Physiology of Finfish and Shellfish (2+1)

2.1.25. Carbon dioxide transported in the blood

The transportation of carbon dioxide is a very significant process of the gas-transfer systems within many animals. There are three main ways in which CO₂ is transported in the blood. A small percentage of the CO₂ that is in the blood is dissolved molecular CO₂. A larger amount of CO₂ reacts with –NH₂ groups of hemoglobin and other proteins to form carbamino compounds. However, most of the CO₂ that is transported in the blood is in the form of bicarbonate (HCO₃^-). The change of bicarbonates into carbon dioxide and water is catalyzed by the enzyme carbonic anhydrase. This enzyme is present in the acidophil cells of the gills, in red blood cells and in other tissues and not in the plasma. In general, CO₂ is diffused into the blood from the tissues. The blood transports CO₂ to the respiratory surfaces of the gills, where it is released into the environment. The blood mainly consists of plasma and erythrocytes (red blood cells). Most of the CO₂ entering and leaving the blood does through erythrocytes.

Mackerel is a fish of high seas that never encounters low levels of oxygen and lives under uniform but low carbon dioxide tensions. The mackerel blood is very susceptible to small changes in carbon dioxide, whereas blood of the carp and the bullhead catfishes often live in stagnant waters is relatively unaffected by changes in carbon dioxide levels.

Factors that influence oxygen consumption include activity, age, reproductive state, nutrition, disease and intrinsic regulatory mechanisms (nervous and hormonal control).

Increases in carbon dioxide tensions or decreases in pH lead to decreased oxygen consumption, but increased ventilation rate. Carbon dioxide is considerably more soluble in water than oxygen.