Blood Clot and Free Moving Clot (Embolus)

What is a blood clot?

A blood clot is a coagulated mass of blood cells and other blood components. It plugs any damaged area of the vessel wall and maintains the integrity of the blood vessel by preventing blood from leaking out.

A host of factors come into play in blood clotting (coagulation). When an area of the vessel lining known as the endothelium is damaged, platelets in the blood attach to his area. It then attaches fibrinogen which are long fibers that help to form a mesh network. Fibrinogen is converted to fibrin by the action of the enzyme thrombin. The platelets adhere to each other to form a tight plug and the fibrin becomes tightly woven to form a clot (platelet plug).

Once the damaged area is sealed, the lining of the vessel wall regrows and the clot dissolves. The integrity of the blood vessel is restored and there is no threat of losing blood at this site.

Sometimes a blood clot forms inside a vessel and continues to grow by trapping red and white blood cells, with further platelet aggregation and fibrin deposition. This affects the flow of blood in the vessel and can break off and lodge elsewhere in the vascular system (embolus). This is known as a thrombus. Technically, a thrombus is a blood clot but in thrombosis, the process of clotting goes beyond what is necessary to seal off a damaged vessel wall.

What is an embolus?

An embolus (plural ~ emboli) is an undissolved, detached mass that travels through the bloodstream to other sites in the body. It may be a solid, liquid or gas. This process of breaking away from the original site and being transported through the circulation is known as embolism or embolization.

Causes of Embolism

In most cases, an embolus is a thrombus (blood clot inside the vessel) that breaks away from its original site and lodges elsewhere.

This free moving blood clot gets impacted in a vessel that is usually smaller and may occlude the vessel partially or almost completely. When a solid embolus like a thrombus lodges in another site of the vascular system, further clotting occurs as platelets and fibrin lodge on the sides of embolus and the surrounding endothelial lining. Blood cells also get trapped and this forms a tight plug around the embolus. Blood flow is disrupted almost entirely and there is a lack of oxygen which can lead to an infarction.

Other substances that may act as an embolus includes :


  • Dislodged pieces of an atherosclerotic plaque which contain lipid debris.


  • Air which may enter the vascular system during childbirth, abortion or injury to the lung or chest.
  • Nitrogen which may enter the gaseous phase due to a sudden drop in pressure as seen in divers who ascend to the surface too rapidly (compression sickness).


  • Small clumps of fat may enter the circulation after severe trauma or fractures.


  • Malignant tumors that enter the blood vessels or a few isolated cells may enter the circulation.

Foreign Bodies

  • From pieces of a catheter, parasites like Schistosomes and its eggs.

Amniotic Fluid

  • A rare condition where amniotic fluid containing fetal cells, hair or other debris may enter the uterine vein and act as emboli.

What happens to an embolus?

An embolus may result in reduced blood flow and diminished oxygen supply to an area resulting in hypoxia and tissue ischemia. If no immediate medical intervention is undertaken, this can lead to an infarct (tissue death).

An embolism may lodge in the pulmonary artery (pulmonary embolism) which may result in sudden death or cause the blockage of an artery supplying oxygen to the brain thereby resulting in a stroke (cerebrovascular accident or CVA).

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