They purify and protect the body from infections. Once an infection is detected in any part of the body, the while cells move in to fight it. There are between 6,000 and 8,000 white cells per cubic millimetre of blood. The white cells have a nucleus and take on various shapes as they circulate in the veins and penetrate all parts of the body. Platelets , or thrombocytes, are smaller than the red and white blood cells.
Platelets play a role in blood coagulation and wound healing.
When a blood vessel ruptures, platelets combine with fibrin, derived from fibrinogen, to form a clot. Red and white cells, and platelets must be replenished. This is the function of the bone marrow , a soft, gelatinous tissue, rich in fats and found in the internal cavities of the bones. Its role is to produce blood cells.