[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Blood, hemostasis and basic immunity](https://durgical.com/en/physiology/#s-blood) › Fibrinolysis and natural anticoagulants [Physiology](https://durgical.com/en/physiology/) · [Blood, hemostasis and basic immunity](https://durgical.com/en/physiology/#s-blood) # Fibrinolysis and natural anticoagulants - Regulation - Exam priority: medium Blood has built-in brakes that keep clots small, and a cleanup enzyme, plasmin, that dissolves them once the vessel has healed. Updated 2026-11-01 Also called: Antithrombin, Protein C and S, Plasmin **See it in 3D:** [Liver](https://durgical.com/en/anatomy/liver/) ## Key points - Antithrombin inactivates thrombin and factor Xa (and IXa); heparin speeds it up about a thousand-fold, so heparin needs antithrombin to work. - Thrombin–thrombomodulin activates protein C; activated protein C with protein S cleaves factors Va and VIIIa. - Factor V Leiden resists activated protein C: the most common inherited thrombophilia in people of European ancestry. - tPA works best on fibrin-bound plasminogen; PAI-1 inhibits tPA and α2-antiplasmin inhibits free plasmin. - D-dimer appears only when cross-linked fibrin is broken down: a normal D-dimer makes recent thrombosis unlikely (high negative predictive value). - Proteins C and S are vitamin K–dependent with short half-lives: the start of vitamin K antagonist therapy can be briefly procoagulant (skin necrosis). ## What it is Coagulation is kept local by natural anticoagulants: antithrombin, the protein C–protein S system, tissue factor pathway inhibitor and the endothelium itself. Once the vessel is repaired, fibrinolysis removes the clot: tissue plasminogen activator turns plasminogen into plasmin, which cuts fibrin into fragments such as D-dimer. Inherited or acquired weakness of these brakes causes thrombosis. ## How it works, step by step - Healthy endothelium expresses thrombomodulin and heparan sulfate and releases tPA, prostacyclin and nitric oxide. - Thrombin escaping the clot binds thrombomodulin → it now activates protein C instead of making fibrin. - Activated protein C with protein S inactivates Va and VIIIa → prothrombinase and tenase slow down. - Antithrombin, bound to heparan sulfate (or heparin), neutralizes thrombin and Xa. - TFPI shuts down the tissue factor–VIIa–Xa complex after the initial burst. - Within the clot, tPA and plasminogen bind fibrin → plasmin forms right where it is needed. - Plasmin cleaves fibrin into degradation products, including D-dimer; free plasmin is neutralized by α2-antiplasmin. ## Regulation - *+* **tPA (endothelium), urokinase**: Convert plasminogen to plasmin - *+* **Heparin and heparan sulfate**: Accelerate antithrombin - *+* **Thrombomodulin**: Turns thrombin into an activator of protein C - *−* **PAI-1 (rises in obesity, inflammation, pregnancy)**: Inhibits tPA, slows fibrinolysis - *−* **α2-antiplasmin, TAFI**: Protect the clot from early lysis ## Clinical notes - Antithrombin deficiency (inherited or nephrotic syndrome loss) causes thrombosis and resistance to heparin. - In DIC, fibrinolysis is intense: high D-dimer, low fibrinogen. - D-dimer is also high in pregnancy, after surgery, in cancer, infection and older age, so a positive result is not specific. ## Memory aid Clotting has an accelerator (thrombin) and three brakes, 'ACT': Antithrombin, protein C (with S) and TFPI; plasmin is the tow truck. ## Test yourself Think of the answer, then tap the question to see it. ## Why does heparin lose effect in a patient with antithrombin deficiency? Heparin has no anticoagulant activity of its own; it acts by binding antithrombin and accelerating its inhibition of thrombin and Xa. Without antithrombin there is little to accelerate. ## How does factor V Leiden increase the risk of venous thrombosis? A point mutation makes factor Va resistant to cleavage by activated protein C, so prothrombinase stays active longer and more thrombin forms. ## Why is D-dimer more useful to exclude than to confirm a thrombosis? Any breakdown of cross-linked fibrin raises it, including surgery, infection, pregnancy and cancer; but a normal value means little fibrin is being lysed, making recent clot unlikely. ## Why can the first days of vitamin K antagonist therapy favour clotting? Protein C has a shorter half-life than most procoagulant factors, so it falls first; until factors II and X fall, the balance tips briefly toward thrombosis. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/fibrinolysis-and-natural-anticoagulants/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Liver](https://durgical.com/en/anatomy/liver/) ## Related topics - [The coagulation cascade](https://durgical.com/en/physiology/the-coagulation-cascade/) - [Thrombosis and bleeding: Virchow's triad and hemostatic defects](https://durgical.com/en/physiology/thrombosis-and-bleeding-virchow-s-triad-and-hemostatic-defects/) - [Coagulation tests: PT/INR, aPTT and platelets](https://durgical.com/en/physiology/coagulation-tests-pt-inr-aptt-and-platelets/) - [Primary hemostasis and platelets](https://durgical.com/en/physiology/primary-hemostasis-and-platelets/) ## Related conditions - [Deep vein thrombosis](https://durgical.com/en/conditions/deep-vein-thrombosis/) - [Pulmonary embolism](https://durgical.com/en/conditions/pulmonary-embolism/) ## In Biochemistry - [Vitamin K (phylloquinone)](https://durgical.com/en/biochemistry/molecules/vitamin-k/) ## In Pharmacology - [Heparin](https://durgical.com/en/pharmacology/heparin/) - [Heparins](https://durgical.com/en/pharmacology/classes/heparins/) - [Warfarin](https://durgical.com/en/pharmacology/warfarin/) [← All of physiology](https://durgical.com/en/physiology/) Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given. Sources: [18.5 Hemostasis](https://openstax.org/books/anatomy-and-physiology-2e/pages/18-5-hemostasis) (OpenStax (Rice University), 2022); [D-Dimer Test](https://medlineplus.gov/lab-tests/d-dimer-test/) (MedlinePlus (U.S. National Library of Medicine), 2024); [Deep Venous Thrombosis](https://www.ncbi.nlm.nih.gov/books/NBK507708/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 37). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/fibrinolysis-and-natural-anticoagulants/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/fibrinolysis-and-natural-anticoagulants/