Introduction to Hemostasis

Normal hemostasis is dependent upon the complex interaction of plasma coagulation and fibrinolytic proteins, platelets, and the blood vasculature. Hemostasis can be divided into three categories, which makes this complex process easier to understand. In addition, each of these categories usually produce different clinical signs, which can aid in narrowing down a differential diagnostic list in a bleeding animal. For example, a defect in primary hemostasis should be considered first in a dog presenting with epistaxis (as shown in the image on the right), whereas a defect in secondary hemostasis is likely in a dog with hemarthrosis. However, it must be realized that all three processes occur simultaneously and not sequentially in vivo. The 3 categories are: Primary hemostasis: This is defined as the formation of the platelet plug. Secondary hemostasis: This is defined as the formation of fibrin through the coagulation cascade. Tertiary hemostasis: This is defined as the formation of plasmin for breakdown of the clot. Inhibitors are important as they serve to limit hemostasis to the site of vessel injury and prevent over-production of the clot, which could lead to pathological thrombosis.

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