaturation and bubble formation]; or in the case of DCI, reducing pathological bubble size and re-oxygenating hypoxic tissues.

Finally, inert gas elimination is independent of depth during 100% oxygen breathing. The inert gas partial pressure gradient for movement from tissue into venous blood is not controlled by ambient pressure in this case; it is controlled by the inert gas partial pressure in the tissue and in arterial blood. As long as the inert arterial gas partial pressure is zero, the diffusion gradient for inert gas removal from tissue is maximal.

 

[Storker]

Well, in theory 21% O2 and 79% argon wouldn't kill you.

Except that the WOB would be bigger than on air, since argon is denser than air.

Fun fact: In the 1989 movie "Dykket" (The dive), they use this as a twist to the story. The characters run out of gas and the only option left for the surface crew is to feed welding argon to their bell.

 

[tursiops]

Except that the WOB would be bigger than on air, since argon is denser than air.

Fun fact: In the 1989 movie "Dykket" (The dive), they use this as a twist to the story. The characters run out of gas and the only option left for the surface crew is to feed welding argon to their bell.

And what happens then?

 

[Storker]

They survive. Finally.

 

[tarponchik]

@Kevrumbo, @Akimbo "Finally, inert gas elimination is independent of depth during 100% oxygen breathing. The inert gas partial pressure gradient for movement from tissue into venous blood is not controlled by ambient pressure in this case; it is controlled by the inert gas partial pressure in the tissue and in arterial blood. As long as the inert arterial gas partial pressure is zero, the diffusion gradient for inert gas removal from tissue is maximal." Exactly! You can't talk of gas percentage and partial pressure like two independents, they are connected.

The author of this quote is mistaken in picking terms, however. There is no such thing as partial pressure of dissolved gas, there is only concentration of dissolved gas. Partial pressure applies only to gases, not to liquids.So change "the inert gas partial pressure in the tissue and in arterial blood" to "the inert gas concentration in the tissue and in arterial blood", and it sounds about right.

 

[Kevrumbo]

@Kevrumbo, @Akimbo "Finally, inert gas elimination is independent of depth during 100% oxygen breathing. The inert gas partial pressure gradient for movement from tissue into venous blood is not controlled by ambient pressure in this case; it is controlled by the inert gas partial pressure in the tissue and in arterial blood. As long as the inert arterial gas partial pressure is zero, the diffusion gradient for inert gas removal from tissue is maximal." Exactly! You can't talk of gas percentage and partial pressure like two independents, they are connected.

The author of this quote is mistaken in picking terms, however. There is no such thing as partial pressure of dissolved gas, there is only concentration of dissolved gas. Partial pressure applies only to gases, not to liquids.So change "the inert gas partial pressure in the tissue and in arterial blood" to "the inert gas concentration in the tissue and in arterial blood", and it sounds about right.

The author of the quote is a Physician (Eddie Brian MD), so physiologically as an applied extension of Henry's and Fick's Law, it's perfectly fine and consistent to use nomenclature like "partial pressure of gas in blood", or "blood gas tensions", or "Arterial Blood Gases" (which is an actual medical diagnostic test). Note the key terms of emphasis here are anything having to do with Tissues, Pulmonary/Respiratory Function and especially Blood. (The effect of hyperbaric oxygen on the oxygen window | Global Underwater Explorers)

For example, the necessary amount of oxygen for human respiration, and the amount that is toxic, is set by the partial pressure of oxygen alone. This is true across a very wide range of different concentrations of oxygen present in various inhaled breathing gases or dissolved in blood.

 

[tarponchik]

@Kevrumbo: So now you know the difference between a physician and a physicist.

 

[KevinNM]

Except that the WOB would be bigger than on air, since argon is denser than air.

Fun fact: In the 1989 movie "Dykket" (The dive), they use this as a twist to the story. The characters run out of gas and the only option left for the surface crew is to feed welding argon to their bell.

Argon is also is much more narcotic than Nitrogen. So do't do it.