Illustration copyright 2000 by Nucleus Communications, Inc. All rights reserved. http://www.nucleusinc.com

A blood clot can form when the valves in the veins do not work properly. If the valves do not open normally, blood cells can collect behind the valve and begin to stick together to form a clot. A collection of clotted blood cells is called a thrombus.

The clot, or a piece of it, can break off and travel through the blood vessels. A blood clot can cause serious problems if it gets stuck in a vein and blocks blood flow (thromboembolus).