Normally, blood remains fluid within a closed vascular system. Abnormalitiesof blood coagulation take two main forms: failure to clot normally (and thus toprevent abnormal degrees of leakage from the vascular system) and failure to prevent excessive clotting (and thus to maintain the patency of the blood vessels). Most emphasis in clinical medicine has been on the diagnosis and treatment of clotting deficiency. To understand the various laboratory tests designed to pinpoint defects in the coagulation mechanism, we must outline the most currently accepted theory of blood coagulation (Fig. 8-1).

Blood coagulation pathways

Fig. 8-1 Blood coagulation pathways. a, activated.