An adult human body contains about 5 litres of blood. This blood is made up of a pale liquid called ‘plasma’ and millions of cells or corpuscles. Corpuscles are tiny red discs that give the blood its colour. The blood also contains white corpuscles. There are about 5 million red corpuscles and between 5,000 to 10,000 white corpuscles in 1 square millimetre of blood. Besides these, there are other blood particles called ‘platelets’, which are responsible for the clotting of the blood. When you get a cut on any part of your body, the platelets pile up around the wound and produce a substance called ‘thromboplastin’ from the blood plasma. Thromboplastin combines with calcium and prothrombin in the blood. These interact with fibrinogen (a protein present in the blood) to form fibrin. Threads of fibrin criss-cross each other to form a kind of dam to trap the blood. The resulting mesh solidifies. The top layer eventually dies, after forming a scab. When the damaged cells are completely replaced, the scab drops off. Platelets also produce a hormone called ‘serotonin’, which contracts blood vessels and stops the flow of the blood. Why don’t platelets always clot the blood flowing in our body? Our blood cells contain an acid called ‘heparin’ that does not allow the blood to clot while flowing inside the body. The time taken for the blood to clot varies from person to person. Those, whose blood clots very slowly or not at all, have a dangerous disease known as ‘haemophilia’. Even a slight injury suffered by a haemophiliac may prove fatal since the blood is incapable of clotting.