Hemostasis

In biology, hemostasis or haemostasis is a process to prevent and stop bleeding, meaning to keep blood within a damaged blood vessel (the opposite of hemostasis is hemorrhage). It is the first stage of wound healing. This involves coagulation, which changes blood from a liquid to a gel. Intact blood vessels are central to moderating blood's tendency to form clots. The endothelial cells of intact vessels prevent blood clotting with a heparin-like molecule and thrombomodulin, and prevent platelet aggregation with nitric oxide and prostacyclin. When endothelium of a blood vessel is damaged, the endothelial cells stop secretion of coagulation and aggregation inhibitors and instead secrete von Willebrand factor, which initiate the maintenance of hemostasis after injury. Hemostasis involves three major steps: vasoconstriction temporary blockage of a hole in a damaged blood vessel by a platelet plug blood coagulation (formation of fibrin clots) These processes seal the injury or hole until tissues are healed. The word hemostasis (ˌhiːmoʊ-ˈsteɪsᵻs, sometimes ˌhiːˈmɒstəsᵻs) uses the combining forms hemo- and -stasis, Neo-Latin from n αἱμο- n haimo- (similar to αἷμα n haîma), meaning "blood", and στάσις n stásis, meaning "stasis", yielding "motionlessness or stopping of blood". Hemostasis occurs when blood is present outside of the body or blood vessels. It is the innate response for the body to stop bleeding and loss of blood. During hemostasis three steps occur in a rapid sequence. Vascular spasm is the first response as the blood vessels constrict to allow less blood to be lost. In the second step, platelet plug formation, platelets stick together to form a temporary seal to cover the break in the vessel wall. The third and last step is called coagulation or blood clotting. Coagulation reinforces the platelet plug with fibrin threads that act as a "molecular glue". Platelets are a large factor in the hemostatic process. They allow for the creation of the "platelet plug" that forms almost directly after a blood vessel has been ruptured.