The cells found in blood are called blood corpuscles. These are of three types,(i) Red blood corpuscles(ii) White blood corpuscles and(iii) Platelets.(i) Red blood corpuscles: Red blood corpuscles are also called erythrocytes. The red colour of erythrocytes is due to the presence of haemoglobin, a red coloured pigment with high affinity for oxygen. The shape and size of RBC’s vary in different animals. The cells may be nucleated or non-nucleated. Actually, the nucleus is present in the RBCs of mammals initially, but when these cells finally differentiate and mature, the nucleus, mitochondria and endoplasmic reticulum degenerate. Such cells accommodate more haemoglobin for transportation of oxygen. About 4.8± 1.0 million RBCs per cubic millimetre of blood are found in adult women and 5.5± 1.0 million per cubic millimetre in adult men.Haemoglobin is a conjugated protein made up of globin and Fe²⁺ containing a  tetrapyrrole prophyrin ring called haeme. One molecule of haemoglobin binds to four molecules of oxygen. Erythrocytes also participate in transporting carbon dioxide from tissues to lungs. Carbon dioxide is mainly carried in both plasma and RBCs as bicarbonate. Carbon dioxide is also partly carried in combination with globin of haemoglobin. Erythrocytes have an average life span of about 120 days.(ii) White blood corpuscles: White blood corpuscles are also called leukocytes, as these are colourless. These are devoid of haemoglobin. Leukocytes are nucleated blood cells. The number of WBCs in adult humans is 7.5 ± 3.5 x10³ per cubic millimeter of blood. WBCs are of two types - granulocytes and agranulocytes. On the basis of staining characteristics of cytoplasmic granules and shape of nucleus, granulocytes are of three types (a) neutrophils, (b) eosinophils and (c) basophils. Granules are in abundance in the cytoplasm. These are phagocytic in action. Eosinophils are larger in size and with bi-lobed nucleus and abundant coarse granules. Basophils release heparin and histamines in the blood. Heparin is a natural anticoagulant. Agranulocytes are non-granular white blood cells. The nucleus is non-lobulated. These are of two types, (a) lympocytes and (b) monocytes. Lymphocytes have a large and rounded nucleus. Due to this reason, cytoplasm forms a thin peripheral film. These are formed in the bone marrow and are differentiated in the same place or in the thymus. The primary function of lymphocytes is to produce antibodies against antigens or to kill the invading pathogens directly in the body.(iii) Blood platelets: Blood platelets are also called thrombocytes because they secrete thromboplastin. These are the smallest blood corpuscles. Blood platelets are non-nucleated, round or oval, biconvex disc-like bodies. Their number normally varies from 0.15 to 0.40 million per cubic millimetre of blood. They bud-off from the cytoplasm of very large megakaryocyte cells of the bone marrow. When a blood vessel is injured, platelets get clumped at the injured spot and release certain chemicals called platelet factors. The thromboplastin, a protein-phospholipid complex, released from the platelets at the site of injury is an important clotting factor in blood coagulation. The oozed out transparent fluid after coagulation of blood is called serum. It means, serum is blood without corpuscles and fibrin protein.