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From chucks post thought I'd make some things clear:
Myoglobin: oxygen binding protein in muscle cells which functions to store and
transport oxygen for mitochondrial oxidation. Has a very high affinity for
oxygen at low partial pressures of oxygen making it excellent for storage,and
terrible for transport.
Hemoglobin:oxygen transport protein in the erythrocytes which can carry up to
four oxygen molecules. Unlike myoglobin, hemoglobin has a low affinity for
binding its FIRST oxygen molecule at low ppO2, higher for the second, even
higher for the third and highest for the fourth. This phenomenon is called
cooperative binding and myoglobin does not do it to any significant extent.
This is what makes Hemoglobin ideal for transport and poor at storage whereas
myoglobin is great at storage and poor at transport.
Heme: an iron porphryrin (small iron containing molecule) which is identical
in both myoglobin and hemoglobin. The ferrous (+2) form of the iron allows the
heme to reversibly bind oxygen, whereas the ferric (+3) form of the iron does
not bind iron at all. Interestingly, free heme groups in an oxygenated
environment are rapidly oxidized to the ferric form.
Transport: The ppO2 in the lungs is roughly 13 kPa (kilopascals) which results
in an approximately 96% saturation of hemoglobin in the lungs with oxygen. As
the oxygen loaded hemeoglobin proceeds to the tissues (arterial system, almost
all exchange occurs in the cappilaries), it passes through regions of
progressively lower ppO2. The cooperative binding allows for the release of
oxygen based on surrounding ppO2, the lower the ppO2 the more oxygen is
released. The ppO2 of working muscle is about 1.5 kPa. The hemeoglobin
frequently returns to the lungs 64% saturated (no working muscle) and is again
replenished to 96%. Exchange occurs into the plasma from the erythrocyte and
then into the surrounding tissue and visa versa in the lungs.In the case of
myoglobin, if it were a transport protein it could only release 1-2% of its
oxygen at a ppO2 of 3 (non working muscle) allowing you to quickly die.
So what causes oxtox? Based on what I told you above I can make an educated
guess. THIS IS JUST A HYPOTHESIS based on the observations explained above.
The high ppO2s result in completely loaded RBCs (red blood cells) and
surrounding tissues. This continued hyper oxygenation converts the heme groups
from ferrous to ferric form, the RBC no longer carry oxygen well. On
conversion the heme would dump all of its o2 resulting in temporary oxygen
starvation and nerve cell death in the brain leading to seizures.
This is just a hypothesis, if you have heard different let me know.
Previous posts:
Hans,
In a message dated 97-11-26 05:56:45 EST, you write:
<< In practice you do metabolize any excess of
oxygen since the body draws from dissolved
oxygen before calling upon that bonded to
hemoglobin. Oxygen becomes a deco liability
at PO2s far into the tox zone only.
>>
Yes, I realize disolved O2 is used but this lowers the PPO2 around the
somatic cells in the intercellular fluids which causes O2 to difuse out of
the plasma to these fluids and then from the Heme bond through the
erythrocyte and plasma to replace that. Right ? Chain reaction stuff ?
Actually all the O2 that reaches the somatic cells comes through this
channel since the Hemoglobin never leaves the red blood cells - it simply
reacts to the chemistry of the plasma through the cell membrane. O2 delivery
to the cells is not an active transport phenomenon, it is by diffusion.
Also hemoglobin's affinity for either O2 or CO2 changes depending on the
chemistry of the local environement (alveolar or tissue capillary bed). I
think it is blood acid levels that partly determine this (carbonic acid ?).
Myoglobin also stores O2 in the muscles but the heme groups there can not
hold 4 oxygen molecules - I think it is only one each.
I have always wondered what part the lymph system plays in moving gasses
around during a dive's swings in partial pressures and changes in gases.
This is essentially a closed ended system but seems to have the ability to
move gasses at least slowly. Perhaps it is able to store some O2 in
solution benignly.
Curiouser and curiouser !
Can you imagine what our body temperature would climb to if we metabolized
the entire compliment of O2 we carried in the tissues on dives that held us
close to 1.2 PPO2 for much of the time ? Maybe we don't actually absorb
enough to cause this but it's kind of a funny thought - underwater
spontaneous human combustion.
So I'll replace it with this one. Is O2 metabolism dependent on the PPO2 of
the tissues or on the demand for energy ? Surely a man who goes to sleep in
a housing on an oil well site does not metabollize the same amount of O2 as
when he is rustling up some sushi.
Hemoglobin also carries CO2 from the tissues. Where does that put this
process of CO2 elimination if all the hemoglobin is bound up to O2 ? This
used to be a suspect in the mechanisms of O2 toxicity but I think it
eventually came under suspicion. I'm not sure if CO2 disolves more readily
in the plasma than O2 (I believe it does) but it would have to move in and
out pretty fast in the case of metabolising most of the O2 present on a dive
to avoid serious CO2 build up. The blood acid would have to rise accordingly
under higher metabolism of O2 regardless of level of activity if we simply
burned all of whatever was available.
One of my dive buddies on AOL reminded me once that the hemoglobin is
usually close to saturated with O2 at all times anyway (on the arterial side)
so that without this change of affinity due to local chemistry in the case of
high O2 content all the CO2 would have to be carried in solution most of the
time. This is not likely the case since the blood turns blue on the veinous
side and we don't seem to suffer from blood chemistry changes during a well
conducted dive.
If all the O2 is metabolized what is left to cause O2 Tox ?
< In other words,
even if you were able to incur oxygen bends
it would resolve as soon as oxygen deprived
tissues absorbed it! >
This seems to be what George is saying in his response -That what you do
get resolves quickly. But such pain as he mentions must be caused by a
localized concentration of O2 bubbles somewhere with a limited surface area.
No doubt it is absorbed quickly but with a molecular size similar to that
of N2 I can not see the dynamics as being much different from that of N2
bubbles.
Obviously I am missing something or am just plain ignorant of how O2 acts
in such cases. There must be some factor other than molecule size and
solubility in fat that makes O2 re-absorb more quickly and considering the
archetecture of the situation
(bubbles around tissue that is surrounded by other oxygenated tissues) I can
not see how metabolism (selective metabolism in this case) would draw from
these bubbles before it would from surrounding tissues and fluids where the
O2 molecules do not have to pass through an interface with a surface tension
to overcome.
This is obviously for bigger people than I to figure out.
Thanks for responding Hans; Doesn't this stuff make your head hurt. Think
I'll go get an Tylanol.
Chuck Boone
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