Physiology of Finfish and Shellfish (2+1)

2.2.9. Blood and Circulation

The blood vascular system in fishes consists of the following components

1. Blood
2. Heart
3. Arterial and venous system

Blood are not in direct contact with the tissues or cells. The blood in fishes is red in colour and consists of fluid plasma and blood cells or corpuscles. About 2-3% of the body weight is of blood. Blood are present in the form of watery fluid (90%) or plasma having solutes in the form of proteins like albumin, globulin and fibrinogen, salts, enzymes, antibodies and hormones and consists of blood cells. It also contains the nitrogenous waste products of the body during their transfer to the organs of excretion. Different minerals like Ca, Fe, Na, K. Mg, P and Cl are present. The blood clotting factors prothrombin is present. The blood clots rapidly in fishes because of presence of thrombokinase in it. In the blood of fish, cells like RBC (erythrocytes), WBC (leucocytes) and thrombocytes (blood platelets) are present. Both RBC (erythrocytes) and WBC are formed from hemocytoblast precursor cells which may originate from a variety of organs but usually mature after they enter the bloodstream. Elasmobranch fishes produce RBC (erythrocytes) from the organ of Leydig (situated in the oesophagus), special tissue around the gonads and especially the spleen. Telostean hemopoietic (blood forming) sites are primarily the kidney and spleen. Fish bone has no marrow for hemopoiesis.