Hemostasis or haemostasis is a complex process which causes the bleeding process to stop. It refers to the process of keeping blood within a damaged blood vessel (the opposite of hemostasis is hemorrhage). Most of the time this includes the changing of blood from a fluid to a solid state. Intact blood vessels are central to moderating blood's tendency to clot. The endothelial cells of intact vessels prevent blood coagulation by secretion of heparin-like molecule and thrombomodulin and prevent platelet aggregation by the secretion of nitric oxide and prostacyclin. When endothelial injury occurs, the endothelial cells cease secretion of coagulation and aggregation inhibitors and instead secrete von Willebrand factor and tissue thromboplastin which initiate the maintenance of hemostasis after injury. Hemostasis has three major steps: 1) vasoconstriction, 2) temporary blockage of a break by a platelet plug, and 3) blood coagulation, or formation of a clot that seals the hole until tissue are repaired.