What happens to blood during exercise?

 

From Washington University

The acid base chemistry of carbonic acid maintains the pH of your blood in a range of 7.35 – 7.45. When carbon dioxide (CO2) gas dissolves in water, it forms carbonic acid:

CO_{2} (g) + H_{2}O (\ell) \rightleftharpoons H_{2}CO_{3} (aq)

Carbonic acid will donate a proton to water (this is the Brønsted-Lowry definition of an acid, bases are proton acceptors, so what’s water?) to form the bicarbonate ion (aka hydrogen carbonate) and the hydronium ion:

H_{2}CO_{3} (aq) + H_{2}O (\ell) \rightleftharpoons HCO_{3}^{-} (aq) + H_{3}O^{+} (aq)

Note that both reactions are written as equilibria (the \rightleftharpoons are the giveaways). Both bicarbonate ion and carbonic acid are present, and can react with excess acid (to produce carbonic acid), or hydroxide ion, resisting changes in pH. Such a system is a buffer.

Combining the two equilibria together produces the simultaneous equilibrium

H_{3}O^{+} (aq) + HCO_{3}^{-} (aq) \rightleftharpoons H_{2}CO_{3} (aq) \rightleftharpoons H_{2}O (\ell) + CO_{2} (g)

Excess CO2 (g) will end up being converted to bicarbonate and hydronium ions (decreasing pH). This is the medical condition known as acidosis. If there isn’t enough CO2 present, hydronium ions and bicarbonate ions in the blood will react to produce more CO2 and increasing blood pH. This results in alkalosis.

Both of these changes are summarized in general as LeChâtlier’s principle – if a system is in chemical equilibrium, it will respond to stresses (adding CO2, not enough CO2) to minimize the stress. Concentrations of species will change so that the equilibirum position is maintained.

Trivia: LeChâtlier is my academic “great grandfather”. My Ph.D. advisor’s Ph.D. advisor did his doctoral work with LeChâtlier. Pretty exciting, I know.

An excellent set of tutorials covering blood chemistry may be found at Washington University.