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Mechanisms Of Blood Coagulation

Blood coagulation refers to the process of forming a clot to stop bleeding. Coagulation is a complicated subject and is greatly simplified here for the student's understanding.

To stop bleeding, the body relies on the interaction of three processes:

Primary Hemostasis

Involves the first two processes:
1. Vasoconstriction. Vasoconstriction is the body's first response to injury in the vascular wall. When an injury occurs, vessel walls constrict, causing reduced blood flow to the site of injury.

2. Platelet plug. Platelets aggregate to the site of the injury. They stick together acting as a "plug." Platelets also activate the process which causes a fibrin clot to form, known as secondary hemostasis.

Secondary Hemostasis

3. Platelets alone are not enough to secure the damage in the vessel wall. A clot must form at the site of injury. The formation of a clot depends upon several substances called clotting factors. These factors are designated by Roman numerals I through XIII. These factors activate each other in what as known as the clotting cascade. The end result of this cascade is that fibrinogen, a soluble plasma protein, is cleaved into fibrin, a nonsoluble plasma protein. The fibrin proteins stick together forming a clot.

The clotting cascade occurs through two separate pathways that interact, the intrinsic and the extrinsic pathway.

Extrinsic Pathway

The extrinsic pathway is activated by external trauma that causes blood to escape from the vascular system. This pathway is quicker than the intrinsic pathway. It involves factor VII.

Intrinsic Pathway

The intrinsic pathway is activated by trauma inside the vascular system and is activated by platelets, exposed endothelium, chemicals, or collagen. This pathway is slower than the extrinsic pathway but more important. It involves factors XII, XI, IX, VIII.

Common Pathway

Both pathways meet and finish the pathway of clot production in what is known as the common pathway. The common pathway involves factors I, II, V, and X.

Instructor's Note:

A diagram may be found in your text illustrating the clotting cascade. The student does not need to be concerned about learning the details of these pathways. The student does need to realize that different factors are involved in each pathway. If a patient does not clot normally, it is usually due to a platelet abnormality or deficiency, or by a defect or deficiency in one of the clotting factors. There are diagnostic tests which test for deficiencies in the intrinsic pathway, the extrinsic pathway, and platelet abnormalities. These tests allow the physician to narrow down and eventually discover the defect which is causing a patient to bleed excessively.

The interested student may want to study a more in-depth chart and explanation of the coagulation cascade. See Coagulation Cascade