Jan Galton age 13, of Marquette, Mich., for her question:

WHAT IS AN EMBOLISM?

An embolism is an obstruction of a blood vessel by an embolus or foreign substance that has been transported by the circulatory system. The embolus may be a blood clot, an air bubble, a fat globule, a clump of bacteria, a tumor cell, a piece of foreign matter such as a bullet fragment or a portion of a parasite such as a tapeworm.

The result of an embolism is an infarct, an area of dead tissue caused by obstruction of the flow of blood to the body cells in an area fed by no collateral system. Infarction may damage the heart or brain, causing heart failure or paralysis. Thrombosis differs from embolism in that a thrombus consisting of a blood clot, or clump of blood cells, forms inside the affected blood vessel. A fragment of a thrombus can become an embolus if it is dislodged and moves through the bloodstream to create an obstruction or embolism.

Thrombosis may be obviated and thrombi prevented from growing by the use of anticlotting drugs, such as coumarin or heparin.