Robert Fryling, age 15, of Cartersville, Ga., for his question:

WHAT CAUSES BLOOD TO CLOT?

It's a serious problem if blood clots within the bloodstream. Two substances, antithromboplastin and antiprothrombin, are contained in the blood to prevent internal clotting.

In addition, there are a number of substances in the blood designed to produce clotting when needed. These include prothrombin, fibrinogen and calcium ions. There are at least 10 other substances involved in the process by which prothrombin is coverted to thrombin, which changes soluble fibrinogen into the insoluble strand of fibrin.

Clotting action occurs when there is external bleeding. Warm towels are sometimes used to speed up the chemical action that causes clotting. Ice packs are also used to stop bleeding since cold causes blood vessels to contract.