dr tsuka
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Hello, this question might sound a little stupid, but anyway.
Suppose Respiratory acidosis: PCO2 = H + HCO3, If PCO2 increases, why is there an overall increase in H and not in HCO3 (HCO3 increases, but slightly) to the point where pH is normal? where is the rest of that HCO3-? What happens in respiratory alcalosis?
If feel ashamed asking this, but itīs a simple fact that always confuses me. Thank you.
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| ssrpk
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hey man! don't be ashamed , we are all on the same boat!
this fact is indeed very confusing, but it goes this way, HCO3- is used up to buffer excess H+ ions therefore the over all pH remains same in normal circumstances! HCO3- in itself is a weak buffer because of it's low pK value around 6.0 , therefore it is continuosly being reinforced by kidney and resipratory rate in order to maintain a normal pH.
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| cyra
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Consider the Henderson- Hasselbach equation:
pH= pK + log [HCO3]/S*PCO2 S=solubility conastant for CO2 in plasma
(.03 mmol/L/mmHg)
From the equation it is clear that the value of arterial pH depends on the RATIO of [HCO3] to S.CO2....not on the individual value of each variable...so if PCO2 increases as in resp. acidosis there is a decrease in the ratio of HCO3/S.PCO2...therefore the decrease in pH.Similarly in resp alklosis there is a a decrease in PCO2...theryby incresing the (HCO3)/S.PCO2 ratio and hence the pH.
I do understand your confusion...from what i had always understood and thought was that any increase in PCO2 would increase production of H2CO3 which should essentially dissociate to form H+...but then there is HCO3 too...and i would end up at the same confusing spot...why the overall increase in H+!!...and what happens to the HCO3!!!!(so you don't need to feel bad!)....until i came across this explaination for it...i know it doesn't really answer the queries we have...but it sure does make a lot of sense...we are probably just thinking too much!!!i hope i was of some help...
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| cyra
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oh and i forgot to add...pH ofcourse is the log of 1/[H+]...so an increase in pH would mean a decrease in [H+] and vice vers...
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| cyra
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Just to add to ssrpk's comment....HCO3 is part of the bicarbonate buffer and is as he said a weak one...it,however doesnot buffer carbonic acid...H2CO3 is buffered by plasma protein and the inorganic phosphate buffer....
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| dr tsuka
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First of all, thank you very much for the answers.
Basically, is it because 1 molecule of PCO2 + H2O DOES NOT GIVE 1 H and 1 HCO3-? Damn it! it does! Back to the same point again! grrrrr.
Kaplan states "For every H+ produced during the development of respiratory acidosis, one HCO3- is also produced. Some rise in HCO3- will not rise out fo its normal range" thatīs it!
They also focus the subject begining from the point that:
PCO2 = H + HCO3
So in respiratory acidosis, as PCO2 increases, the reaction shifts to the right, and in resp alk. it shifts to the left, I mean, what da hell?
I think I gonna let this one drop and continue with the remaining whole bunch. I donīt have extra disk space in my brain to keep processing this data. I think weīre thinking too much, this has become a more Philosophical cuestion, cause everybody just accepts the fact that increase PCO2 equals decrease in pH, and I am going to do that too!
Thanks again.
Edited by dr tsuka on 08/16/05 - 06:07 PM
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| mildus
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Thank you CYRA, I really like your explanation.
And about bicarbonate buffer: it is chemical buffer, but it is very significant although its pK isn't high enough because its components are regulated by physiological buffers (CO2 by lungs and bicarbonate anions by kidney) - it is connected with external environment thanks to lungs and kidney
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| p53
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I think you overcomplicate the matter. Actually, using normal values for CO2 tension, [H+] and [HCO3-]:
[H+]=24 PCO2 / [HCO3-]
From this equation it is apparent that:
1. An increase in the [H+] can be reduced toward normal by a decrease in CO2 tension, an increase in plasma [HCO3-], or both.
2. Vice versa.
Edited by p53 on 09/02/05 - 09:28 AM
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| mildus
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nice and simple
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| p53
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dr tsuka, I found best explanation for your question. First of all, let me say, that your equation PCO2 = H + HCO3 isn't right, actually, as cyra and I already have said, PCO2 = [HCO3-][H+]. You have confused it with another equation CO2 + H2O = H2CO3 = H+ + HCO3-.
OK, but your question is again ununswered - why equal increase in H and HCO3 due to rise in PCO2 results in respiratory acidosis, where is HCO3 to compensate H? you said that 'HCO3 increases, but slightly'. It isn't right - HCO3 increases exactly as much as H. Then why doesn't it compensate H? BECAUSE THEIR STARTING CONCENTRATIOS IN ECF ARE VERY DIFFERENT - the [H+] in the normal state is 40 nanoequivalents (nEq/L) per liter, whereas the [HCO3-] is regulated to 24 milliequivalents (mEq/L). Note that mEq is 1000 times more than nEq. A change of pH from 7.4 to 7.2 requires a rise in H from 40 to 60 nEq/L (a 50% increase). The total amount of HCO3 also rises by the same amount (20 nEq/L), however because the amount of HCO3 in the ECF is much more large, this change is relatively insignificant. The actual change in [HCO3-] is on the order of a 0.001% increase.
Any more questions?
Edited by p53 on 09/02/05 - 09:27 AM
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| cyra
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Thankyou p53!..it makes a lot of sense now!!...just one more thing....so if the starting concentrations of H+ and HCO3 are different...how is it that we have a neutral pH...is it because of the fact that the excess HCO3 forms part of the bicarb buffer of the body?
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| p53
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You should notice that pH is created only by concentration of H+. As you said 'pH is the log of 1/[H+]'. Other cations (for example Na+ and K+) and anions (for example HCO3- and Cl-) don't have any business with pH. For pH it doesn't matter how much [HCO3-] you have, matters only how much [H+] you have - more than in water (pH falls), less than in water (pH rises), or exactly the same as in water (neutral pH, =7).
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| cyra
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My bad!!... ..must' ve been the mid-day brain saturation setting in!!...thanx once again..appreciate it!
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| p53
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You're welcome. Just to clarify this point more - addition of the new HCO3- to ECF increases pH because it reacts with H+ and creates H2CO3, which then forms CO2 and H2O. CO2 goes to the lungs and is exhalled - you are left only with water with its neutral pH, i.e. the net result of one HCO3- addition is the loss of one H+. This means increased pH.
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| mildus
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p53, thanks very much, I really like your explanation.
You entered the essence of the problem!
Edited by mildus on 08/31/05 - 01:37 AM. Reason: change
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| ssrpk
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kool man, u have truely nailed the topic!
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