For transport of carbon dioxide in the form of bicarbonate, the dissolved carbon dioxide in the blood reacts with water to form carbonic acid (H2CO3). This reaction is very slow in blood plasma, but occurs very rapidly inside RBCs because a zinc containing enzyme, the carbonic anhydrase is present in RBCs. It accelerates its rate about 5000 times. This enzyme not only speeds up the formation of H2CO3 but also rapidly converts it back to carbon dioxide and water when blood reaches the lungs.