A blood clot is a normal reaction of the body that occurs if a blood vessel is injured. 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 can form in the blood stream because of an injury to the normal smooth lining of a blood vessel, because of sluggish blood flow or because of an abnormal clotting mechanism (usually the result of genetic abnormalities in the clotting mechanism). If plaque deposits in the blood vessel wall rupture, a blood clot can form, which can then block the flow of blood and lead to a heart attack or stroke. A blood clot in an artery is called anArterial 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 in patients recovering from surgery or with heart failure). If clots occur in veins, a Venous Thrombosis is said to occur. If an arterial or venous thrombosis breaks loose and travels through the bloodstream, it is called an embolus.