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Causes of thrombosis

NHS Choices Medical Reference

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Thrombosis occurs when a blood clot forms within a vein or an artery, slowing or stopping the flow of blood.

How the blood clots

Your blood contains cells called platelets and proteins, known as clotting factors, which together make up the blood-clotting mechanism.

When a blood vessel is cut, the platelets and clotting factors in your blood mesh together to form a solid clot at the site of the wound. This clot acts as a plug to stop the wound bleeding.

Normally, the blood-clotting mechanism is only triggered when a blood vessel is damaged and bleeds, such as when you cut yourself. Sometimes the blood may start to clot even when a blood vessel has not been damaged. If this happens, a blood clot can form within a vein or artery (thrombosis).

Why does thrombosis occur?

Thrombosis can occur for many different reasons. The main risk factors are:

  • getting older,
  • being inactive or immobile for long periods of time,
  • having a previous blood clot,
  • having a previous heart attack or stroke,
  • having a member of your family with a blood clot,
  • cancer,
  • taking the combined contraceptive pill or hormone replacement therapy (HRT),
  • a condition such as thrombophilia that makes your blood more likely to clot,
  • high blood pressure (hypertension),
  • high cholesterol levels,
  • pregnancy,
  • smoking,
  • a poor diet,
  • being obese, and
  • having an operation on the lower half of your body.

However, the three main causes of thrombosis are:

  • slow blood flow,
  • blood clotting too easily, and
  • blood vessel damage.

In some cases, thrombosis may occur due to a combination of these factors. The three main causes of thrombosis are described in more detail below.

Slow blood flow

When you are inactive, your blood tends to collect in the lower parts of your body, often in your lower legs. This is usually nothing to worry about because when you start to move around, your blood flow increases and moves evenly around your body.

If you are inactive or immobile (unable to move at all) for a long period of time, such as after an operation or on a very long journey, your blood flow can slow down considerably. A slow blood flow makes it easier for the clotting mechanism to be triggered and for a blood clot to form.

The blood clots too easily

In some cases, your blood may be 'stickier' than usual, which means that it has an increased tendency to clot.

One possible reason why the blood may clot too easily is a condition called thrombophilia. Thrombophilia can be inherited (passed down through family members), or it can result from defects in your blood that you are born with.

It is also possible to develop thrombophilia later in life. This can happen if you have another condition, such as Hughes syndrome (which occurs when your immune system attacks a particular fat in your blood).

Having cancer can also make your blood more likely to clot, and this can be made worse by chemotherapy and radiotherapy treatment.

Your blood may also be more prone to clotting if you are taking the combined contractive pill, or some types of hormone replacement therapy (HRT). Being pregnant also makes your blood 'stickier', which is your body's way of preventing too much blood loss during childbirth.

Blood vessel damage

If the wall of a blood vessel is damaged, it may become narrowed or blocked, which can result in the formation of a blood clot.

Blood vessels can be damaged by injuries such as broken bones or severe muscle damage. Sometimes, blood vessel damage that occurs during surgery can cause a blood clot, particularly in operations on the lower half of your body.

However, in most cases, damage to blood vessels is due to atherosclerosis (narrowing and hardening of your arteries). Atherosclerosis is caused by the build-up of fatty deposits on the walls of your arteries and is the main cause of arterial thrombosis.

Arteriosclerosis may occur for many reasons, such as poor diet or being obese. The major risk factors are:

  • smoking,
  • high blood pressure (hypertension), and
  • high cholesterol levels.

Cholesterol is a fatty substance that sticks to the walls of your arteries and makes them narrower.


  • Cholesterol: Cholesterol is a fatty substance made by the body that lives in blood and tissue. It is used to make bile acid, hormones and vitamin D.
  • Arteries: Arteries are blood vessels that carry blood from the heart to the rest of the body.
  • Veins: Veins are blood vessels that carry blood from the rest of the body back to the heart.
  • Blood vessels: Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
  • Blood: Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
  • Heart attacks: A heart attack happens when there is a blockage in one of the arteries in the heart.
  • Inflammation: Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Medical Review: October 04, 2008

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