Carbon Dioxide

[turnerjd]

OK other medical people....

In one of the other threads there was a link to olympus diving, in north carolina, US. They have an article on why they want people to use lower ppO2. ( http://www.olympusdiving.com/po2s.pdf ) On page 2 is sais 'It [haemoglobin] is also responsible for transporting CO2, the by-product of metabolism, back to the lungs for exhalation. In the presence of both O2 and CO2, Hemoglobin will bind with the oxygen rather than the carbon dioxide.'

I haven't forgotten my basic biochemistry have I, but surely about 95% of the CO2 is carried back as dissolved bicarbonate?

If I am correct (which I hope I am) then why have they written such rubbish? It is hopelessly wrong. Oxygenated blood (ie with O2 bound to haemoglobin) at ppO2=0.21ata will be exactly the same as the oxygenated blood at elevated ppO2 since the binding is efficient (The figure I remember is 99% of binding sites occupied at ppO2=0.21ata). All that the higher ppO2 will do is make the transfer of O2 from the air sac of the alvaeoli to the haemoglobin molecule more efficient (which considering it is pretty darn efficient to start with is going to be a minimal effect). Muscles will still be using O2 at the same rate, freeing up Haemoglobin molecules at the same rate (which will only depend on work done, not ppO2)

If I am correct, that negates one of their main argument for a lower ppO2.

Jon T

Purely out of interest, I've heard of Joel Silverstein in some diving related context, but who is he? and what does he do? Robert Decker on the other hand is some-one I have never heard of, again does anybody know who he is?

 

[Rick Murchison]

From the encyclopedia...
"In the lungs, each iron atom combines reversibly with a molecule of oxygen. Each hemoglobin molecule also has attached a single cysteine amino acid, which attracts nitric oxide from the lungs. The enriched hemoglobin circulates and is carried through the body to the tissues, where the nitric oxide dilates the small capillaries, allowing hemoglobin to deliver its oxygen to the tissues. Then the oxygen- and nitric oxide-free hemoglobin molecule picks up carbon dioxide and free nitric oxide and transports both back to the lungs, where they are exhaled as waste."
Rick

 

[Katharina]

Hi!
Take a look at http://www.nitroxdiver.com!
He has been instructor for technical diving for many years already. Also author/co-author of books on nitrox and tec. Afaik a pretty famous person among tec divers.
Katharina

 

[large_diver]

Yes -- well-known tech diver and frequent publisher of books/papers on mixed gases and other tech diving topics.

Check his bio on his site --
http://www.joelsilverstein.com/about.html

He also received some press in the recent book "Deep Descent" about diving on the Andrea Doria.

 

[large_diver]

Here is some more info on Robert Decker. Some of the stuff he has published on mixed gas diving has been done in conjunction with J. Silverstein and one of the true pioneers of mixed gas diving, Billy Deans.

http://www.sportdiverhq.com/profile.htm

 

[turnerjd]

His diving techniques etc... might be good, but I think his basic biology / biochemistry is sort of screwed up.

I looked up a book (in french) by Dr P Forester(?) who is a doctor and diving medicine / hyperbaric physiology specialist, and in his book it clearly sais that CO2 elimination from the body isn't thought to be a major problem as 95% of it is transported from the muscle tissue to the lungs as dissolved bicarbonate.

If Ric has correctly quoted the PADI encyclopedia then some-one ought to tell them that it is wrong there too......

Jon T

 

[Rick Murchison]

The quote was from the "Electronic Encyclopedia" (encyclopedia.com)
Here's one from Encarta:
"Hemoglobin, iron-protein compound in red blood cells that gives blood its red color and transports oxygen, carbon dioxide, and nitric oxide. Hemoglobin is present in all but the least complex of animals."
Looks like CO2 is transported by hemoglobin as well as carbonate in solution.
Rick

 

[scubadoc]

Approximately 40% of plasma CO2 is either dissolved or bound to hemoglobin; most CO2 is carried via bicarbonate ions.

Remember this equation?
CO2 + H20 =>Carbonic anhydrase<= H2CO3 =><= (HCO3) + (H+)

Carbonic anhydrase is found in the cytosol of the RBC and catalyzes the formation of carbonic acid. This dissociation occurs forward and backward in the tissues and lungs.

So - the right answer is that both are correct. CO2 exists in many forms in blood all in equilibrium. These forms include CO2, H2CO3, and HCO3- in ratios of 1,000:1:21,000. The reaction CO2 + H2O -> H2CO3 is relatively slow and requires about 3 sec to reach equilibrium. In RBC,s carbonic anhydrase speeds this reaction to nearly instantaneous. H2CO3 -> H+ + HCO3- is very rapid without enzymes.

In systemic capillaries, CO2 diffuses into the blood plasma and into RBC's. In the RBC's, it may form carboxyhemoglobin or carbonic anhydrase may convert it to H2CO3 which breaks down into HCO3- and H+. The H+ also reversibly attaches to Hb forming HHb (reduced hemoglobin) which also reduces its affinity for O2 and adds to the Bohr effect. Because of the presence of the enzyme, this can all happen very quickly, long before the 0.1 sec it takes for blood to traverse the capillary.