Once haemostasis is restored and the tissue is repaired, the clot or thrombus must be removed from the injured tissue. This is achieved by fibrinolytic pathway. The end product of this pathway is the enzyme plasmin, a potent proteolytic enzyme with a broad spectrum of activity. Plasmin is formed by activation of the proenzyme, plasminogen by either plasma or tissue activators. Tissue plasminogen activators are found in most tissues, except the liver and the placenta, where they are synthesized by endothelial cells and are found concentrated in the walls of blood vessels. The two best characterized are vascular activator (commonly known as tissue plasminogen activator -- tPA) and urokinase. There is great interest in using tPA as a therapeutic agent for dissolving blood clots: the gene for tPA has now been cloned and the expressed gene product is avaible for clinical trials. Plasminogen activator is also a product of macrophages. The level of tissue activator in the plasma is normally low, but can be increased by exercise and stress.
Two forms of plasminogen are present in the plasma; one has a glutamic acid at the N-terminal of the polypeptide chain, and is called native or glu-plasminogen, and the other a lysine. The latter form arise as a result of partial degradation of the parent molecule by autocleavage.
Triggering of fibrinolysis occur when the plasminogen
activator, plasminogen, and fibrin are all in close proximity.
Both plasminogen and its activator bind avidly to fibrin as the
clot forms. This close association prevents inhibition of plasmin
activity by inhibitor, and allows proteolysis of the fibrin to
proceed after the production of lys-plasminogen. Plasmin
inhibitors ( antiplasmins) which can control plasmin activity
include: 
-antitrypsin,
-antiplasmin, C1 inhibitor,
antithrombin III.
Plasmin attacks fibrin at a number of different sites, at least 50, reducing its size such that it no longer has haemostatic activity. Many fragments are formed during this process, and some retain the capacity to polymerize, thus some of the early degradation products can compete with fibrinogen for thrombin and act as inhibitors of clot formation. This may prevent the clot being removed before the tissue is repaired.