Why doesn't oxygen cause decompression sickness?

[Smache]

We all know decompression sickness is caused by nitrogen (or helium) absorbed by tissues in the body. If a diver ascends too quickly, the gas doesn't have time to leave the body safely, and forms bubbles. I'm wondering why oxygen doesn't have the same problem.

One explanation I've gotten is that oxygen doesn't have a chance to get absorbed because it's metabolized. But any rebreather diver can confirm that most of the oxygen in each breath is not metabolized, and gets exhaled along with the nitrogen.

I've also heard that it's because oxygen is just not soluble in body tissues. That also seems unlikely, since there's at least enough oxygen dissolved in sea water for marine animals to survive.

Anyone have an explanation?

 

[Lorenzoid]

Oxygen in the blood is mostly bound to hemoglobin.

 

[lowwall]

Oxygen in the blood is mostly bound to hemoglobin.

mostly ≈ 98%

Excerpt from Physiology, Oxyhemoglobin Dissociation Curve by Daniel P. Kaufman, Patricia F. Kandle , Ian V. Murray, Amit S. Dhamoon

https://pubmed.ncbi.nlm.nih.gov/29762993/

"Oxygen is primarily transported throughout the body in red blood cells, attached to hemoglobin molecules. Oxygen is also dissolved directly in the bloodstream, but this dissolved fraction contributes little to the total amount of oxygen carried in the bloodstream. Henry’s Law states that the dissolved fraction is proportional to the atmospheric pO2, but the solubility of oxygen is so low that only 3ml O2/L of blood is dissolved at atmospheric oxygen tension. Hemoglobin carries 98% of the oxygen in the blood in the protein-bound form, approximately 197 ml/L. ... Arterial oxygen content is approximately 20 g/dL, the venous oxygen content is 15 g/dL, and dissolved oxygen contributes 0.1 g/dL in each case..."

Which means dissolved O2 may bubble if the ascent is fast and long enough and there isn't sufficient local deoxygenated hemoglobin to grab it, but the contribution from oxygen relative to nitrogen (or helium) is so small that it can effectively be ignored.

 

[Smache]

I guess I don't understand the connection. Why does the fact that oxygen is mostly carried by hemoglobin mean it doesn't cause decompression sickness?

 

[lowwall]

I guess I don't understand the connection. Why does the fact that oxygen is mostly carried by hemoglobin mean it doesn't cause decompression sickness?

Bubbles are made up of O2 molecules that come out of solution and clump together. Oxygen molecules that are bound to hemoglobin simply aren't affected by the pressure changes.

 

[Smache]

Bubbles are made up of O2 molecules that come out of solution and clump together. Oxygen molecules that are bound to hemoglobin simply aren't affected by the pressure changes.

That makes sense.

What about the 2% or so that isn't carried by hemoglobin? It seems like that 2% is still a significant amount of oxygen dissolved, it's just a small percentage of the total because of how great hemoglobin is at carrying oxygen.

My current mental model is that tissues are limited in how fast they can on or off gas because they have a limited blood flow and blood can only carry so much inert gas. But oxygen doesn't have this limitation and can equalize pretty much immediately because of how great blood is at carrying oxygen. Sound correct?

 

[ColinT3]

Need to consider if gases are metabolically active (O2) or not, ie inert gases in this context (N2, He). The Wiki on Decompression theory goes into detail around solubility and dissolved gas dynamics etc etc -https://en.wikipedia.org/wiki/Decompression_theory

 

[Miyaru]

That makes sense.

What about the 2% or so that isn't carried by hemoglobin? It seems like that 2% is still a significant amount of oxygen dissolved, it's just a small percentage of the total because of how great hemoglobin is at carrying oxygen.

My current mental model is that tissues are limited in how fast they can on or off gas because they have a limited blood flow and blood can only carry so much inert gas. But oxygen doesn't have this limitation and can equalize pretty much immediately because of how great blood is at carrying oxygen. Sound correct?

Maybe look at it this way:
Hemoglobine consists of 4 iron atoms: Fe
When oxygen attaches to hemoglobine, those 4 atoms oxidize: 4 Fe + 3 O2 → 2 Fe2O3
It is not a dissolved gas anymore.

When pressure goes up, additional oxygen can dissolve into the blood like other gasses. As @ColinT3 mentioned above, it's now about metabolic versus inert gas - nitrogen (and helium) are inert gasses which are not used (metabolized) in our body. Excess oxygen will be metabolized by your body, but the inert gasses stay at the same level (partial pressure). And that's why they can cause DCI.

But any rebreather diver can confirm that most of the oxygen in each breath is not metabolized, and gets exhaled along with the nitrogen.

It's a little different:
You breathe in 21% oxygen at the surface. Not all of that oxygen is going into your blood - the air you breathe out still contains about 16% oxygen. That 16% just went in and out of your lungs, not into your bloodstream.

 

[dmaziuk]

We all know decompression sickness is caused by nitrogen (or helium) absorbed by tissues in the body. If a diver ascends too quickly, the gas doesn't have time to leave the body safely, and forms bubbles. I'm wondering why oxygen doesn't have the same problem.

It does. I don't have the link but there is a study from the 50's where they managed to bend goats on pure O2. It should be findable on PubMed. It was probably the first time I saw the word "miraculous" in a scientific paper: the symptoms spontaneously resolved within minutes.

One speculation is that O2 is recognized as "good" and does not trigger immune response and inflammation, unlike inert gases.

 

[kinoons]

Which means dissolved O2 may bubble if the ascent is fast and long enough and there isn't sufficient local deoxygenated hemoglobin to grab it, but the contribution from oxygen relative to nitrogen (or helium) is so small that it can effectively be ignored

This explanation for the win. In short, yes O2 could form bubbles during assent. However the burden of bubbles from N is so much greater that and extra produced by the freely circulating O2 can be ignored.