The Extrinsic System

Abnormal bleeding

Animals with disorders of hemostasis are often presented as emergency patients and, as such, offer a challenge to the attending clinician. This article reviews the basic physiology of hemostasis and laboratory tests used for diagnosis. Guidelines for the evaluation and treatment of patients with bleeding disorders are provided.

An overview of hemostasis

The normal hemostatic system is complex yet exquisitely well regulated. Interrelationships exist between responses of the vasculature, circulating platelets, coagulation proteins, and fibrinolytic mechanism. These relationships serve to limit blood loss, preserve tissue perfusion, and stimulate local repair processes. Natural inhibitors of coagulation and fibrinolysis modulate these systems to prevent uncontrolled thrombosis or hemorrhage following pathologic stimuli. Vascular endothelial cells play an important role in the maintenance of a thromboresistant luminal interface with circulating cells and proteins. Normal hemostasis also requires the synthetic, metabolic, and repair processes of the vascular endothelium. The initial vascular response to injury produces brief vasoconstriction and exposes subendothelial substances that attract circulating platelets and activate coagulation proteins. Platelets respond by adherence and aggregation at the site of injury, with subsequent release of substances that mitigate blood loss. Platelet adherence to collagen requires von Willebrand's factor, fibrinogen, fibronectin, and specific glycoprotein receptors on platelet surfaces. Platelet-to-platelet interactions (aggregation) recruit additional platelets to the primary hemostatic plug. Aggregation requires fibrinogen, energy, and calcium. Release of ADP, serotonin, and the contents of intracellular granules as well as generation of prostaglandins prepares platelet surfaces for reactions with the coagulation proteins. Activation of the intrinsic or extrinsic coagulation pathway, or of both, causes formation of fibrin from fibrinogen by means of an elaborate and intricate system that also entraps platelets and activated coagulation proteins. The intrinsic system is activated by the contact of factor XII with a negatively charged surface, most likely collagen. Through a series of reactions with prekallikrein, HMWK, and factors XI, IX, and VIII, the common coagulation pathway is propagated. The extrinsic, or tissue factor, system stimulates both the common pathway and factor IX the intrinsic pathway. Discovery of this stimulation of the intrinsic pathway by factor VII in the extrinsic pathway has stimulated reassessment of the biologic importance of the extrinsic system. The common pathway includes factors X and V and causes thrombin to convert fibrinogen to fibrin. Calcium and platelet phospholipids are substances that have important roles in steps in the coagulation scheme. Once fibrin is formed, factor XIII interacts with the substance, providing a stabilizing effect.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased factor VII coagulant activity in the rabbit following diet-induced hypercholesterolaemia. Evidence for increased conversion of VII to alpha VIIa and higher flux within the coagulation pathway

Factor VII coagulant activity (VIIc) is considerably higher in rabbits fed a 1% cholesterol-supplemented diet than in rabbits fed a standard diet. This increase was statistically significant 4-6 days from the beginning of treatment and rose to about 300% during the 100 days of treatment. Treatment is also associated with a 20-fold increase in plasma cholesterol concentration with the major fraction of excess cholesterol associated with the very low and intermediate density lipoprotein fractions. In both groups of rabbits, the direction and extent of variation in VIIc generally coincided with variation in cholesterol, so that over time there were significant and positive correlations between plasma cholesterol concentration in both the rabbits fed the standard diet and the rabbits fed the cholesterol-supplemented diet. The increase in VIIc was due to a higher proportion of the more active alpha VIIa in the plasma of hypercholesterolaemic rabbits rather than to an increase in the concentration of the single-chain protein. The plasma concentration of factor X and prothrombin had increased in the hypercholesterolaemic rabbits by 10 days from the beginning of treatment and both proteins were maintained at 150-200% of the concentrations in the plasma of rabbits fed the standard diet. However, these differences were only seen when the factor X and prothrombin were assayed using synthetic substrates. The specific coagulation assays for these two factors revealed no differences between the groups of animals up to 100 days.