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.
Blood clots are essential to stop bleeding after injury, but harmful blood clots can also form, causing serious damage. Most heart attacks and strokes result from the sudden formation of a blood clot on a 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).
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