The prevention of blood loss following rupture of blood vessels, which is effected by several physiological processes. Initially, bleeding is restricted by constriction of the damaged vessels, whose endothelial surfaces also stick together. The damage to the vessel endothelium exposes collagen, which attracts platelets to the site. These become sticky and release the vasoconstrictor serotonin and arachidonic acid, which is converted to thromboxane A₂. This attracts other platelets to the site, so that a plug forms. Neighbouring undamaged areas continue to release prostacyclin, which inhibits platelet aggregation and so prevents unnecessary enlargement of the plug. The various stages of blood clotting then ensue.