The prime function of the coagulation mechanism is to protect the integrity of the blood vessels while maintaining the fluid state of blood. Serious medical problems or even death may occur with the inability to stem the loss of blood or with the inability for a normal clot to form.
Hemostasis and coagulation tests are generally done for patients with bleeding disorders, vascular injury or trauma, or coagulopathies. Reflex vasoconstriction is the normal response to vascular insult once the first-line defenses (skin and tissue) are breached. In larger vessels, vasoconstriction may be the primary mechanism for hemostasis. With smaller vessels, vasoconstriction reduces the size of the area that must be occluded by the hemostatic plug. Part of this cascade of sequential clotting events relates to the fact that platelets adhere to the injured and exposed subendothelial tissues. This phenomenon initiates the complex clotting mechanism whereby thrombin and fibrin are formed and deposited to aid in intravascular clotting (Table 2.7).
The entire mechanism of coagulation and fibrinolysis (removal of fibrin clot) is one of balance. It may best be understood by referring to Figures 2.2 and 2.3. Abnormal bleeding does not always indicate coagulopathy in much the same way that lack of bleeding does not necessarily indicate absence of a bleeding disorder.
The most common causes of hemorrhage are thrombocytopenia (platelet deficiency) and other acquired coagulation disorders, including liver disease, uremia, disseminated intravascular coagulation (DIC), and anticoagulant administration. Together, they account for most hemorrhagic problems. Hemophilia and other inherited factor deficiencies are seen less frequently. Bleeding tendencies are associated with delays in clot formation or premature clot lysis. Thrombosis is associated with inappropriate clot activation or localization of the blood coagulation process. Finally, clotting disorders are divided into two classes: those caused by impaired coagulation and those caused by hypercoagulability.
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