Nitrogen and Oxygen

[Akimbo]

Wikipedia actually uses respiration as an example to describe diffusion, bulk flow and Fick's Law. It's a good, light read: Diffusion - Wikipedia

My post was a little unclear. I think Mark's Powell's explanation of diffusion is especially good from the perspective of decompression, but the link I included was to Wikipedia. I felt that quoting that much from his book would be excessive and unfair to him. He uses the drop of ink in a glass of water analogy which is an excellent visualization tool.

I think Mark Powell's Deco for Divers does a pretty good job of explaining the concept of diffusion starting on page 30.

 

[KevinNM]

Yes, the key is the difference between the nitrogen percentage in the body and in the gas you are breathing. No nitrogen at all in the gas you are breathing gets rid of the most nitrogen each breath. You could breath 100% argon and get rid of nitrogen just as fast as breathing 100% oxygen. But you'd die.

Well, in theory 21% O2 and 79% argon wouldn't kill you. But you'd be so damn narced...

 

[boulderjohn]

Diffusion occurs in one direction or the other depending on the concentration gradient. Molecules diffuse from higher concentration to lower concentration until they are equal. The higher the gradient, the quicker the diffusion. Thus, breathing a higher concentration of oxygen, 100% being the highest, increases the concentration gradient for nitrogen and facilitates diffusion outward, off-gassing.

I will try to say the same thing in a different way that may help some readers.

The movement of the molecules is random. It goes in all directions at all times. When the body is at equilibrium, as it likely is now unless you have been diving in the last day or recently changed altitude, the randomness means that there is approximately the same number of molecules leaving the tissues as entering them. It is just a matter of the law of probabilities.

If you descend to 99 FSW, every lungful of air you inhale now contains 4 times as many molecules as it did on the surface. That means, again by pure chance, 4 times as many nitrogen molecules are entering the tissues as are leaving it. So the tissues get more and more and more nitrogen molecules moving around in them. If the diver stays at that depth as long as a specific tissue's halftime, then there will only be twice as many molecules entering as there are leaving. that slows down the rate at which the tissue gains more nitrogen. As it gets closer and closer and closer to equilibrium, the rate of increase gets slower and slower and slower.

When the diver ascends, there will come a time that there is more nitrogen in the tissues than in the air being breathed. When that happens, there will be more nitrogen leaving the tissues then entering them.

If a diver starts the dive with nitrox instead of air, the diver is breathing fewer nitrogen molecules, so the rate at which nitrogen enters the tissues will be lower.

It the diver breathers a high oxygen or even pure oxygen mix when ascending, the diver will have fewer nitrogen molecules in the mix, and the rate at which the tissues lose oxygen is increased.

 

[tarponchik]

How does oxygen promote evacuation of nitrogen? What is the mechanism?

Your question is about hyberbaric oxygen therapy, right? The right answer was instantly given by @victorzamora but oh, well, this is Scubaboard and everyone is entitled to say something.

In DCS treatment, you have to dissolve N2 bubbles that had formed already. And to dissolve them, you have to get rid of the N2 dissolved in body fluids. This will shift the equilibrium between the bubble and the body fluids towards bubble dissolving.

Let's say initially we have a 3-component system, "100% N2-bubble--air-saturated liquids--air". What happens if the patient breathes air? Since the system evolves toward equilibrium, some N2 in the bubble will dissolve but also some O2 will come into the bubble from the liquid. The system evolution dead-ends once the stage "air-bubble--air-saturated liquids--air" is reached. Therefore, if the patient breathes air, the bubble stays forever.

Now, if we change the system to "air bubble--air saturated liquids--O2", we'll create N2 concentration gradient that needs to be undone. The system can undo the gradient in one way only, by gradually moving N2 from the liquids, then from the bubble into O2 atmosphere. The end of the system evolution will be "O2-bubble--O2-saturated liquids--O2". Of course, now we have an O2-bubble instead of N2-bubble, but oxygen is our friend, right?

 

[Kevrumbo]

<Bump>

 

[scubadada]

Your question is about hyberbaric oxygen therapy, right? The right answer was instantly given by @victorzamora but oh, well, this is Scubaboard and everyone is entitled to say something.

In DCS treatment, you have to dissolve N2 bubbles that had formed already. And to dissolve them, you have to get rid of the N2 dissolved in body fluids. This will shift the equilibrium between the bubble and the body fluids towards bubble dissolving.

Let's say initially we have a 3-component system, "100% N2-bubble--air-saturated liquids--air". What happens if the patient breathes air? Since the system evolves toward equilibrium, some N2 in the bubble will dissolve but also some O2 will come into the bubble from the liquid. The system evolution dead-ends once the stage "air-bubble--air-saturated liquids--air" is reached. Therefore, if the patient breathes air, the bubble stays forever.

Now, if we change the system to "air bubble--air saturated liquids--O2", we'll create N2 concentration gradient that needs to be undone. The system can undo the gradient in one way only, by gradually moving N2 from the liquids, then from the bubble into O2 atmosphere. The end of the system evolution will be "O2-bubble--O2-saturated liquids--O2". Of course, now we have an O2-bubble instead of N2-bubble, but oxygen is our friend, right?

I don't believe the original post mentioned anything about HBO therapy or anything about the treatment of DCS
"How does oxygen promote evacuation of nitrogen? What is the mechanism?"

 

[boulderjohn]

I don't believe the original post mentioned anything about HBO therapy or anything about the treatment of DCS
"How does oxygen promote evacuation of nitrogen? What is the mechanism?"

Unfortunately, the question was ambiguous enough for a bunch pf people to understand its context differently and answer differently. I wish the OP had come back and clarified the question.

 

[Johnoly]

...Unfortunately, the question was ambiguous enough for a bunch pf people to understand its context differently....

Glad you said that......so here's my "de-rail" question

Why does helium make us talk funny when we inhale a balloon?

 

[scubadada]

Glad you said that......so here's my "de-rail" question

Why does helium make us talk funny when we inhale a balloon?

Decreased density of gas increases the speed over your vocal cords and raises the pitch.

 

[Texasguy]

To clarify, my question was about giving a suspected patient of DCS a 100% oxygen vs any other gas mixture.

@tarponchik Тарпончик, не срывай крышечку.