Chapter 4: Hemostasis

Hemostasis is the complex interaction of proteins, cells, and blood vessels with a goal of arresting hemorrhage. Blood clot formation occurs at the site of vascular injury and this must occur quickly, remain localized, and be tightly controlled. With an imbalance or dysfunction at any level of the hemostatic process, excessive bleeding or clotting may occur, both of which can be life-threatening. In health, a balance is maintained so that animals experience neither hemorrhaging nor thrombosis through hypocoagulation or hypercoagulation, respectively. This balance is maintained in several ways: intact blood vessel endothelium is anticoagulant, whereas, injured endothelium is procoagulant; many of the coagulation factors must be first activated in order to participate in clotting; and potent anticoagulants are present both in the peripheral blood and bound to vascular endothelium. The normal sequence of events when vascular injury occurs is: vasoconstriction, platelet plug formation, and coagulation, followed by clot retraction and fibrinolysis (Figs 4.1 and 4.6). There is clinical relevance in distinguishing between primary hemostasis (vasoconstriction and platelet plug formation) and secondary hemostasis (coagulation), although both are closely associated and often occur simultaneously, depending on the degree of injury.