haemostasis 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 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.

haemostasis Process by which bleeding stops. Blood vessels constrict, platelets aggregate, and plasma coagulates to form filaments of fibrin.

haemostasis (hee-moh-stay-sis) n. the arrest of bleeding, involving the physiological processes of blood coagulation and the contraction of damaged blood vessels. The term is also applied to various surgical procedures used to stop bleeding.