Biochemistry at The Biology Project

Clinical Correlates of pH Levels
Problem Set

Problem 1: What is blood bicarbonate?

Tutorial to help answer this question

Blood bicarbonate can be thought of as:
A. the ionized (deprotonated) form of carbonic acid, which is derived from CO2 and water.
B. the protonated form of carbon dioxide.
C. a transient species that cannot be measured in body fluids.
D. being formed from hydrogen peroxide via the action of catalase.


The Bicarbonate Ion
Bicarbonate ion (HCO3-)
The most important single buffer in human blood is the bicarbonate ion.

Bicarbonate, HCO3-, is in equilibrium with CO2 and H2CO3 (carbonic acid) as follows:

CO2 + H2O <--> [H2CO3] <--> H+ + HCO3-
from metabolism (volatile) always available unstable intermediate the pH term major species in blood

The central species in these reactions (H2CO3) is a weak acid with a pKa value of 6.14. However, H2CO3 is unstable, either breaking down to CO2 and H2O at pH values below its pKa, or (as is usually the case),to HCO3- and H+ in pH conditions above its pKa. Thus, only CO2, HCO3- and H+ can be measured effectively.

Carbonic anhydrase

The rate of formation of H2CO3 from CO2 and H2O, normally slow, is greatly enhanced by the enzyme carbonic anhydrase, found in red blood cells.

[Problem 1] [Answer] [Problem 2]

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Department of Biochemistry and Molecular Biophysics

The University of Arizona
January 1999
Revised: October 2004
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