When you have a cut, blood will flow out from the wound. Putting it very simply, when a blood vessel injury occurs, three mechanisms swing into action.
First, blood vessels immediately contract, so reducing the rate at which blood is being lost. Second, the platelets adhere to the cut edges and quickly form a soft plug that further checks the loss of blood. Third, the platelet plug is reinforced by the production of fibrin, forming a hard, solid plug. In this way, within a few minutes, the blood loss can be satisfactorily stopped in most cases.
However, the final clot product, which is called fibrin, has to be manufactured when needed. This comes from a substance called fibrinogen, normally present in the plasma, the fluid part of the blood. But the fibrinogen must be activated to form the clot, and this is done by the action of another substance called thrombin. But, thrombin in turn comes from a substance named prothrombin, also normally present in plasma. When injury occurs, tissue extract is produced, causing two other normally occurring blood elements, thromboplastin and calcium, to work on the prothrombin to form thrombin, which combines with the fibrinogen to form fibrin, and so a clot is born that effectively seals off the break.
Blood clots occur at different speeds in different persons. People whose wounds clot slower than normal speed or don’t clot at all have a disease call hemophilia disease. A man who has hemophilia can transmit the disease to his wife’s off springs.