Hemostasis is the term applied to the process that regulates the loss of blood from the circulatory system following injury. It involves three interrelated physiological mechanisms: constriction of blood vessels, aggregation of blood platelets to damaged subendothelial surfaces, and the formation of fibrin clots. Together these produce the vascular plug that prevents further bleeding. Abnormal function of one or more of the separate mechanisms may result in excessive bleeding or hemorrhage.