olena_r
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I have a question. I just read in kaplan notes that bicarbonate is the major (90%) form of CO2 in the blood. CO2 is converted to HCO- and H+ in the erythrocytes by carbonic anhydrase, H+ stays in the erythrocyte binded to Hb, and HCO- goes back to the blood streem.
So, logically , if CO2 production by tissues increases , there should be increase of blood bicarbonate, which is alkalosis, but in reality it's the opposite. Could someone please explain me this?
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| vrach
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Thanks for asking the Q Olena. This always bothered me, but I never got down to the bottom of it. After a little bit of searching I think I've cleared the confusion for myself atleast. So here it goes:
CO2 is transported in 3 forms- dissolved in plasma, attached to Hb, and the major form of HCO3-
CO2 in the presence of water spontaniously converts to H2CO3 (carbonic acid)- this is a weak acid.
CO2 in RBC is converted first to H2CO3, which then breaks down into H+ and HCO3-. The H+ is bound to Hb, and the HCO3- is transported into the blood in exchange for CL-, which enters the RBC. So there is no net increase in the anionic (negative charge/base) in the plasma/blood.
So when metabolic rate increases, this buffer system increases its conversion of CO2, and thus prevents acidosis (that could be caused by excess H2CO3), without any increase in the alkalinity of blood.
And CO2 is always expelled in the lungs, so even if there is and inrease in metabolic rate, there will be a corresponding increase in ventilation.
Here is the article that helped me understand this better:
http://www.nda.ox.ac.uk/wfsa/html/u13/u1312_01.ht...
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| mdwannabe
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Ooops
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| mdwannabe
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that is a very nice outline of the CO2 handling. Vrachebnie comments are quite usefull. :-) I guess I would add only that acidosis is rearly caused by overproduction, rather its the problem with buffering system, or CO2 elimination. Also, CA has to be mentioned as present only in RBC in relationship to plasma. Also consider the spontaneuity of the reaction in plasma.
I guess thats should explain the rest of your concerns.
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| olena_r
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Thank you very much.That was very helpful. I've also found an interesting fact that although CO2+H2O -> H2CO3 can go spontaineously, carbonic anhydrase increases the rate of the reaction 25000-30000 times
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| mdwannabe
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absoltely... also don't forget, it is working in reverse as well, catalizing just as well. :-)
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| sr
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this is a funny question. i had the same q and asked my prof. the answer i got wasnt helpful. basically she said you know that co2 is an acid, you know that from undergrad.
but i dont! if co2 is converted to h+ and hco3- and hc03- is reabsorbed and h+ is recycled then that means you keep making more hc03-.
i am confused as you can tell.
another prof said that every acid dissociates to h+ and a conjugate base and that my question is silly.
help please...
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| ramo7
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hey, its true what you are saying. the co2 reacts with the water to form an acid which then dissociates into h+ and hco3-(conjugate base) but the h= is buffered by the deoxyhaemoglobin so it wont form hcl(acid) with the cl- that comes into the rbc inplace of the hco3-. at the lungs the process is reversed and the co2 is regenrated to be expired. hope that answered your doubts.
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