When carbon dioxide dissolves in water, it can do so as a gas dissolved in water or by reacting with water to produce carbonic acid. In the cells of your body, the rate of carbonic acid production is accelerated by the enzyme carbonic anhydrase, as indicated in the following figure.
Carbonic acid is known as a weak acid because it partially dissociates into the positive Hydrogen ions and negative bicarbonate ions. All of the above reactions are reversible and subject to the laws of mass action.
For example, when excess hydrogen ions are added to the system the equilibrium is shifted to the left. This means that some of the added hydrogen ions will react with the bicarbonate ions to produce carbonic acid and the carbonic acid will dissociate into carbon dioxide and water as shown:
When hydrogen ions are removed from the reaction, the equilibrium will shift to the right. More carbon dioxide will combine with water and more carbonic acid will be produced and more hydrogen ions and bicarbonate ions will be produced.
When hydrogen ions are added or removed from the reactions, the equilibrium shift to maintain a relatively constant hydrogen ion concentration.
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