Formed elements: RBCs, WBCs and platelets

Erythrocytes, leucocytes and platelets together are the formed elements, making up nearly 45 per cent of blood.

RBCs are the most numerous blood cells: on average 5 to 5.5 million per mm³ of blood in a healthy adult man. In adults they are made in the red bone marrow.

In most mammals RBCs lack a nucleus and are biconcave. Their red colour and name come from haemoglobin, an iron-containing complex protein; a healthy person has 12–16 g of haemoglobin per 100 mL of blood, and it is central to carrying the respiratory gases.

An RBC lives on average 120 days and is then destroyed in the spleen, which is therefore called the graveyard of RBCs.

WBCs (leucocytes) are colourless because they lack haemoglobin, have a nucleus, are fewer (on average 6000–8000 per mm³) and are generally short-lived.

Granulocytes are neutrophils, eosinophils and basophils; agranulocytes are lymphocytes and monocytes.

Neutrophils are the most abundant WBCs (60–65 per cent) and basophils the least (0.5–1 per cent). Neutrophils and monocytes (6–8 per cent) are phagocytes that destroy invading organisms.

Basophils release histamine, serotonin and heparin and are involved in inflammation. Eosinophils make up 2–3 per cent; they help the body fight infection and are associated with allergy. Lymphocytes (20–25 per cent) are of two main kinds, B and T, and both are responsible for immune responses.

Platelets (thrombocytes) are cell fragments budded off from megakaryocytes, special cells of the bone marrow. Blood normally holds 1,50,000–3,50,000 platelets per mm³.

Platelets release a range of substances, most of them involved in clotting. Too few platelets lead to clotting disorders and excessive blood loss.