Regulation of blood coagulation Regulation of blood coagulation. Coagulation is initiated by a tissue factor (TF)–factor VIIa complex that can activate factor IX or factor X. At high TF concentrations, factor X is activated primarily by the TF-VIIa complex, whereas at low TF concentrations, the contribution of the factor IXa–factor VIIIa complex to the activation of factor X becomes more pronounced. Coagulation is maintained through the activation by thrombin of factor XI. The coagulation system is regulated by the protein C pathway. Thrombin activates protein C. Together with protein S, activated protein C (APC) is capable of inactivating factors Va and VIIIa, which results in a downregulation of thrombin generation and consequently in an upregulation of the fibrinolytic system. The activity of thrombin is controlled by the inhibitor antithrombin. The solid arrows indicate activation and the broken arrows inhibition. Source: Hereditary Thrombophilia, Williams Hematology, 9e Citation: Kaushansky K, Lichtman MA, Prchal JT, Levi MM, Press OW, Burns LJ, Caligiuri M. Williams Hematology, 9e; 2015 Available at: http://accessmedicine.mhmedical.com/DownloadImage.aspx?image=/data/books/1581/kau_ch130_f001.png&sec=108084639&BookID=1581&ChapterSecID=108084622&imagename= Accessed: November 08, 2017 Copyright © 2017 McGraw-Hill Education. All rights reserved