They are both what might be called "pathological blood clots." Remember, not all clots are problems. In fact, there are plenty of situations in which you want your blood to clot quickly, but obviously you want the clot stay put and form only where it is needed. A thrombus is a clot in the intact vascular system. The embolus is the clot that breaks free and is floating along in the intact vascular system. Of course, it's going to eventually come to a vessel to small to pass through and get stuck, causing serious shortage of blood downstream. Emboli are not necessarily vagrant blood clots. Any space occupying lesion moving along in the blood flow could be considered an embolus. For example: pieces of bone following a really bad fracture, amniotic fluid, bullets and even air.
 

Mark Braun, MD braunm@indiana.edu
Copyright 1998, the Trustees of Indiana University