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Blood Clots

What Causes Blood Clots?

The process of blood clotting is triggered whenever flowing blood is exposed to certain substances. There are many different such substances, which are called thrombogenic because they promote formation of thrombus (another name for a clot). Many thrombogenic substances are located in the skin or in blood vessel walls. Normally safely separated from flowing blood, their contact with blood usually means the blood vessel wall is ruptured and bleeding. Examples of these thrombogenic substances are tissue factor, collagen, and von Willebrand factor.

Most heart attacks and strokes result from the sudden formation of a blood clot on a waxy cholesterol plaque inside an artery in the heart or brain. When the plaque ruptures suddenly, thrombogenic substances inside the plaque are exposed to blood, triggering the blood clotting process.

Blood clots may also form when blood fails to flow properly. Atrial fibrillation is an abnormal heart rhythm in which blood pools in the heart, potentially forming blood clots. If a blood clot dislodges and travels to the brain, it can cause a stroke. Prolonged immobilization can reduce blood flow in the legs, increasing the risk for blood clots in leg veins (deep venous thrombosis, or DVT).

Alterations to the Blood Clotting Process

Medical conditions and medications can alter the process of blood clot formation, making blood clots more or less likely. Some examples include:

Aspirin, clopidogrel (Plavix), prasugrel (Effient), and ticagrelor (Brilinta), ticlodipine (Ticlid), and dipyridamole (Persantine). Drugs that interfere with platelet function, making blood clots less likely.

Apixaban (Eliquis), Warfarin (Coumadin), dabigatran (Pradaxa), rivaroxaban (Xarelto), and edoxaban (Savaysa). Oral drugs that reduce production of clotting factors, reducing blood clotting.

Heparin. An intravenous or injectable drug that interferes with thrombin, preventing blood clot formation.

Tissue plasminogen activator (tPA). A clot-dissolving drug that activates plasmin, and is occasionally given as a treatment for heart attack or stroke.

Hypercoagulable state. An improper balance between clotting factors and clot-reducing substances that results in increased likelihood of abnormal blood clots.

Hemophilia. A genetic deficiency of certain functioning clotting factors results in poor blood clotting and excessive bleeding.

von Willebrand factor deficiency. A relatively common condition resulting in slow blood clot formation, which is usually mild.


WebMD Medical Reference

Reviewed by Jennifer Robinson, MD on April 17, 2015
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