A blood clot is a normal reaction of the body that occurs if a vessel wall is injured. Blood clots are formed by a chain of molecular and cellular events causing the blood to form a clot. This is desirable if it occurs to stop the bleeding caused by an injury. However, blood clots can become very dangerous if they occur within the blood stream. Blood clots form within blood vessels because of an injury to the normal smooth lining of the blood vessel, because of sluggish flow or because of an abnormal clotting mechanism (usually the result of genetics). The arteries of the body can be damaged by atherosclerotic plaque (fatty deposits in the vessel wall), especially if the plaque ruptures. Blood clots that form in arteries can cause heart attacks and strokes. This is called arterial thrombosis. The veins of the body are also prone to blood clots, especially if there is injury to the vein and if there is sluggish flow (such as caused by surgical recovery or heart failure). If clots occur in veins, a venous thrombosis occurs. If an arterial or venous thrombosis travels from the site in which it formed, it is called an embolus and it is said to have embolized. A pulmonary embolus, for instance, is a blood clot in the lung that has traveled there through the circulation, most commonly forming in the legs.
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