Blood Clots

Blood has a seemingly impossible job: It must flow continuously and smoothly throughout your body for an entire lifetime, but quickly shut off to prevent spills when you get a cut or injury.

Blood clots are healthy and lifesaving when they stop bleeding. But they can also form when they aren't needed and cause a heart attack, stroke, or other serious medical problems.

How Does Blood Clot?

The life cycle of a normal blood clot depends on a series of chemical interactions.

1. Platelets form a plug. Tiny bits in your blood called platelets get "turned on" by triggers released when a blood vessel is damaged. They stick to the walls in the area and each other, changing shape to form a plug that fills in the broken part to stop blood from leaking out.

When activated, platelets also release chemicals to attract more platelets and other cells, and to set off the next step.

2. The clot grows. Proteins in your blood called clotting factors signal each other to cause a rapid chain reaction. It ends with a dissolved substance in your blood turning into long strands of fibrin. These get tangled up with the platelets in the plug to create a net that traps even more platelets and cells. The clot becomes much tougher and more durable.

3. Reactions stop its growth. Other proteins offset extra clotting factor proteins so the clot doesn't spread farther than it needs to.

4. Your body slowly breaks it down. As the damaged tissue heals, you don't need the clot any more. The tough fibrin strands dissolve, and your blood takes back the platelets and cells of the clot.

What Causes Blood Clots?

The process begins whenever flowing blood comes into contact with specific substances in your skin or in blood vessel walls. When they touch, it usually means the skin or blood vessel wall is broken.

Waxy cholesterol plaques that form in arteries have these things inside, too. If the plaque breaks open, they'll start the clotting process. Most heart attacks and strokes happen when a plaque in your heart or brain suddenly bursts.

Blood clots can also form when your blood doesn't flow properly. If it pools in your blood vessels or heart, the platelets are more likely to stick together. Atrial fibrillation and deep vein thrombosis (DVT) are two conditions where slowly moving blood can cause clotting problems.


Medications Affect the Clotting Process

Some drugs stop platelets from signaling each other so they won't stick together.

Medicines called blood thinnersmake it hard for your body to make clotting factors, or they prevent proteins in the clot-forming process from working.

Clot-dissolving drugs such as altplase, streptokinase, and tenecteplase activate the protein that breaks down the fibrin strands. Sometimes doctors prescribe it as a treatment for heart attack or stroke.

WebMD Medical Reference Reviewed by Louise Chang, MD on January 9, 2018



Merck Manual: "How Blood Clots."

World Federation of Hemophilia: "The clotting process."

American Stroke Association: "Anti-Clotting Agents Explained."

Lipe, B. Circulation, Oct. 4, 2011.

Furie, B. New England Journal of Medicine, 2008.

Hoffman, R. Hematology: Basic Principles and Practice, 5th edition, Churchill Livingstone Elsevier, 2009.

Fiumara, K. Circulation, 2009.

© 2018 WebMD, LLC. All rights reserved.