Physiology · Blood, hemostasis and basic immunity

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

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.

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.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

Liver

In Biochemistry

Vitamin K (phylloquinone)

In Pharmacology

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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 (OpenStax (Rice University), 2022); D-Dimer Test (MedlinePlus (U.S. National Library of Medicine), 2024); Deep Venous Thrombosis (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).

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