An enzyme, present in red blood cells and kidney cells, that catalyses the reaction between carbon dioxide and water to form carbonic acid, which subsequently dissociates:

This reaction is one of the fastest known and controls the elimination of carbon dioxide from the body and the pH of urine. It also facilitates the transfer of carbon dioxide from the tissues to the blood and from the blood to the alveoli (air sacs) of the lungs. See also chloride shift; haemoglobinic acid.