Haemostasis: Basic physiology and diagnosis

Blood coagulation and fibrinolysis are mechanisms that, in parallel with in flammatory and repair responses, help to protect the integrity of the vascu lar system and to maintain the fluidity of blood. The initial response to v ascular injury is contraction of vessels. The normal endothelium maintains blood fluidity by producing inhibitors of blood coagulation and platelet ag gregation, by modulating vascular tone and permeability, and by providing a protective envelope separating haemostatic blood components from reactive subendothelial structures. Thus, loss or injury of endothelium results in a shift towards increased blood coagulation. Adhesion of platelets and plate let aggregation requires von Willebrand factor, and platelet glycoproteins as well as formation of thromboxan from arachidonic acid are subsequent ste ps which result in a platelet plug at the site of injury, Fibrin formation, through the activation of plasma coagulation occurs on this initial platel et plug and stabilises the thrombus, Several plasma procoagulant factors ar e involved in this reaction. Specific inhibitors regulate fibrin formation in order to limit the reaction and to prevent excessive fibrin formation. L oss of procoagulant factors results in a bleeding diathesis, loss of these inhibitors in thrombophilia, The fibrinolytic system is able to lyse clots or thromboseses, It is similarly organised by promoters and inhibitors. Thi s system is also regulated by hormones and cytokines, which can induce or i ncrease expression, synthesis and release of haemostasis factors. A suffici ent function of haemostasis is essential in critical care patients, since b leeding and thrombosis are frequent and relevant complications in critical care patients. Laboratory evaluation of haemostasis focuses on platelet cou nts and plasma coagulation profiles. The detailed knowledge of the test sys tems is important since therapy with antithrombotic drugs, blood components or coagulation factor concentrates is frequent, expensive and not without risks in these patients.