Hemostasis is arrest of bleeding. It has three phases: the vascular phase, platelet phase, and coagulation phase. In the vascular phase, endothelial cells contract (vascular spasm) to minimize blood loss.
In the platelet phase, attachment of platelets to endothelial surfaces (platelet adhesion) seal off the area. These platelets (thrombocytes) are cell fragments derived from (large) megakaryocytes in the red bone marrow. When activated, platelets release cytokines that attract more platelets, forming a platelet plug.
In the coagulation phase, activation of a cascade of enzymes converts fibrinogen to an insoluble fibrin network.
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