The enzyme carbonic anhydrase (CA) catalyzes the reversible hydration/dehydration of CO(2) and water, maintaining a near-instantaneous equilibrium among all chemical species involved in the reaction. CA is found in association with all tissue and organ systems involved in the transport and excretion of CO(2), from the site of CO(2) production, metabolically active tissue such as muscle, to circulating red blood cells in the vasculature, to the various organs of gas exchange, the lungs and gills. The presence of the enzyme in every fluid compartment along the pathway of CO(2) transport appears necessary in order to allow the dehydration of HCO(3)(-) to keep pace with the rapid diffusion of CO(2) across biological membranes. Within the actual organ of gas exchange, CA is compartmentalized in multiple subcellular fractions, with the specific subcellular localization determining the enzyme's physiological function.