Fibrinolysis

When a blood clot is initially formed, it is a semisolid mass consisting of platelets and a fibrin mesh, which also traps red and white blood cells and contains plasma. The clot solidifies as platelets contract and squeeze out plasma water.

The protein plasminogen is among the serum proteins that are adsorbed into the clot at the time of its formation. The cleavage of plasminogen produces the protease plasmin, which breaks down fibrin and fibrinogen.

The breakdown products of the fibrin mesh are released from the blood clot and are scavenged by macrophages. Fibrinolysis begins soon after a clot is formed. The activation of plasminogen is mainly regulated by two factors, which are released from capillary endothelial cells: tissue plasminogen activator and urokinase-type plasminogen activator. It is important that plasmin is not active in the circulation because it can also break down fibrinogen.