TRANSPORT AND EXCHANGE OF RESPIRATORY GASES IN THE BLOOD | Carbon Dioxide Transport and Excretion

Abstract

Carbon dioxide excretion varies between fishes. Excretion of CO2 in teleost fish is limited by the rate of bicarbonate–chloride exchange across the erythrocyte membrane. Carbon dioxide and oxygen transfer are tightly coupled in the red-blood-cell (RBC) membrane with carbonic anhydrase activity restricted to the RBC and not the plasma. Coupling is much less in elasmobranchs with a broad distribution of carbonic anhydrase in both RBCs and plasma. Lampreys and hagfishes lack band-III protein and there is no chloride shift associated with carbon dioxide excretion. In general, CO2 levels are low in fish because of the countercurrent nature of water and blood flows in the gills, the major site of CO2 excretion. Air-breathing fishes have an air-breathing organ to take up oxygen but excretion of CO2 is usually still across the gills into water. Some marine fish excrete CO2 across the gut, forming calcite and white chalk-like feces. Acid–base balance is not regulated in fish through modulation of CO2 excretion but rather through control of bicarbonate and proton excretion.