Blood clots can form in both arteries and veins, but the causes, dangers, and prevention strategies are quite different depending on where they occur.
In Sweden, researchers at Lund University have discovered three new gene variants that significantly increase the risk of blood clots in the veins—specifically in the legs—by up to 180%.
Blood clots in arteries are usually caused by hardened, narrowed blood vessels filled with plaque. If a plaque bursts, the body treats it like an injury, triggering platelets to form a clot.
If the clot blocks blood flow to the heart or brain, it can cause a heart attack or stroke. These are called arterial blood clots and are more widely known and studied.
Venous blood clots, on the other hand, tend to get less attention—even though they are among the most common causes of death worldwide. A venous clot usually forms in the leg when blood flow slows down too much.
The body’s clotting system can then activate unnecessarily, forming a blockage. If the clot breaks off and travels to the lungs, it can cause a pulmonary embolism, a life-threatening condition.
In Sweden alone, over 10,000 people suffer from venous thromboembolism each year, and the number is rising. Age is a major risk factor—about 10% of people over 80 will experience a blood clot at some point. Other risk factors include obesity, inactivity, and being tall.
According to Professor Bengt Zöller of Lund University, tall people are more prone to venous clots because their longer veins slow down blood return to the heart. Gravity makes it harder to push blood back up from the legs, especially if a person is sitting still for too long.
Arteries and veins also behave differently due to blood pressure. Arteries have higher pressure, which contributes to plaque buildup and atherosclerosis. That’s why high blood pressure, high cholesterol, and smoking are big risks for arterial clots.
But veins are low-pressure systems and do not develop plaque in the same way. Instead, factors like obesity, poor muscle activity, and damaged vein valves play a greater role in clot formation in veins.
Diet and exercise also influence clot risk. Processed foods may slightly increase the risk, while plant-based diets and omega-3-rich foods—like those eaten by commercial fishermen—seem to offer protection.
Staying active is important, especially during long periods of sitting or bed rest. Traveling by plane or recovering from surgery are high-risk times for clots, and doctors may recommend blood thinners in those situations.
Pregnancy is another high-risk time. During pregnancy, blood naturally becomes more prone to clotting, and some protective proteins are reduced. That’s why doctors are extra cautious about blood clots in pregnant women, especially if other risk factors are present.
A major discovery in Sweden has been the genetic causes of venous thrombosis. About half of all cases have some genetic link. One well-known inherited condition is Factor V Leiden, which causes resistance to activated protein C (APC), a natural anti-clotting protein.
About 10% of Swedes carry this mutation. While this mutation may have once helped people survive injuries by bleeding less, it now poses a risk in our modern, less active lives.
In a recent study using data from 30,000 participants in the Malmö Kost Cancer Study, researchers looked at 27 genes related to clotting. They found three gene variants—ABO, F8, and VWF—that increased clot risk by 10 to 30 percent each.
People who had multiple variants saw their risk rise dramatically. Someone with five of these risk genes was found to have an 180% higher risk of venous clots. Unlike Factor V Leiden, which is mostly found in people of European descent, these three new gene variants are found worldwide in up to 50% of people.
This finding is important because it could change how long people need to take blood thinners after a clot, based on their personal genetic risk. As Professor Zöller puts it, “Tailoring treatment based on risk assessment will become increasingly important.”
So what can you do to lower your risk of a blood clot?
Keep moving. Avoid sitting for long periods, especially on flights or during recovery from illness.
Wear support stockings if you’re at risk and need to sit or stand for long stretches.
Take blood thinners if your doctor recommends them in high-risk situations, such as after surgery or during cancer treatment.
Avoid estrogen-based birth control or hormone therapy if you or your family have a history of blood clots.
Make lifestyle changes: stop smoking, lose weight, eat a healthy diet, and exercise regularly.
Stay protected against infections, which can trigger clotting.
A blood clot forms when blood thickens and clumps together, blocking blood flow. Whether in arteries or veins, these clots can be dangerous—but many are preventable with a healthy lifestyle, awareness of risk factors, and, increasingly, personalized medicine based on genetics.
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The research findings can be found in Research and Practice in Thrombosis and Haemostasis.
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