Since fibrin is formed in the final step of the coagulation cascade to reinforce the platelet plug, the body must have a mechanism to eventually dissolve this structure and re-establish blood flow through the vessel. The major enzyme responsible for the breakdown of fibrin is plasmin.
Plasmin is circulating in the blood in the inactive form plasminogen. Tissue plasminogen activator (tPA) and urokinase plasminogen activator (uPA) are enzymes involved in the reaction that causes plasminogen to become plasmin.
Plasmin will break down fibrin into small "fibrin degradation/split products." D-dimers are the result of the breakdown of fibrin that cross-linked together during the coagulation cascade.