Chapter 17 - Blood - Review Questions - Page 656: 20a

Fibrinolysis is the dissolution or digestion of a blood clot by the hydrolytic actions of certain enzymes. Fibrin fibers are what hold the clot together as a single structural unit. When fibrin is hydrolysed the clot dissolves or breaks up into very small pieces. This opens up the blood vessel, restores free passage of blood, and obviates or reduces the risk of an embolic stroke.

Step by Step After a blood vessel is breached and begins to bleed, the body initiates the process of clotting. The function of coagulation is to stop blood loss, and to establish the conditions for blood vessel wall repair. Clot formation begins seconds after blood vessel injury . A soft clot forms in about five minutes. In about 45-50 minutes after clot formation is initiated clot retraction begins. This is accomplished by the contraction of the actin and myosin molecules in the fibrin fibers. The result is a firmer clot. The clot stabilizes conditions to allow the endothelial and smooth muscle to multiply and restore the damaged blood vessel wall. The healing process is promoted by platelet derived growth factor(PDGF) and vascular endothelial growth factor (VEGF). The clot has a job to do but it is not a permanent structure. Therefore,when healing of the blood vessel wall has been accomplished, it is essential that the clot be removed. The process by which a clot is digested and removed is called fibrinolysis . The main enzyme in the process of fibrinolysis is plasmin. Plasmin is produced from a precursor, plasminogen, which is part of the fibrin clot. The transformation of plasminogen to plasmin is accomplished by the action of thrombin and the enzymes tissue plasminogen activator (tPA) and activated factor XII. Fibrinolysis begins about 50 hours after clot stabilization for the purpose clearing the blood vessel . The removal of the clot is important for the following reasons: 1. Clot removal facilitates blood vessel wall repair. 2. Clot removal aids in the restoration of normal blood flow. 3. Clot dissolution reduces or obviates the risk of an embolic stroke