Illustration of the condition known as deep vein thrombosis which has led to the formation of a blood clot (thrombus) in a subsidiary vein of the larger common iliac vein. Thrombi tend to form where the blood flow is sluggish and are helped by some condition which increases the natural tendency of the blood to clot. Atherosclerosis, the build-up of fat deposits on blood vessel walls, is the most common condition which encourages the clotting process. If a piece of clot is shed it could be carried to the heart or lungs where it may block an artery. This may lead to a heart attack or a pulmonary embolism.