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And the saga continues... (g) >Scott, > >Let me check, but I do not think that the statement: > >>4) CO binding is so thermodynamically favorable that binding is, >>practically speaking, permanent, essentially removing the bound >>hemo site from the pool of available transport. > >is quite accurate. At sealevel, with only 0.21 ATA of oxygen partial >pressure, this statement is essentially true, but in the presence of >high enough partial pressures of oxygen, monoxide will unbind from >hemoglobin. "Further more, it bind with about 230 times as much tenacity >as oxygen, which is illustrated by the carbon monoxide-hemoglobin >dissociation curve in Figure 41-12 (this is take from Guyton's "Textbook >of Medical Physiology", page 511 and 512). This curve is almost identical >with the oxygen-hemoglobin dissociation curve, except that the pressures >of the carbon monoxide shown on the abscissa are at a level 1/230 of those >on the oxygen dissociation curve. Therefore, a carbon monoxide pressure >of only 0.4 torr in the alveoli, 1/230 that of the alveolar oxygen, allows >the carbon moxide to COMPETE equally wit# the hemoglobin and causes half to >the hemoglobin in the blood to become bound with monoxide instead of oxgyen." Aren't we really saying the same thing? That ratio adds up to an overwhelming thermodynamic favorability for the CO binding over O2 in my book. And, barring administration of unusual and artificial driving forces to that reaction, the hemoglobin would certainly remain bound for the servicable lifetime of the molecule. Unusual and artificial driving forces such as high-pressure O2, aka one treatment for CO posioning. >"A patient severely poisoned with carbon monoxide can be advantageously >treated by administering pure oxygen, for OXYGEN at high alveolar pressures >DISPLACES carbon monoxide from its combination with hemoglobin far more >rapidly than can oxygen at the low pressure of oxygen present in atmospheric >air." Not possible in naturally occuring quantities and pressures of the gas. Hence my mention of unusual and artificial circumstances. >And, as it is stated in Mountcastle's "Medical Physiology", "The combination >of CO and Hb is freely reversible when the partial pressure of CO becomes >less in the alveolar air than it is in the mixed venous blood. The most >rapid elimination of CO is accomplished by (1) reducing inspired CO to >zero by removing the victim from contact to CO to insure a maximum partial >pressure gradient for elimination of CO, (2)increasing alveolar ventilation >by any appropriate means NOT including CO2 administration, and (3)employing >high inspired Oxygen tensions to facilitate the dissociation of CO from >HB. This is most effective if oxygen administration is accomplised in >a hyperbaric chamber, which allows alveolar oxygen partial pressure of >greater than 1.0 atmospheres absolute, up to a maximum of 2 - 3 ATA's >for short durations to accelerate the CO elimination.' (Mountcastle, " >Medical Physiology", volume 2, 14th edition.) But medical intervention is not what I was talking about. This is treatment, not what would happen to an end-user who unknowingly gets low-grade levels of CO in his mix. >4) CO binding, still the one-way trip, is driven less by the >>lowered pressure, but the bound molecules remain the same. > >So, as you can see, CO binding is not the "one-way trip" as you described >it. I'd hold to the contrary, John. Without medical intervention, or administration of the previously mentioned special and artificial environments, I'd say that it basically is. As you have eloquently presented, it is, however, certainly possible to reverse the process by administering high-pressure O2 and removing the CO. If the diver were to proceed to use his O2 bottle at the 20ft stop, this would happen, or if he gets treated by his doc. Barring either of those, wouldn't the 1/230 favorability ratio result in a no-go on CO disassociation from the hemoglobin binding sites? Again, I do not see how anything either of us has put forth contradicts that of the other. What say you? p.s. are you enjoying this as much as I am? ------------------------------------------------------- scot@bt*.co* Scot Anderson pp000082@in*.co* Voice: 703/761/6536 CIS:74147.2357 Fax: 703/556/9290
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