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Anticoagulants

Anticoagulantia

For medical students2 min readUpdated 2026-10-10

Anticoagulants are vital physiological components of the anticoagulant system that maintain blood in a fluid state. They prevent excessive thrombus formation by strictly localizing the clotting process to the site of vascular wall injury.

Main RoleMaintaining blood fluidity and controlling thrombus formation

Antithrombin IIIThe key thrombin inhibitor in blood plasma

HeparinSynthesized by basophils and mast cells in the body

Fibrin (Antithrombin I)Capable of adsorbing and inactivating up to 90% of thrombin

Primary Anticoagulants

Primary anticoagulants form the basis of the primary anticoagulant system. Their main distinctive feature is that they are constantly present in circulating blood. It is thanks to them that blood in intact vessels maintains its rheological properties and does not clot spontaneously.

The most important components of this group include:

Antithrombins. These are specific proteins that inhibit clotting system proteases. The most significant among them is antithrombin III, which acts as the major physiological inhibitor of thrombin.
Heparin. This substance is actively produced by mast cells and basophils. Heparin has a broad spectrum of action: it inhibits platelet hemostasis and suppresses the activity of most plasma factors, thereby blocking virtually all phases of the coagulation cascade. A crucial property of heparin is its ability to dramatically enhance the inhibitory effect of antithrombin III.
Antithromboplastins. These compounds purposefully inhibit the formation of prothrombinase and block its action, preventing the active phase of coagulation from triggering.

Secondary Anticoagulants

Unlike primary ones, secondary anticoagulants do not circulate in the blood constantly in an active form. They form the secondary anticoagulant system because they are generated directly during the process of blood coagulation and subsequent fibrinolysis. Their physiological task is to restrict thrombus formation strictly to the site of vascular injury and prevent the uncontrolled spread of the thrombus through the bloodstream.

This group includes the following components:

Fibrin (antithrombin I). Formed as a natural outcome of hemostasis. By forming a clot, fibrin strands adsorb onto their surface and inactivate up to 90% of all synthesized thrombin, preventing it from acting further.
Fibrinogen peptides. During fibrin formation, the thrombin molecule cleaves specific peptides from fibrinogen. Once in the plasma, these fragments begin to exhibit their own pronounced anticoagulant activity.
Fibrin and fibrinogen degradation products. These appear at the stage of fibrinolysis (clot dissolution). They are potent anticoagulants that simultaneously work in three directions: inhibiting thrombin, halting further fibrin formation, and preventing the aggregation of new platelets.

Comparative Characteristics of the Systems

To better understand the functioning of anticoagulant mechanisms, let us examine their key differences in a table:

Characteristic	Primary System	Secondary System
Time of Appearance	Constantly present in blood	Formed during hemostasis and fibrinolysis
Main Function	Maintaining the initial fluid state of blood	Limiting the formed thrombus to the site of vascular injury
Key Agents	Antithrombin III, heparin, antithromboplastins	Fibrin, fibrinogen peptides, degradation products
Relation to Thrombin	Direct inhibition (antithrombins)	Adsorption (fibrin) and inhibition by breakdown products

Mnemonic

To quickly memorize the components of the secondary anticoagulant system, use the "Three Fs" rule: Fibrin itself, Fibrinogen peptides, and Fibrin degradation products.

Frequently asked questions

Which specific blood clotting factors does antithrombin III suppress?

Antithrombin III is a plasma protein that forms complexes with serine proteases and inactivates them. Its specific targets include:

thrombin;
factor IXa;
factor Xa;
factor XIIa.

In addition to clotting factors, antithrombin III targets also include plasmin and kallikrein. In certain sources, antithrombins as a whole are also noted to inhibit thrombin and factors X, V, and VIII.

What is the anticoagulant role of the vascular endothelium?

The anticoagulant role of the intact vascular endothelium is to prevent blood cell adhesion to the wall and participate in limiting thrombus formation.

The endothelium:

synthesizes prostacyclin (PGI₂) and endothelium-derived relaxing factor (NO), which possess antiplatelet activity; prostacyclin also inhibits the adhesion and aggregation of erythrocytes and platelets;
expresses/synthesizes tissue factor pathway inhibitor (TFPI), which binds to factor Xa and inactivates factor VIIa as part of a complex;
expresses thrombomodulin on the surface of intact endothelial cells; the "thrombin + thrombomodulin" complex activates protein C.

Which proteins are part of the protein C anticoagulant system?

The protein C anticoagulant system directly includes:

protein C — a vitamin K-dependent plasma protein; activated by the "thrombin + thrombomodulin" complex, causing proteolysis of active factors VIIIa and Va, which leads to the blockade of thrombin generation;
protein S — a vitamin K-dependent plasma protein; acts as a cofactor in the proteolytic reaction and enhances the action of protein C.

Additionally, thrombomodulin expressed on the surface of intact endothelial cells participates in protein C activation.

What is the fundamental difference between primary and secondary anticoagulants?

Primary anticoagulants are constantly present in the bloodstream, preventing blood from clotting under normal conditions. Secondary anticoagulants appear only as a consequence of the clotting process or clot dissolution to halt its excessive growth.

How does fibrin stop the clotting process if it is itself the basis of a thrombus?

Formed fibrin strands act as a powerful sorbent (antithrombin I). They absorb and permanently inactivate up to 90% of the generated thrombin, depriving it of the ability to trigger further coagulation.

What is the synergy between heparin and antithrombin III?

Heparin not only independently inhibits coagulation and platelet hemostasis, but also acts as a catalyst: it can sharply enhance the inhibitory effect of antithrombin III on clotting factors.

Go deeper

Mechanisms of clotting protease inhibition by antithrombin III
The process of peptide cleavage from the fibrinogen molecule by thrombin
The role of basophils and mast cells in the synthesis and secretion of endogenous heparin
The effect of fibrin degradation products on platelet aggregation processes
Biochemical pathways of blocking prothrombinase formation