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Thrombosis is the formation of a blood clot, or thrombus, in a blood vessel. The process is an exaggeration of a normal and useful event by which the body prevents the loss of blood from veins and arteries as a result of an injury. During thrombosis, the blood clotting process goes beyond the construction of a blockage for a damaged blood vessel and actually produces a solid clump (the clot) that reduces or even interrupts the flow of blood in the blood vessel.

Thrombosis most commonly occurs for one of three reasons. First, the blood's normal clothing system may become more active with the result that clots within a vein or artery are more likely to form. After childbirth, for example, a woman's clotting mechanism becomes more active to reduce her loss of blood. If the system becomes too efficient, however, clots may form within a blood vessel.

A reduction in blood circulation can also result in the production of a clot. Primitive clots that might otherwise be swept away by the normal flow of blood may be left undisturbed on a vessel wall, allowing them to develop in size.

Finally, the normal aging of blood vessels may contribute to the formation of clots. The first signs of atheroma, or degeneration of blood vessels, can be observed as early as childhood. The more advanced stage—hardening of the arteries—usually does not become a medical problem until old age, however. At this state, scarring of the interior walls of veins and arteries provides sites on which clots can begin to form.

A variety of medical problems can result from thrombosis depending on the location of the thrombus. When thrombosis occurs in a coronary artery of the heart, the result is a myocardial infarction, otherwise A scanning electron micrograph (SEM) of a thrombus (clot) completely occluding a major pulmonary blood vessel in a human lung. At center is the blood clot. The thick wall of the pulmonary vessel surrounds it. On the outside is the fibrous tissue and alveoli of the lung. Moredun Animal Health LTD/National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. known as a heart attack. If a thrombus forms in the brain, a stroke occurs. Thrombosis in a leg vein, especially the long saphenous vein, causes the condition known as venous thrombosis or phlebitis.

A secondary consideration in thrombosis is the possibility that a piece of the thrombus may become detached and carried away by the blood. The embolus thus formed may lodge in another part of the body, such as the lungs, where it may cause further medical problems.

Researchers have identified a number of conditions that may lead to thrombosis. These include one's genetic make-up, bodily injury, surgery, pregnancy, smoking, stress, hypertension, and arteriosclerosis.

The most suitable form of treatment depends on the form of thrombosis that occurs. With phlebitis, for example, bed rest and elevation of the limb is recommended. In some instances, by-pass surgery may be required. For many cases of thrombosis, anticoagulant drugs—drugs that reduce the rate of blood clotting—can also be effective.

See also Circulatory system.

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