Clotting is the body's normal response to a bleeding injury. To control bleeding from vessels larger than capillaries a clot must form at the injury site. Platelets, one of the three main cellular components of human blood, serve as the body's first line of defense to prevent blood loss. When an injury such as a cut occurs, platelets become activated platelets. They change their shape, become sticky and build up on a blood vessel wall to form a plug. Platelets are also involved in the secretion of chemical platelet factors into the blood plasma. In a complex series of reactions fibrinogen is converted into fibrin, an insoluble protein that forms an intricate network of tiny threads called fibrils. Blood cells and plasma are tangled in the network of fibrils to form the clot.

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