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What is thrombosis?

Thrombosis is when a blood clot (thrombus) forms inside a blood vessel, potentially blocking the flow of blood through the circulatory system and causing serious health issues.

Blood carries the oxygen necessary for survival through the blood vessels to all organs of the body, so any blockage of this blood flow can be very dangerous. Blood clots are often the underlying cause of heart attack and stroke, if they occur in the blood vessels leading to the heart and brain respectively.

Blood clots normally only form if there is a cut to the skin and bleeding starts. The blood thickens and forms a clot at the site of the cut to stop the bleeding.

Types of thrombosis

There are two main types of thrombosis, depending on which blood vessels are blocked.

Arterial thrombosis

Arterial thrombosis is when a blood clot blocks an artery. Arteries carry oxygen-rich blood from the heart around the body, so arterial thrombosis can be dangerous. If a blood clot occurs in the main arteries of the heart, this can lead to heart attack. If a blood clot occurs in the arteries to the brain, it can cause a stroke.

Venous thrombosis

Venous thrombosis is when a blood clot blocks a vein. Veins carry oxygen-depleted blood from the body back to the heart, and blockages can cause serious problems. An example of venous thrombosis is deep vein thrombosis (DVT), which most commonly affects veins in the legs, such as the femoral vein. In DVT, there is the risk of a pulmonary embolism – when a clot breaks off, travels through the blood stream, and becomes lodged in the pulmonary artery (the main blood vessel to the lungs).

Superficial thrombosis

Blood clots can also occur in a vein near the skin. This is known as superficial thrombosis, and while it can be painful, is usually not serious.

Thrombosis vs embolism

An embolism occurs when a bodily substance or foreign object becomes lodged in a blood vessel and blocks the flow of blood in the body. When the embolism is specifically caused by a blood clot, it is known as a thromboembolism. For example, a piece of blood clot can break off and travel through the blood stream around the body, become stuck in a blood vessel and block blood flow.

How do blood clots form?

Blood has the ability to clot, which is important to stop excessive bleeding in the case of an injury, such as a cut. In these instances, a blood clot forms to seal up the wound and stop the bleeding.

However, unwanted blood clots can form within the circulatory system as a result of atherosclerosis. This is the process by which plaques that are made up of fat, cholesterol, calcium and other substances build up in the walls of the arteries.

Over time, these plaques harden, narrow the opening of the arteries and restrict the blood flow. If these plaques break open, they form a blood clot that can further limit or even block the flow of blood to organs and other parts of the body.

Atherosclerosis is the main underlying cause of cardiovascular disease (CVD). If it occurs in the arteries that supply blood to the heart, it can result in a heart attack. If it occurs in one of the arteries to the brain, it can cause a stroke. If it occurs in arteries in the limbs, it can lead to peripheral artery disease.

Thrombosis symptoms

The symptoms of thrombosis can vary between people and may also appear similar to those of other health problems and blood disorders. They include:

  • chest pain
  • swelling in the leg or arm
  • pain in the calf or inner thigh
  • weakness or numbness in one side of the body
  • feeling mentally unclear.

Symptoms of superficial thrombosis

A superficial thrombosis can be painful around the site of the clot, and show swelling and redness.

Symptoms of DVT

While DVT can occur without symptoms, it can cause pain and swelling in the affected leg or pelvis. Pain in the leg often starts in the calf and can feel like a cramp. The affected leg may also show red or discoloured skin, and there may be a feeling of warmth in the leg.

The impact of thrombosis

Thrombosis can lead to heart attack and stroke, amongst other cardiovascular diseases. Cardiovascular disease is Australia’s – and the world’s – number 1 killer, while 85 per cent of all cardiovascular deaths are due to heart attack and stroke.1

Every year, 30,000 Australians develop thrombosis.2, 3 Up to 10 per cent of all deaths in Australia are due to thrombosis, and up to 70 per cent of these cases are preventable.4

The risk factors for thrombosis

There are many risk factors for thrombosis, and the risk of developing thrombosis is about the same between men and women. However, women are more at risk if they are pregnant or taking particular oral contraceptives.

You may have a higher risk of developing thrombosis if you:

  • have other medical conditions such as cancer, heart disease, diabetes or a blood disorder
  • are unable to move around or are immobile for long periods, such as through hospitalisation or bed rest, or long-term sitting such as during travel
  • have had a major injury or surgery, especially to the hips and legs
  • are on certain medicines that may increase your risk of clotting
  • are on medications that contain oestrogen, such as birth control and hormone replacement therapy
  • are pregnant or 6 weeks post-birth
  • have varicose veins
  • are over 50
  • have a family history of thrombosis.

Lifestyle factors also increase your risk of thrombosis. These include:

How to prevent thrombosis

There are some lifestyle factors that you can take charge of to lower your risk of developing thrombosis.

How is thrombosis diagnosed?

Your doctor will take your medical history, talk about your overall health and lifestyle, and conduct a physical examination.

If thrombosis is suspected, you might have tests such as an ultrasound to check the blood flow in your blood vessels, a blood test to see how well your blood can clot, or an X-ray to see what type of blood clot you have and where it is located.

How is thrombosis treated?

Thrombosis treatment will depend on which type of thrombosis you have, the severity and location of the blood clot, and your overall health condition.

Medications can be prescribed to dissolve existing blood clots, and anticoagulation medication is also available that can thin the blood to make it less likely to clot and help prevent blood clots.

In more severe cases, surgery may be required to remove or bypass the blood clot, or to widen the affected blood vessels and hold them open.

How is HRI fighting thrombosis?

HRI is conducting groundbreaking research from a broad range of angles to understand thrombosis and to find innovative ways of preventing, detecting and treating the resultant cardiovascular disease.

Our Vascular Complications Group is investigating ways to stimulate blood vessel growth to bypass blocked arteries in peripheral artery disease, one of the cardiovascular diseases caused by atherosclerosis leading to thrombosis.


  1. WHO Organization. Cardiovascular diseases (CVDs).
  2. Ho WK et al. The incidence of venous thromboembolism: a prospective, community-based study in Perth, Western Australia. Med J Aust. 2008 Aug 4;189(3):144-7. doi: 10.5694/j.1326-5377.2008.tb01947.x.
  3. Szabo F et al. Venous thromboembolism in tropical Australia and in Indigenous Australians. Semin Thromb Hemost. 2014;40(7):736-40. DOI: 10.1055/s-0034-1390002.
  4. Guyatt GH et al. Antithrombotic therapy and prevention of thrombosis, 9th ed: American College of Chest Physicians evidence-based clinical practice guidelines. Chest 2012;141:7S–47S.

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