They are also termed as thrombocytes. These are small disc-shaped cells without a nucleus about one third the size of the red blood cells. There are about 2 to 5 lakhs of platelets in each cu.mm. of blood. They play an important part in the control of bleeding after injury and in the clotting of blood.
Blood maintains its liquid state as long as it remains in the blood vessel. If it is drawn from the body, it thickens and forms a jelly. This jelly or clot contracts or shrinks and a straw-coloured fluid called serum is squeezed out from it. If shed blood is microscopically examined, very fine threads will be seen. These threads entangle the blood cells and together with them form the clot.
The process of clotting is called blood coagulation or haemostasis. Its purpose is to prevent blood loss when a blood vessel is ruptured. The normal coagulation time varies from 3 to 8 minutes.
Four substances are necessary for the coagulation of blood: prothrombin, thromboplastin, calcium and fibrinogen. Fibrinogen, prothrombin and calcium are present in the circulating blood.
Thromboplastin is present in the tissues. When the blood is shed thromboplastin is liberated from the injured tissue. The thromboplastin acting upon the prothrombin in the presence of calcium converts into active thrombin.
Thrombin acts in turn upon the soluble protein fibrinogen converting it to insoluble fibrin which is deposited as fine threads to form the framework of the clot.
When more that 40 percent of the blood is lost from the body over a short period of time, the body is unable to repair the loss unaided. Some artificial means of replacing the lost fluid must be resorted to. The intravenous injection of blood is called blood transfusion.
Direct method i.e. direct from donor's vein to that of recipient. It is however, not undertaken in practice.
Indirect method : In this method the blood is collected, tested and stored. For transfusion purpose, blood banks are established. The blood is used as and when needed.
Plasma Transfusion is given in cases of burns etc.
Body conditions which need blood transfusion are (1) major operations (2) accidents involving blood loss (3) various blood diseases i.e. leukemia, severe anaemia and agranulocytosis. (4) In case of haemophilia with severe bleeding and (5) in carbon - monoxide poisoning cases.