Julie Kenney, age 13, of Springfield, I11., for her question:

WHAT CAUSES A THROMBUS?

A blood clot that stays in the blood vessel at the place where it was formed is called a thrombus. A thrombus may be formed by a physical injury or because of a disorder of blood composition.

The process of forming a thrombus is called thrombosis. More than one thrombus are thrombi.

When the blood flow is blocked by thrombosis, oxygen and nourishment for the body tissues carried by the blood is stopped. Prolonged stoppage can result in tissue death and possible gangrene.

Doctors often use drugs to retard the clotting of blood. Often patients suffer only temporary disability from thrombosis because blood circulation continues through other vessels.

If the thrombus is dislodged and moves to another site, where it causes another obstruction, it is called an embolus and blood stoppage is then called an embolism. Surgery is sometimes used to remove an embolus.