Help me understand the bicarbonate buffer system

Nursing Students Student Assist

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I understand, at least in theory, the carbonic acid equation (H2O + CO2 H2CO3 H + HCO3), but I'm not understanding how the buffer system works in practice.

So, CO2 build-up is associated with acidosis...but CO2 is itself a neutral molecule, correct? Why is it associated w/ acidosis? When it dissociates, it turns into a hydrogen ion AND a bicarb...don't those two balance each other out?

I read that most CO2 in the body is carried as bicarb; in many instances, a bicarb level is the same as the carbon dioxide level. I really don't get this...again, doesn't CO2 dissociate into both bicarb AND hydrogen? So wouldn't the hydrogen concentration also reflect the CO2 level? Why is it even carried as bicarb?

This is giving me dyspepsia...please help!

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Hmm, let me see if my chemistry skills are on point! lol Well CO2 is neutral, but in the presence of water in our bodies it dissolves to form bicarbonate/carbonic acid. So if you have an excess of CO2 in the presence of water, you will yield more product (by Le Chatelier's Principle), an acidic molecule, leading to respiratory acidosis.

I don't know if it's OK with allnurses to post websites, but I found this one that explains it perfectly IMO: http://www.chemistry.wustl.edu/~courses/genchem/Tutorials/Buffers/carbonic.htm

The equation on that site gives a better picture of what goes on.

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I understand that CO2 combines with water to create hydrogen ions, but it also creates bicarbonate molecules...so why don't the H+ and HCO3- balance each other out? Why does the dissociation of H2CO3 lead to acidosis, as through no bicarb were produced at all?

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I understand that CO2 combines with water to create hydrogen ions, but it also creates bicarbonate molecules...so why don't the H+ and HCO3- balance each other out? Why does the dissociation of H2CO3 lead to acidosis, as through no bicarb were produced at all?

I think part of your confusion is in how acid actually exists in an aqueous (water) solution. The H+'s are chemically happier to be floating around by themselves than they are to be connected to something (that's why they're an acid). So, if you have a solution of hydrochloric acid (HCl) you wouldn't have HCl floating in water, you'd have H+'s and Cl-'s floating in water. That's due to the stability of the individual ions, they're attraction to each other, the other substance and water.

I guess the easiest way to explain it is that the H+'s are happiest (most stable) floating around by themselves until there are lots of HCO3's around and the draw is irristible (they're negative and the H's are positive)so the H+s finally join the HCO3 and become H2CO3.

Does that help? Or just further confuse you?

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Yes, thank you, that's going in the direction I need...is it then fair to say that, when H2CO3 dissociates, the resulting hydrogens are "stronger" than the bicarb in terms of the impact they have on pH? I was wondering if that was the case.

I also read that, in order to maintain a pH of 7.4, there must be 20 bicarbs to every one CO2...is that the case for the same reason?

Thank you!

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Yes, thank you, that's going in the direction I need...is it then fair to say that, when H2CO3 dissociates, the resulting hydrogens are "stronger" than the bicarb in terms of the impact they have on pH? I was wondering if that was the case.

I also read that, in order to maintain a pH of 7.4, there must be 20 bicarbs to every one CO2...is that the case for the same reason?

Thank you!

At the heart of it, a buffer system is a dynamic equilibrium, so it's going to constantly be adjusting to stay in it's most stable concentrations or proportions. So, it's not like none of the bicarbonate and hydrogen ions get together and suddenly they all of a sudden do. They're getting together and breaking apart and getting together and breaking apart, you get the idea. The dynamic equilibrium bit comes in with how many are more happy apart than together at any given time. So maybe instead of thinking about strength or weakness....think about concentrations. if you add something to one side.....it's going to push to the other side in order to maintain it's equilibrium (I think of one of those kids toys they're like a balloon tube filled some kind of gel goo, when you squeeze one end it all shoots to the other...same concept here). So, if metabolic acidosis adds more H+'s to the right side of the equation.....it's going to push through the equilibrium to the left until equilibrium is balanced again. If it can't do that....it will keep pushing till the buffer "breaks." In the body's case, it then starts to get rid of CO2 (Kussmaul breathing)....which then hopefully helps the body compensate and regain equilibrium (ie normalizes the pH).

The way they measure the "strength" of an acid, pH, is basically a measure of the concentration of hydrogen ions that are happy to be by themselves (it's an inverted log so the lower the pH the higher the concentration of H+ ions that are dissociated from another molecule). So, really strength is about concentrations.... but I think the image of one kind of ion being "stronger" than the other is messing you up a bit.

clear as mud? :-) Hope I didn't make it worse....let me know if I did!

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