Page "Bohr effect" Paragraph 9

Although the reaction usually proceeds very slowly, the enzyme family of carbonic anhydrase, which is present in red blood cells, accelerates the formation of bicarbonate and protons.

This causes the pH of tissues to decrease, and so, promotes the dissociation of oxygen from hemoglobin to the tissue, allowing the tissue to obtain enough oxygen to meet its demands.

Conversely, in the lungs, where oxygen concentration is high, binding of oxygen causes hemoglobin to release protons, which combine with bicarbonate to drive off carbon dioxide in exhalation.

Since these two reactions are closely matched, there is little change in blood pH.