How the Blood Clots

Blood clot illustration
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The clotting mechanism is one of the most important and complex of physiologic systems. Blood must flow freely through the blood vessels in order to sustain life. But if a blood vessel is traumatized, the blood must clot to prevent life from flowing away. Thus, the blood must provide a system that can be activated instantaneously –and that can be contained locally to stop the flow of blood. This system is called the clotting mechanism.

To treat or prevent abnormal blood clotting, doctors must understand the multifaceted aspects of the clotting mechanism. The following explanation is greatly simplified, but is designed to provide a basic understanding of how the many drugs used to treat clotting problems work, and some basis for assessing the treatments your doctor may prescribe for you.

How does the blood clot?

There are two major facets of the clotting mechanism:– the platelets, and the thrombin system.

The platelets are tiny cellular elements, made in the bone marrow, that travel in the bloodstream waiting for a bleeding problem to develop. When bleeding occurs, chemical reactions change the surface of the platelet to make it “sticky.” Sticky platelets are said to have become “”activated.”” These activated platelets begin adhering to the wall of the blood vessel at the site of bleeding, and to each other. Within a few minutes the sticky platelets form what is called a “white clot.” (A clump of platelets appears white to the naked eye.)

The thrombin system consists of several blood proteins that, when bleeding occurs, become activated. The activated clotting proteins engage in a cascade of chemical reactions that finally produce a substance called fibrin. Fibrin can be thought of as a long, sticky string. Fibrin strands stick to the exposed vessel wall, clumping together and forming a web-like complex of strands.

 Red blood cells become caught up in the web, and a “red clot” is said to be present.

A mature blood clot consists of platelets and fibrin strands, as well as trapped red blood cells. The strands of fibrin bind the platelets together, and “eventually tighten” the clot to make it stable.

In arteries, the primary clotting mechanism depends on platelets. In veins, the primary clotting mechanism depends on the thrombin system. But in reality, both platelets and thrombin are involved, to one degree or another, in all blood clotting.

How Can the Clotting Mechanism Produce Problems?

The clotting system, like all complex physiologic systems, can produce problems. 

Obviously, if either the platelets or the thrombin system do not work adequately, episodes of abnormal bleeding can occur. Low platelet counts can occur with chemotherapy, for instance, or with leukemia. Several genetic disorders, including hemophilia, can cause the thrombin system to malfunction. Any of these conditions can cause serious bleeding problems.

The clotting mechanism can also cause clots to form where they can do harm, a condition called thrombosis. Thrombosis can occur in a coronary artery (or an artery to the brain) when an atherosclerotic plaque ruptures This arterial clot can block blood flow and produce damage to the heart (a heart attack) or to the brain (a stroke).

Abnormal clots can also occur in the veins, most often the leg veins, producing a condition called deep venous thrombosis, or DVT. The venous clots can break off (emboiize) and travel to the lungs, causing a dangerous condition called pulmonary embolus.

So a normally functioning clotting system is necessary to prevent both excessive bleeding, and excessive clotting.

How Can Abnormal Blood Clotting Be Treated?

Excessive bleeding caused by a platelet deficiency is usually treated with platelet transfusions while the cause of platelet problem is sought. Disorders of the thrombin system can usually be reversed, temporarily at least, with a plasma infusion (which replaces missing or malfunctioning clotting factors).

Drugs aimed at stopping the formation of blood clots can be directed either at inhibiting platelet function, or the thrombin system. While all treatments meant to inhibit blood clots have their own profile of adverse effects, one problem common to all these treatments is excessive bleeding. They must all be used with appropriate precautions.

Read about the drugs that are meant to prevent thrombosis and embolism.

A Word From Verywell

The normal functioning of the blood clotting system is vital to life. Unless the clotting mechanism works within its normal, narrow specifications, either excessive bleeding occurs, or blood clots form where they are not supposed to form, damaging vital organs. 

The treatment of clotting abnormalities, therefore, is a critically important aspect of medical practice.


Brass LF. Thrombin and Platelet Activation. Chest 2003; 124:18S.

Furie B, Furie BC. Mechanisms of Thrombus Formation. N Engl J Med 2008; 359:938.

Goldhaber SZ. Risk Factors for Venous Thromboembolism. J Am Coll Cardiol 2010; 56:1.

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