Medical Information: Basics of Coagulation

Blood coagulation plays a crucial role in haemostasis though it only occurs through the interaction of numerous factors. The human body, through a complex mechanism that causes blood to clot if a wound occurs, protects itself from excessive blood loss on one hand and from excessive clotting (thrombosis) on the other hand (1).

Primary haemostasis - white thrombus

Blood vessels (arteries and arterioles) contract and so constrict the flow of blood. At the same time, damaged endothelial cells release substances that attract thrombocytes and activate clotting factors. The thrombocytes accumulate around the edges of the wound, aggregate and so close the wound up (white thrombus). The vascular and cellular components of blood coagulation are referred to as primary haemostasis.

Secondary haemostasis - stabilising the thrombus

To stop the thrombus being washed away in the blood stream, a second mechanism is simultaneously activated: activated clotting factors circulating in the blood stabilize the white thrombus with a network of protein fibres (fibrin), to which other blood cells adhere. The stable blood clot (red thrombus) that then forms permanently closes the damaged blood vessel. This process is known as secondary haemostasis which includes the formation of fibrin from fibrinogen through activation of the coagulation cascade.

Following thrombus formation, the contraction of vessels in the injured area declines. In healthy humans this process takes about 1-3 minutes.

Clotting factors - plasmatic coagulation

Clotting factors are mainly produced in the liver and are released into the plasma. Vitamin K plays a crucial role in the synthesis of factors II, VII, IX and X.

Roche Diagnostics graphic (data on file)

This is the rationale for treatment of thromboembolic diseases with coumarins, which antagonise the action of vitamin K and so inhibit blood clotting (Vitamin K antagonists - VKAs). This process is termed anticoagulation.

Fibrinolysis

After the wound has healed the fibrin itself is dissolved in a process known as fibrinolysis. The fibrin clot is sometimes replaced by a permanent framework of scar tissue but mostly completely resolves.

Under physiological conditions blood coagulation and fibrinolysis always occur simultaneously in the blood stream, normally being in a dynamic equilibrium, ensuring that blood remains liquid within the vascular system. An upset in this delicate balance can lead to bleeding as a result of diminished coagulation or increased fibrinolysis and, conversely, formation of blood clots as a result of increased coagulation and diminished fibrinolysis.

Statements based on references

Lutze G. Useful facts about coagulation. Mannheim: Roche Diagnostics GmbH, 2000. (data on file) Please do not hesitate to contact your local Roche representative if you need further information or references.

Please do not hesitate to contact your local Roche representative if you need further information or references.