(i) RBCs and are of five types. Main

(i) Red blood cells or erythrocytes.

(ii) White blood cells or leucocytes.

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(iii) Thrombosis (platelets).

(i) Red blood cells or spherical, disc shaped. Human RBC’s are biconcave and are devoid of nucleus. Their number is about 5 million per cubic mm of blood. Their red colour is due to the presence of a pigment called hemoglobin.

Main function of Races is to transport oxygen with the help of hemoglobin.

(ii) White blood cells or leucocytes do not have a definite shape but possess a nucleus. These are bigger than RBCs and are of five types. Main function of leucocytes is to protect the body from microorganisms and provide immunity to body.

(iii) Blood platelets or thrombocytes are small bodies numbering about 2, 00,000 to 4, 00,000 per cubic millimeter mm3 of blood. They perform an important function of forming a clot on the skin or blood vessel in the event of an injury and thus prevent excess flow of blood from the body. Clot formation also prevents a loss of blood pressure which otherwise would reduce the efficiency of the pumping system.