Tissue plasminogen activator (t-PA) is a serine esterase that plays a key role in the fibrinolysis system. It is present in plasma, 95% bound to PAI-1, in platelets and in some tissues. In plasma, the enzymatic activity of t-PA on plasminogen is very low, it is amplified 200 to 400 times when t-PA and plasminogen are adsorbed to fibrin.

Plasminogen activator inhibitor (PAI-1) is a glycoprotein, the primary inhibitor of t-PA and u-PA. It plays an important role in controlling excessive fibrinolysis. PAI-1 is mainly synthesized by vascular endothelial cells, as well as by other cells (hepatocyte, CML, fibroblasts, etc.). It circulates in plasma in 3 forms: an active form linked to vitronectin, a latent free form and an inactive form. By inhibiting t-PA and u-PA, PAI-1 limits the activation of plasminogen and controls the degradation of fibrinous thrombus.