Platelets function both mechanically and biochemically in the process of hemostasis. When injury to a blood vessel occurs, platelets aggregate forming a plug which helps to stop the flow of blood. They release certain substances, among them serotonin and Platelet Factor 3. Serotonin causes the blood vessels in the area to constrict, thereby further stopping the flow of blood. Platelet Factor 3 catalyzes the coagulation reaction whereby a fibrin clot is formed, completing the seal. Platelets also maintain the integrity (leak-free) state of blood vessels.