Blood clotting is a natural process by which the body protects itself. When a blood vessel is severed, the blood thickens and forms a clot of blood cells and fibrin (a protein) to stop the bleeding. If we did not have this ability, people would bleed to death after even minor cuts.

However, when a blood clot (thrombus) forms in an artery, blocking the blood flow to the heart or the brain, it can cause a heart attack or stroke.

A clot can also form if the blood slows and pools in the heart’s chambers. A piece of this clot might then travel in the bloodstream to lodge in an organ or artery, cutting off the blood supply. A travelling clot is called an embolus.

Normally, clot formation stops once the clot has served its purpose, and the clot dissolves. If this mechanism fails, clots may persist or grow excessively, causing complications.


In some patients, clots form because their blood contains excess platelets (cells which clump together to stop bleeding) and clotting factors (proteins in the blood that help clot formation).

If the inner lining (endothelium) of the blood vessels or heart is damaged, this attracts platelets and activates clotting factors. The thrombus formed may be large enough to obstruct the blood vessel.

Slow or turbulent blood flow also encourages clot formation, much as water pooling in a river can get clogged up.

These triggers may be caused by a number of medical conditions. People at risk of thrombi include those with:

    • High blood pressure

    • Recent heart attack or stroke

    • Heart-valve disease

    • Raised platelet levels or genetic clotting disorders

    • Atrial fibrillation (irregular heart rate) or heart failure

    • Trauma to blood vessels

    • Atherosclerosis (fat deposits on the artery lining)

    • Infections

    • Inflammatory bowel disorder

    • Pregnancy

    • Renal disease

    • Auto-immune disease

    • Some cancers

    • Problems with veins, e.g. varicose veins or aneurysms

In addition, lifestyle choices can increase the risk of blood clots:

    • Smoking

    • Obesity (more than 10kg overweight)

    • Lack of exercise

    • Sitting or lying in one position for a long time (eg. on long flights)

    • Oral contraceptives, especially high-dose pills

    • Hormone-replacement therapy


Symptoms depend on where the clot is located:

    • Pulmonary embolism (in the pulmonary artery to the lungs): Sharp chest pain, rapid heartbeat, coughing blood, shortness of breath, low-grade fever.

    • Deep-vein thrombosis (in the leg or arm veins): Sudden pain, tenderness, swelling, slight blue colour.

    • Stroke (in the carotid artery of the neck, or in the brain): Visual disturbances, weakness, seizure, speech impairment.

    • Coronary thrombosis (in the coronary arteries):  chest pain, heart attack.

    • Abdomen: Severe abdominal pain, vomiting, diarrhoea.

    • Retinal vein occlusion (in the eye): a painless decrease in vision or “blind spot” in one eye.


If you have any of these symptoms, see your doctor urgently (especially if you have a heart condition).

Your doctor will take a complete medical history and do a physical examination. A diagnosis can usually be made quickly. However, specific tests may be needed to identify where the thrombus is located.

In the most commonly used test, venography or arteriography, a dye is injected into the blood vessel. X-rays can then accurately show the flow of the dye through the bloodstream.

Other, less invasive test are better suited to certain body areas, or for certain patients (eg. pregnant women). These may include blood tests, and various kinds of imaging such as ultrasound, ECG or magnetic resonance venography. Coronary angiography may be used in urgent cases of arterial thrombosis, and computerised tomography (CT) scans are used to establish if a stroke is due to an artery blockage or a brain bleed.


Treatment aims to remove the clot where possible, reverse or minimise damage, and prevent complications and recurrence.  Early treatment can stop the thrombus enlarging and prevent emboli. The type of treatment depends on whether the clot is in a vein or artery, its location and size, and the patient’s general health.

If the thrombus is in an artery and has caused a heart attack or stroke, then “clot-busters” (thrombolytic medications, e.g. streptokinase) may be given by drip to dissolve the clot. These medications must be given within three hours of the attack. This treatment may have side effects (in some cases, excessive bleeding) and is not suitable for all patients.

If a clot forms in a vein, it may cause a life-threatening pulmonary embolism. These thrombi are treated with a combination of painkillers and anti-clotting drugs. The affected area will be elevated, bandaged or put into an anti-embolism stocking to reduce swelling. In severe cases, a surgeon may remove a clot before it travels to the lungs.

All thrombosis patients will be placed on some form of anti-clotting medication to prevent more blood clots from forming. If there is an underlying cause for the clot formation, eg. atrial fibrillation, the patient may be put permanently onto anticoagulants, which may have side-effects.

Treatment is generally successful, if begun within a few hours. Lingering problems are usually due to delay in getting medical attention. Dissolving most of a coronary artery clot may avert a major heart attack, but the patient may still have a smaller attack. Brain damage may be minimised, but there may be residual neurological problems. Although thrombosis in the legs, eg. among air travellers, can very successfully treated, persistent leg swelling is a common problem.


If you have one of the underlying conditions mentioned above as risk factors, you should be vigilant about blood clots. Patients with cardiovascular disease should have regular check-ups, and and take their heart medication correctly. Patients with recent physical trauma, as well as certain cancers and auto-immune diseases, should be monitored for clot formation. Pregnancy is also a high-risk condition.

Patients who are immobilised after surgery may need anti-clotting medication and / or support stockings. Long-haul air travellers should be careful to drink enough water and to move around on the plane. They may also consider aspirin and use support stockings.

In general, weight loss, not smoking and regular exercise are of benefit to everyone. Patients over 50 often use aspirin for other reasons, and this may also help prevent clots.

Taking an omega-3 supplement such as fish oil or krill oil may help inhibit blood clotting.  Chia seeds are an alternative for vegetarians looking for an omega-3 supplement option.  Vitamin E and coenzyme Q10 may also possess blood thinning effects.  Talk with your doctor before you take omega-3 supplements with anti-clotting drugs, such as warfarin or clopidogrel.

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