A blood clot is a jelly-like mass of blood tissue formed by coagulating factors in the blood. Clots are a normal reaction to stop the bleeding if a blood vessel is injured. Blood clots can also form when plaque deposits in the blood vessel walls rupture. Clots can become dangerous if they get into the bloodstream, because they can block the flow of blood to the heart or brain and cause a heart attack or stroke. A blood clot in an artery is called an arterial thrombosis. A blood clot in the vein is called a venous thrombosis.