MCAT Content / Circulatory System / Coagulation Clotting Mechanisms

Coagulation, clotting mechanisms

Topic: Circulatory System

Coagulation is the process by which a blood clot forms to reduce blood loss after damage to a blood vessel.

Coagulation is the process by which a blood clot forms to reduce blood loss after damage to a blood vessel. Several components of the coagulation cascade, including both cellular (e.g. platelets) and protein (e.g. fibrin) components, are involved in blood vessel repair.

When the lining of a blood vessel breaks and endothelial cells are damaged, revealing collagen proteins from the extracellular matrix, thromboxane causes platelets to swell, grow filaments, and start clumping together, or aggregating. The platelets release chemicals known as clotting factors to begin the chemical cascade process.

These clotting factors take part in a cascade of chemical reactions that eventually create a mesh of fibrin within the blood. Each of the clotting factors has a very specific function. Prothrombin, thrombin, and fibrinogen are the main factors involved in the outcome of the coagulation cascade. Prothrombin and fibrinogen are proteins that are produced and deposited in the blood by the liver.

When blood vessels are damaged, vessels and nearby platelets are stimulated to release a substance called prothrombin activator, which in turn activates the conversion of prothrombin, a plasma protein, into an enzyme called thrombin. Thrombin facilitates the conversion of a soluble plasma protein called fibrinogen into long, insoluble fibers or threads of the protein, fibrin. Fibrin threads wind around the platelet plug at the damaged area of the blood vessel, forming an interlocking network of fibers. This net of fibers traps and helps hold platelets, blood cells, and other molecules tight to the site of injury, functioning as the initial clot. This temporary fibrin clot can form in less than a minute and slows blood flow before platelets attach. Next, platelets in the clot begin to shrink, tightening the clot and drawing together the vessel walls to initiate the process of wound healing. Usually, the whole process of clot formation and tightening takes less than a half-hour.


Key Points

• The coagulation cascade is a series of reactions, which leads to the formation of a clot

• Platelets release chemicals that cause a cascade reaction

• Prothrombin, thrombin, and fibrinogen are the main factors involved in the outcome of the coagulation cascade. Prothrombin and fibrinogen are proteins that are produced and deposited in the blood by the liver.

• Prothrombin is converted into thrombin which converts fibrinogen into fibrin.

• Fibrin forms an interlocking network of fibers that traps and helps hold platelets, blood cells, and other molecules tight to the site of injury, functioning as the initial clot.

• Coagulation involves a complex cascade in which a fibrin mesh is cleaved from fibrinogen.

• Fibrin acts as a “molecular glue” during clot formation, holding the platelet plug together.


Key Terms

fibrin: an elastic, insoluble, whitish protein produced by the action of thrombin on fibrinogen and forming an interlacing fibrous network in the coagulation of blood.

endothelium: a thin layer of flat epithelial cells that lines the heart, serous cavities, lymph vessels, and blood vessels

thrombin: the end product of the coagulation cascade, which cleaves fibrin from fibrinogen

fibrinogen: is converted enzymatically by thrombin to fibrin and then to a fibrin-based blood clot

platelets: tiny blood cells that help your body form clots to stop bleeding



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