The blood flow will then carry the embolus (via blood vessels) to various parts of the body where it can block the lumen (vessel cavity) and cause vessel obstruction or occlusion. The free-moving thrombus is called an embolus.[4] A thrombus is always attached to the vessel wall and is never freely moving in the blood circulation. This is also the key difference for pathologists to determine the cause of a blood clot, either by thrombosis or by post-mortem blood clot.