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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 pK_a, 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.