The main roles of platelets are: hemostasis, coagulation, vasoconstriction and repair of broken blood vessels. Platelets are small pieces of cytoplasm shed by megakaryocyte cytoplasm in bone marrow, with normal value of 100-300×10^9 cells/L, which have various roles: 1. Hemostasis: platelets have the role of adhesion and aggregation, when injured, the inner wall of the blood vessel is no longer flat, platelets can be directly adhered to the broken blood vessel wall, aggregated into clusters to form a thrombus, thrombus blocking the blood vessel fissure, maintaining the integrity of the blood vessel wall, to help stop bleeding. 2. Coagulation: platelets can release substances that promote blood coagulation, promote fibrinogen into insoluble fibrin, and form clots at the rupture of blood vessels to help stop bleeding. At the same time, platelets can release anti-fibrinolytic factors, inhibiting the activity of the fibrinolytic system, so that the formation of blood clots do not collapse. 3. Vasoconstriction: Platelets can release vasoconstrictors such as 5-hydroxytryptamine, catecholamines, etc., which make the damaged blood vessels tighten to varying degrees, while the blood flow in the tubes decreases, preventing blood loss. 4. Repair of broken blood vessels: platelets also have the role of nutrition and support of capillary endothelium, to help rebuild the endothelium of broken blood vessels. Platelets have the functions of hemostasis, coagulation, vasoconstriction and repair of broken blood vessels, and are an important member of the human coagulation system.