The coagulation cascade is essential for life, but it is only half the story of homeostasis. Once a platelet or fibrin clot is made, there must be a mechanism to eventually break down the clot, once the tissue has been repaired. The breakdown of the clot must occur in a regulated way so as to avoid large insoluble clots being released into the bloodstream. These clots could cause blockages in the vessels and cause strokes, heart attacks, and other ischemic events.
The fibrinolytic cascade refers to the breakdown of clots. It is the reversal of the coagulation cascade. As can be seen in the accompanying figure, once fibrin is made, plasminogen is activated by tissue plasminogen activator (TPA), forming plasmin. The plasmin enzyme then breaks down the fibrin polymer. The coagulation and fibrinolytic cascades are finely regulated by endothelial cells and blood cells that release a myriad of cytokines and protein activators.
Clinically, the fibrinolytic pathway can be stimulated by giving a patient TPA. This is sometimes done therapeutically in cases of stroke. Alteplase is a tissue plasminogen activator used as an emergency thrombolytic drug. It is given intravenously to promote clot breakdown. If this drug is given within 2 hours of onset, it can help save patients from the devastating effects of a stroke.
