Carbon dioxide forms carbonic acid (H2CO3)
in water (or blood).
Carbonic acid is in dynamic equilibrium with hydrogen ion (H+) and bicarbonate ion (HCO3-).
The HCO3- removes excess H+ in the blood by forming H2CO3.
If blood is low in H+, the H2CO3 dissociates to restore H+ concentrations.
Carbonic acid is a buffer that maintains blood pH near neutral.