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I'm trying to understand CO2 dissolved in blood, carbonic acid and the bicarbonate ion. I think of 'dissolving' as in NaCl dissociating into Na+ and Cl- ions sloshing around in water. CO2 isn't going to break apart. Do some CO2 molecules just float around in water/blood, or are they immediately latched on to by an H2O to become carbonic acid, H2CO3?

Or does it readily become H+ and HCO3- (bicarbonate) (Is it HCO3 or HCO3- as an ion to make the + - math work out?) My understanding is that some magical buffering operation is going on (in the body), where the balance between carbonic acid and bicarbonate ions shifts -- perhaps aided by an enzyme.

I'm an EMT trying to understand how 'respiratory acidosis' (presumably carbonic acid) is different from 'metabolic acidosis' (presumably CO2 in the blood in the form or bicarbonate?) These seem to be two things, not just 'CO2 dissolved in the blood'. When I ask medical doctors about this, they usually say 'Gee, I used to know that 30 years ago when I went to medical school'. I'm hoping that real chemists can describe accurately what is going on.

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    $\begingroup$ Gases can be dissolved without any changes in their chemical structures. For example, oxygen from air can be dissolved in water : about $10$ mg per liter. It is not much, I know, but when dissolved, it remains as $\ce{O2}$ molecules in solution. This is the same for $\ce{CO2}$, although a small proportion of these $\ce{CO2}$ molecules are transformed into $\ce{H2CO3}$, and then into ions. Unfortunately, it is difficult to calculate these proportions. $\endgroup$
    – Maurice
    Commented Apr 28, 2022 at 21:28
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    $\begingroup$ It would be better if you picked one - water, as for blood it's more complicated. Red cells bind not only oxygen but also CO2. $\endgroup$
    – Mithoron
    Commented Apr 28, 2022 at 22:11
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    $\begingroup$ Respiratory acidosis is indeed caused by excess carbonic acid in blood, but metabolic acidosis is other acids, eg lactic acid $\endgroup$
    – Andrew
    Commented Apr 28, 2022 at 22:31
  • $\begingroup$ @Mithoron. Lets pick blood, as that is where the respiratory vs metabolic acidosis is important. I understand now that CO2 is transported in blood in 3 ways, as carbonic acid, as bicarbonate ions, and as you said, bound to red blood cells. $\endgroup$
    – Ribo
    Commented Apr 28, 2022 at 22:34
  • $\begingroup$ @Andrew, 'other acids like lactic acid are part of metabolic acidosis' sounds like part of the answer. Good. Is another part of the answer the 'buffering' action of the bicarbonate ion 'changing to or from' carbonic acid? $\endgroup$
    – Ribo
    Commented Apr 28, 2022 at 22:38

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There are three relevant reactions here:

  1. Dissolution of gaseous carbon dioxide into aqueous solution: $\ce{CO2 (g) <=> CO2(aq)}$

  2. Hydration of carbon dioxide to carbonic acid: $\ce{CO2 (aq) + H2O <=> H2CO3 (aq)}$

  3. Deprotonation of carbonic acid to bicarbonate: $\ce{H2CO3 (aq) <=> HCO3- + H+}$

All three are in rapid equilbrium, with rxn 2 aided in the body by the enzyme carbonic anhydrase. In the blood, all three species ($\ce{CO2 (aq)}$, $\ce{H2CO3}$ and $\ce{HCO3-}$) are potentially present, but at quite different levels. The carbon dioxide can also be bound to hemoglobin, but I'm ignoring that.

The apparent pKa of carbonic acid is about 6.4, below the typical pH of blood (roughly 7.5), so most of the dissolved carbon dioxide is in the form of bicarbonate, which necessarily means that a lot of protons have been released into the blood, lowering its pH a bit (ie making it more acidic). That is important for respiration because it promotes release of oxygen from hemoglobin. The lower pH is also one of the primary mechanisms by which the body recognizes the need for oxygen.

Under typical conditions, about 5% or so of the carbon dioxide would be present as $\ce{CO2 (aq)}$ and the rest as bicarbonate or bound to hemoglobin.

When the acidic blood passes the lungs (ie get exposed to air with low $\ce{CO2}$ content, the rapid equilibria noted above result in the bicarbonate being converted back to $\ce{CO2}$ and released into the gas phase to be exhaled, restoring the blood to the original slightly higher pH.

"Respiratory acidosis" occurs when the lungs are not functioning properly, so that the carbon dioxide/bicarbonate is not effectively removed from the bloodstream and the pH remains low.

"Metabolic acidosis" is unrelated to carbon dioxide and instead results from buildup of other acidic compounds, such as lactic acid or acidic ketone bodies, the presence of which also results in a lower than normal pH of the tissue and bloodstream.

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