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What are blood clots and what causes them?

When we’re injured, our body stops the bleeding by forming a clump of blood (a blood clot) that closes the wound. The blood clot also prevents germs from getting into the wound.

Sometimes blood clots form in the bloodstream even though there are no external injuries. If a blood vessel is blocked by a blood clot, dangerous complications like thrombosis or a stroke can occur. That very rarely happens in healthy people. But certain illnesses and risk factors increase the likelihood of blood clots forming and causing complications. Blood-thinning medications (anticoagulants) can reduce that risk.

How does blood clot?

If you get injured and start to bleed, this is what happens:

  1. Your blood vessels become narrower. This reduces the flow of blood to the injured tissue, limiting the loss of blood.
  2. Blood platelets (thrombocytes) in the bloodstream 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 tissue called clotting factors or coagulation factors. These substances form a special protein that makes the clump harder and attaches it to the wound.

There are a total of 13 different 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 can't make vitamin K, so we need to get it from our diet.

Why else might blood clots form?

Blood clots can also develop because of something besides an external injury, such as:

  • blood flowing too slowly (for instance, if you have to rest for a long time after breaking a bone)
  • a natural temporary increase in blood-clotting activity after major surgery (for instance, after getting an artificial knee)
  • damage to the inner walls of blood vessels, where platelets and other substances can build up (for example in atherosclerosis)
  • a disease that is already present at birth, in which the clotting factors have a stronger effect or the body produces more platelets than normal (for instance, congenital protein S or protein C deficiency)
  • the use of certain medications (like hormonal contraceptives that contain the estrogen and progestin)
  • cancer

The medical term for a blood clot is “thrombus.”

When can blood clots become dangerous?

Clots in the leg veins can become dangerous if they break away, travel to the lungs and block a blood vessel there. The medical term for this condition is pulmonary embolism. The typical signs of pulmonary embolism include sudden breathing difficulties, coughing, coughing up blood, and chest pain.

If blood clots form in arteries, the organs that they usually supply may no longer get enough blood, or might not get any blood at all. One example of this is blood clots in the coronary arteries: If one of these arteries is blocked, it causes a heart attack.

Blood clots that develop in the heart itself could cause a stroke if they travel to the brain and block blood vessels there. This is more likely to happen in people who have . Atrial fibrillation is a particular type of irregular heartbeat where two of the heart's chambers (called atria) beat very fast and irregularly. This means that the blood flows more slowly there, so it might build up and clot.

The risk of blood clots forming in the heart is also higher in people who have artificial heart valves. Blood platelets may stick to the valves and clump together there.

Brandes R, Lang F, Schmidt R. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2019.

Kasper DL, Fauci AS, Hauser SL et al. Harrison's Principles of Internal Medicine. New York: McGraw-Hill; 2015.

Menche N. Biologie Anatomie Physiologie. München: Urban und Fischer; 2016.

Pschyrembel Online. 2022.

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What are blood clots and what causes them?

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Updated on October 25, 2022

Next planned update: 2025

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

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