<P> Hemostasis or haemostasis is a process which causes bleeding to stop, 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, blood changing 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 endothelial injury occurs, 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 has three major steps: 1) vasoconstriction, 2) temporary blockage of a break by a platelet plug, and 3) blood coagulation, or formation of a fibrin clot . These processes seal the hole until tissues are repaired . </P> <P> The word hemostasis (/ ˌhiːmoʊˈsteɪsɪs /, sometimes / ˌhiːˈmɒstəsɪs /) uses the combining forms hemo - and - stasis, New Latin from Ancient Greek αἱμο - haimo - (akin to αἷμα haîma), "blood", and στάσις stásis, "stasis", yielding "motionlessness or stopping of blood". </P>

The stimulus for the formation of a platelet plug or a blood clot is