Merck Manual

Please confirm that you are not located inside the Russian Federation

Quick Facts

How Blood Clots


The Manual's Editorial Staff

Last full review/revision Sep 2020| Content last modified Sep 2020
Click here for the Professional Version
Get the full details
NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Topic Resources

What is a blood clot?

A blood clot is a clump of material that your body makes to plug up a bleeding blood vessel. Blood clots look like dark purple jelly. They're made up of things in your bloodstream:

  • Platelets (cell-like blood particles)

  • Red blood cells

  • Special proteins called clotting factors

After your blood vessel heals, the blood clot isn’t needed anymore. Then your body breaks up (dissolves) the clot.

How does blood clot?

Platelets, red blood cells, and clotting factors circulate in your bloodstream. That way, they can be right there when a blood vessel gets cut or damaged. When a blood vessel is cut, a blood clot forms:

  • The blood vessel narrows to slow the flow of blood

  • Platelets stick to the damaged area of the blood vessel

  • The platelets release substances that activate the clotting factor proteins

  • The clotting factors form a net that traps red blood cells and more platelets

The clump of material quickly grows until it's big enough to plug the blood vessel.

Blood Clots: Plugging the Breaks

When an injury causes a blood vessel wall to break, platelets are activated. They change shape from round to spiny, stick to the broken vessel wall and each other, and begin to plug the break. They also interact with other blood proteins to form fibrin. Fibrin strands form a net that entraps more platelets and blood cells, producing a clot that plugs the break.

Blood Clots: Plugging the Breaks

What can go wrong with the clotting process?

Too much clotting

Some health problems cause your body to form too many blood clots (excessive blood clotting). Your clotting factors may be overactive. Or there may be a problem with your clot-dissolving system.

Too much clotting can lead to other problems such as:

  • Stroke from blocked vessels in the brain

  • Heart attack from blocked vessels leading to the heart

  • Pulmonary embolism, a problem that happens when your blood carries clots from your legs, pelvis, or belly area into your lungs

Not enough clotting

Some health problems keep your body from clotting enough. Even small damage to a blood vessel may lead to serious bruising and bleeding. You may not have enough platelets, or they may not work properly. Or you might not have enough clotting factors. Some medicines affect your ability to form clots.

How do doctors treat blood clots?

If your blood is clotting too much, doctors may give you:

  • Drugs such as aspirin that block platelets

  • Drugs that block your clotting factors

Drugs that block your clotting factors are sometimes called "blood thinners."

If you have a dangerous blood clot in your brain or heart, doctors may give you a clot-dissolving drug. Clot-dissolving drugs can also dissolve helpful blood clots, so you may start bleeding (but this is rare).

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
Others also read

Also of Interest


View All
Sickle Cell Disease
Sickle Cell Disease
Sickle cell disease is a blood disorder that is inherited from both parents. This disorder...