What are blood clots and what causes them?

When we injure ourselves and start to bleed, our bodies make sure that the bleeding soon stops by forming a clump of blood (a blood clot) that closes the wound. This reaction is very important, because it ensures that we lose as little blood as possible, stops germs from getting into the wound, and allows the wound to heal.

But sometimes blood clots form in the bloodstream even though there are no external injuries, and blood vessels may become blocked as a result. This can lead to dangerous complications such as a or stroke. These kinds of blood clots only occur very rarely in healthy people. But certain illnesses and genetic factors can increase the risk of blood clots forming. Many people who have this higher risk take anti-clotting medication as prevention.

When we injure ourselves and start to bleed, this is what happens:

  1. Our blood vessels become narrower. This reduces the flow of blood to the injured tissue, limiting the loss of blood.
  2. Blood platelets in the bloodstream, known as thrombocytes, attach to the damaged area of the blood vessel and clump together to reduce the bleeding.
  3. The body then activates a number of substances in the blood and the tissue. These substances solidify the clump by forming a special protein and fix the clump at the wound. These substances are called clotting factors or coagulation factors. There are 13 clotting factors in human blood and tissues. Most of them are made in the liver. The liver needs vitamin K to make some of these clotting factors. Our bodies cannot make their own vitamin K, so people have to get it in their diet.

Blood clots can also form even if the person does not have any external injuries. For instance, if blood flows too slowly and it starts to build up, large numbers of blood platelets may group together and stick to each other, forming a blood clot. It is also not uncommon for them to form because the inner walls of blood vessels are damaged, for example in atherosclerosis. If clotting factors are stronger, that can also increase the risk of blood clots forming for no identifiable reason. There are a number of reasons that this can happen, including a genetic predisposition, a tumor, or because somebody is taking a particular kind of medication. The medical term for a blood clot is “thrombus”.

When can blood clots become dangerous?

If a blood clot forms in a vein it is called venous thrombosis. Venous thrombosis usually affects the veins in the legs. The main reason people get this kind of thrombosis in the legs is because they do not get enough movement over a long period of time – perhaps because they have had major surgery, or have a serious illness or injury. A blood clot in the legs can become dangerous if part of the clot (called an embolus) breaks off and blocks a blood vessel in the lungs. The medical term for this condition is pulmonary embolism. Typical signs of pulmonary embolism include sudden breathing difficulties, coughing, coughing up blood, and chest pain.

If blood clots form in arteries, the tissues and organs that they usually supply no longer get enough blood, or might not get any blood at all. This kind of thrombus usually develops in the coronary (heart) arteries or inside the heart. If a thrombus blocks a coronary artery, it can cause a . Blood clots that develop in the heart itself could cause a stroke if they move to the brain and block blood vessels there.

People who have have a higher risk of blood clots in the heart. Atrial fibrillation is a particular type of irregular heartbeat, where two of the chambers (called atria) beat very fast and irregularly. This means that blood does not flow through the heart as quickly and steadily. Artificial heart valves also increase the risk of a thrombus: Their surface is not as smooth as that of natural valves, so blood platelets are more likely to attach to them and form a blood clot.

Particular medications or illnesses such as cancer or genetic coagulation disorders can also increase the risk of blood clots developing.

Andreae S. Lexikon der Krankheiten und Untersuchungen. Stuttgart: Thieme; 2008.

Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson JL, Loscalzo J. Harrison’s Principles of internal medicine. New York: McGraw-Hill Companies. 18th ed; 2011.

Pschyrembel W. Klinisches Wörterbuch. Berlin: De Gruyter; 2014.

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Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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Updated on December 27, 2016
Next planned update: 2021


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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