Lionel Head, age 14, of Dugald, Manitoba, Canada, for his question:

WHAT IS THROMBOSIS?

A blood clot that stays in the blood vessel at the place where it was formed is called a thrombus. Several are called thrombi. The process of forming a thrombus is called thrombosis.

Arterial thrombosis is the blocking of an artery by a thrombus.

When the flow of blood is blocked by a thrombosis, oxygen and nourishment for the body tissues carried by the blood are also stopped. This can cause a blood deficiency called ischemia. Prolonged ischemia can result in gangrene, a critical condition of tissue destruction.

When there is a thrombosis in the brain, it may provoke a stroke.