A blood clot is a mass of blood cells and blood components that form to stop the bleeding that occurs when a blood vessel is injured. When a blood vessel is broken, platelets in the blood become sticky and clump together at the site of the injury. They begin to form a mass to stop the flow of blood.
Clotting is the body's normal response to a bleeding injury. It is a necessary function to prevent a person from losing too much blood. Most blood clots dissolve back into the blood when the body has healed the vessel. Blood clots, however, can be potentially dangerous if they occur within healthy blood vessels, or if they do not dissolve when their work is done. A thrombus is a blood clot that forms along the wall of the heart or a blood vessel. This type of clot can slow blood flow, and if the clot becomes large enough, it may stop the flow of blood in the vessel. An embolus is a clot that forms in one area of the body, travels through the bloodstream, and lodges in another vessel in the body. Emboli are less common and more dangerous, because they can cause a sudden blockage in blood flow (embolism), which could be fatal. An embolism occurring in an artery will block blood flow to an organ or tissue, and could cause tissue damage or death. An embolism in: