Hemostasis is a complex process which changes 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 thrombus formation by secreting tissue plasminogen activator (t-PA) and by inactivating thrombin and adenosine diphosphate (ADP). Injury to vessels overwhelms these protective mechanisms and hemostasis ensues. Hemostasis proceeds in two phases: primary and secondary hemostasis.

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Vascular spasm, platelet plug formation, and clot formation all help to stop excessive blood loss after tissue injury.