Thrombosis is the process of formation of a blood clot, known as thrombus, in a blood vessel which obstructs or block the blood flow from an injured blood vessel or organ of the body. The vessel may be any vein or artery. The movement of clots to the crucial parts of the circulatory system may cause serious complications. Blood clots can cause heart attack or stroke; or pain, swelling, redness and tenderness in the area of the clot.