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PaCO2 levels dnt rise as fast as PaO2 levels fall...

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PaCO2 levels dnt rise as fast as PaO2 levels fall...

Postby phuturephysician » Fri Jan 30, 2009 12:57 am

My professor made this statement and I'm trying to make sense of it, any help would be appreciated.

PaCO2 levels dont rise as fast as PaO2 levels fall b/c of a greater diffusibility of CO2.

Does it have to do w/ the different partial pressure gradients of the two gases when loading/unloading at the tissues?
I've got myself all wound up here. There is a difference in the way both gases travel in the blood. The vast majority of O2 binds to hemoglobin (w/ a weak, reversible bond) and approx 70% of CO2 is bound to HCO3-. But I do know that approx. 23% of CO2 is bound to hemoglobin. Am I in the right direction at all??? And then I think of the Bohr effect...
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Postby mith » Fri Jan 30, 2009 6:20 am

Your prof is saying that since o2 consumed equals co2 generated you'd expect same amounts in blood, however since partial pressure is only a measurement of dissolved gas in plasma, you can use the other (HCO3-) transport etc to account for the "missing" partial pressures.
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Postby favorina » Fri Jan 30, 2009 7:14 pm

The body tries to eliminate the CO2 as much as possible as it can cause a fall in the pH. He could have said that CO2 is eliminated at a faster rate than it is produced. This creates a constant and large conc gradient for more efficient removal of waste CO2 from cells as accumulation could be fatal.
Why the does it fall faster...?
+Constant flow of blood
+carbonic anhydrase in red blood cells (not in plasma!) speeds up formation on hydrogencarbonate ion. (although HCO3- formed through formation of the acid, this is not free in plasma and H+ readily taken up by heamoglobin in the red blood cell...bohr effect bla bla)
+CO2 combines with the protein part of haemoglobin.

only a small part (5%) of the original Co2 produced actually dissolves in the blood.

then what mith said :P
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