The reaction of carbon dioxide with water requires carbonic anhydrase, an enzyme in the red blood cell. Carbonic acid is a weak acid and partially dissociates into hydrogen and bicarbonate ions. The reaction goes from left to right in the systemic tissues where carbon dioxide is produced, and right to left in the lungs where it is eliminated through respiration. Because the concentration of hydrogen ions (H+) fluctuates the pH decreases slightly in the tissues and increases slightly in the lungs. The pH range of blood is 7.35 to 7.45 and is maintained by the buffering action of the dissociation products of H2CO3 as well as by the blood's protein buffers.