Help w bicarbonate buffering system

[theliman]

Hi there,

I've always thought I had a reasonable grasp on ABG's, but I frequently find myself coming back to the same question, thinking I've resolved it, then thinking about it again.

If we get into an acidotic state and our kidneys are functioning, after some time they will be able to help compensate by holding onto/secreting bicarb (in addition to excreting H+). We seem to mean a few different things when we say bicarb sometimes (serum CO2, ABG HCO3-), but in the context of the gas it would be specifically HCO3-.

So, with compensation, we will see elevated HCO3-, yes? Now, my main question is, if the buffering system involves this equation:

CO2 + H2O H2CO3 HCO3- + H+

then I would think that HCO3-'s buffering action is to combine with the H+ to create H2CO3 (and perhaps then move towards CO2 + H2O?). Which would use up (bring back down HCO3-), wouldn't it?

I haven't taken chemistry since 10th grade (15 years ago) so I need to brush up on that and plan on taking it soon.

But, in summary, I'm wondering: If our kidneys increase HCO3- to compensate, why is it elevated in the blood if its mechanism of compensation is to combine with H+ and form something else? Or do I have the mechanism wrong?

Thanks!

[HeartRN13]

I think you're overthinking it, if you're coming from a nursing perspective. Sorry to answer the question in a way that isn't helpful, but I'm an ICU nurse as well and I wouldn't lose sleep or overthink this!

[HeartRN13]

Good luck!

[Greenclip]

This situation applies to long-term compensated respiratory acidosis. The compensation equation is never going to tip back to the left (CO2 and H2O) because the person is not blowing off their CO2...they are retaining it. The HCO3- mops up the extra H+ in the bloodstream, yes, but because it's a weak acid it is easily broken apart when it gets to the kidneys. The H+ is excreted via the kidneys by combining with ammonia and phosphate. NH4+ and H2PO4- go out in the urine. The bicarb is re-generated by the kidneys, and goes back into the bloodstream to get to work all over again.

I think that taking college level chem will help. But it will also help to get a pathophys book and review urine formation. A good book which covers details will probably answer your question much better than I can!

[theliman]

ugh, post deleted for some reason.

thank you Greenclip for your help, I did some more looking into this last night and what you say falls in line with the main ideas I gathered.

This website was especially helpful, and the graphic 2/5 down Electrolytes and Acid-Base Balance - UCSD Lab Medicine "renal reclamation of bicarbonate." other useful ideas were that the increased bicarb is also a marker of H+ excretion (see graphic). also, acids/bases/etc. are constantly associating/dissociating so it's not like all of the bicarb floating around is immediately "used up" or bound to H+ to buffer.

thanks again

[Dodongo, APRN, NP]

The arterial blood gas machines that the RTs use measures pH and pCO2 and then calculates a bicarbonate value using the Henderson–Hasselbalch equation. What is actually being measured is the serum total CO2, which includes bicarb, but also all available forms of carbon dioxide.