Causes of Blood Clots in the Leg

Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Do you know someone who has had a blood clot in their leg? Did you know there's more than one reason for blood clots to form in the legs? This lesson will discuss the three general reasons for them and provide specific examples of each.

Sitting Disease

Sitting disease. Have you heard of it? It's not a disease in the real sense, but it is a humongous problem. Sitting disease basically implies that if you don't get enough exercise because you sit for too long at work, at home, in a car, on a plane, and so forth, you're setting yourself up for numerous health problems. Those problems include obesity, diabetes, arthritis, heart disease, stroke, and blood clots in the legs.

Today we'll focus on the last of these, the causes of blood clots in the legs.


Blood clots in the blood vessels of our body are called thrombi, which is the plural for 'thrombus'. These particular blood clots form in the veins of the leg.

This image shows a thrombus within a vein of a leg.

There are three overarching reasons for why blood clots in the leg may form. One of them is trauma. Imagine that the vein is like a sticker. If you pull off the backside of the sticker it becomes, well, sticky! That's similar to what happens with an injured vein. The injury exposes the sticky side of the vein and causes cells and proteins that form blood clots to now stick to the vein, forming a thrombus.

Real world examples of trauma that can lead to this include:

  • Surgery
  • Car accidents
  • A broken bone

Venous Stasis

Another major reason for blood clots forming in the legs is called venous stasis. You know how fast moving rivers are unable to freeze as quickly, if at all, compared to slow moving rivers or still ponds? Well, your blood is much the same. Except we're not talking about freezing, we're talking about clotting. Blood that is moving and travels well and quickly is far less likely to clot than blood that's barely moving at all.

Real world examples of what can lead to or exacerbate venous stasis include:

  • Lack of exercise in daily life
  • Long flights where a person sits in one spot for too long
  • Surgery under general anesthesia, where the person is obviously not moving
  • Being bed-ridden because of surgery or a serious medical illness, like a stroke

As you can tell, these reasons all tie into one form of sitting disease or another, whether by choice or due to a physical restriction.


The final major reason for blood clots in the leg is hypercoagulability. Hyper- means excessive (abnormally high) and 'coagulability' means tendency to clot. You know how some things become stickier over time, like spilled soda on a counter that is left to dry over time? The soda isn't sticky when it's wet, but it sure is when it's dry! Well, various factors can also increase the stickiness of blood over time and thus its tendency to clot.

Real world things that can lead to this include:

  • Obesity
  • Diabetes
  • Some cancers
  • Certain medications, like oral contraceptives
  • Inflammation within the body or as a result of a blood infection
  • Sickle cell anemia
  • Immobility (there's that pesky sitting disease again)

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