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Blood clotting is an important process that prevents excessive bleeding, but sometimes, blood clots in places where it shouldn’t. In this lesson, learn disorders that affect blood clotting.


Hello, I’m Pat the Platelet! Welcome to my home. We’re actually inside a blood vessel inside your body.

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This is where I spend all my time! I’m here today to show you how I, together with the other platelets inside your body, help your blood to clot. I’ll also show you what happens when platelets make a mistake and cause your blood to clot in places where it shouldn’t.

What Do Platelets Do?

First, let’s talk about what platelets are and how they work.

As a platelet, I am one of three general types of cells that live in your blood, and I am sometimes also known as a thrombocyte. When I am inside your blood vessels, I float around doing pretty much nothing, but if your blood vessel is damaged, such as when you get injured, proteins in your tissues cause me to become activated. Then, a cascade of events happens that results in me being stuck to many other platelets to form a blood clot, or thrombus.This process is really important and prevents excessive bleeding when you get hurt, but can sometimes also happen in places where it is not helpful, like inside your blood vessels.

How Many Platelets Do You Need?

As a platelet, I am really tiny, only about two to three micrometers in diameter, and there are a lot of other cells like me in your blood.

Normally, there are about 150,000 to 400,000 platelets in every cubic millimeter of human blood.Sometimes, you can have certain medical conditions that cause the number of platelets to be either too high or too low. If there are less than 50,000 platelets per cubic millimeter, this means that you have too few platelets for your blood to clot normally, a condition called thrombocytopenia.

This can happen following a severe infection, or can be caused by genetic conditions, cancer treatment, or other medications. On the other hand, if your platelet count is above 750,000 platelets per cubic millimeter, this is a condition called thrombocytosis. This can develop because of autoimmune disorders, severe inflammation, disorders of the spleen, or after taking certain drugs.

If you have too many platelets, it makes it more likely that your blood will clot inappropriately, so thrombocytosis is usually treated with anticoagulant drugs.


Although platelets like me are really important for helping your blood to clot when you are injured, sometimes we make a mistake and cause blood clots to form inside your blood vessels instead of outside. The formation of blood clots inside blood vessels is called thrombosis. If the thrombus forms in a small blood vessel, it often blocks the flow of blood completely. When a thrombus completely blocks, or occludes, blood flow in a vein or artery, it is called a thrombotic occlusion. This often happens in the coronary arteries, which are small and commonly affected by atherosclerosis, increasing the risk of thrombosis.

When a coronary artery is blocked by a thrombus, it is called coronary thrombosis, and this is the main cause of heart attacks. Other blood vessels that are commonly affected by thrombosis are the deep veins in the legs. If you do not move your legs for long periods of time, the blood can pool in your deep leg veins, increasing the risk that a blood clot will form. When a thrombus forms in the deep veins and blocks blood flow, it is called a deep vein thrombosis.Deep vein thrombosis is more likely on long airplane rides where you may not move around enough, and in people who have had surgery or have to keep their legs immobilized after a fracture.

In all of these cases, the cause of the thrombosis is the lack of blood flow in the deep leg veins.


Thrombotic occlusion at the location where the thrombus forms is not the only danger that you face when a thrombus forms inside a blood vessel. In many cases, a thrombus may form inside the heart or in a large blood vessel where it does not block blood flow or cause any problems locally. However, pieces of the thrombus can break away and float in the blood to other areas of the body. Any free-floating mass, such as a piece of a thrombus, that can travel in the circulation and block small vessels is known as an embolus, and the blockage of a blood vessel by an embolus is called an embolism. Just like thrombosis, if a blood vessel is blocked by an embolus, it can cause heart attacks, strokes, and tissue damage in other organs and tissues that are not receiving enough blood.

Risk Factors for Thrombosis and Embolism

I and your other platelets do not want to block your blood vessels, but sometimes we have a hard time figuring out exactly where we need to form a clot and where we don’t. Are there things that you can do to reduce your risk of thrombosis and embolism? Yes! First, thrombosis often occurs when atherosclerotic plaques in arteries rupture. Anything that you can do to reduce your risk of developing significant atherosclerosis would also reduce your risk of thrombosis and embolism.

This includes keeping your blood pressure, blood lipid levels, and weight low and your activity level high.Smoking also increases your risk of both atherosclerosis and thrombosis, so refraining from smoking is something else you can do. In addition, if you have atrial fibrillation or a heart valve replacement, your risk of developing a thrombus is elevated, so you should take anticoagulant medications to reduce your risk. The risk of thrombosis is also very high after you have surgery, or if you have broken bones, so anticoagulants may be necessary in those cases as well. When you are traveling, your risk of deep vein thrombosis goes up if you sit still for long periods of time, so get up and walk around as much as possible. You may even want to wear some compression stockings to help prevent blood pooling in your legs.

Of course, sometimes, you may do everything you can and still develop thrombosis. It’s important to treat it right away if you have any symptoms, such as swelling and pain in your legs, shortness of breath, or chest pain.

Lesson Summary

Thanks for joining me to learn about what platelets do and how they can sometimes form a blood clot, or thrombus, inside your blood vessels.

Let’s take a moment and review all the terms that we just learned about blood clotting disorders:

  • Thrombocytopenia is a condition in which there are too few platelets for your blood to clot normally (less than 50,000 per cubic millimeter).
  • Thrombocytosis is a condition where the platelet count is above 750,000 platelets per cubic millimeter.
  • Thrombosis is the formation of blood clots inside blood vessels.
  • Thrombotic occlusion is a thrombus that completely blocks, or occludes, blood flow in a vein or artery.
  • Coronary thrombosis is a coronary artery that is blocked by a thrombus.
  • Deep vein thrombosis is a thrombus that forms in the deep veins and blocks blood flow.
  • Embolus is any free-floating mass, such as a piece of a thrombus, that can travel in the circulation and block small vessels.
  • An embolism is the blockage of a blood vessel by an embolus.

Learning Outcomes

Once you are finished, you should be able to:

  • Recall what a platelet is and how a thrombus is formed
  • Name and describe blood clotting disorders

Medical Disclaimer: The information on this site is for your information only and is not a substitute for professional medical advice.

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