Carbon dioxide excretion in fishes

Abstract

The pattern and control of carbon dioxide excretion in fish is reviewed with particular emphasis on the site(s) of bicarbonate dehydration, the involvement of diffusive and convective processes, and the relationship with ionic and acid–base regulation. The principal route for carbon dioxide excretion in fish involves the catalysed dehydration of plasma bicarbonate within erythrocytes to form physically dissolved CO2 and the subsequent diffusion of physically dissolved CO2 across the gill epithelium. It is likely that bicarbonate entry into the erythrocyte in exchange for intracellular chloride, rather than branchial CO2 diffusion or blood/water convection, is the rate-limiting process in carbon dioxide excretion, although a change in any one of these factors will affect overall CO2 elimination. Additionally, a relatively minor amount of CO2 is hydrated within gill epithelial cells to form H+ and HCO3− ions that are exchanged for Cl− ions and Na+ ions, respectively. Evidence is presented indicating that branchial and erythrocytic HCO3−/Cl− exchanges are under adrenergic control and that modulations of these processes by elevated levels of circulating catecholamines may be important in regulating acid–base disturbances.