A blood clot, also known as a thrombus is the act of blood cells preventing further external bleeding also referred to as homeostasis. Blood clots are formed by platelets, which are cells that do not contain a nucleus. The platelets form a wall known as a platelet plug which prevents the flow of blood cells and further helps the coagulation process of blood cells. Small blood clots that are caused by superficial wounds generally don't last very long unless the person has a known blood disorder. Serious blood clots which are caused by heart failure or vein disorder usually last an entire lifetime.