Understanding blood clotting

PDF download is not available for Arabic and Urdu languages at this time. Please use the browser print function instead

Teens living with hemophilia can learn how the body forms clots to stop bleeds.

There are three different types of blood vessels in your body; arteries, veins and capillaries. When a blood vessel breaks the body forms a clot to repair the injured vessel.


Arteries take red blood cells that carry oxygen (oxygenated blood) from the heart to the body.


Veins take red blood cells that no longer carry oxygen (deoxygenated blood) from the body to the heart.


Capillaries are small, thin blood vessels. Inside capillaries, red blood cells carry and deliver oxygen into tissues of the body; the capillaries then carry the deoxygenated red blood cells back into veins.

Networks of veins and arteries run through your joints to supply blood to all parts of your body.

How does the body stop a bleed?

When a blood vessel breaks it starts to bleed. To prevent blood loss, blood cells called platelets stick to the damaged area and to each other, forming a plug. This platelet plug triggers a healing process called blood clotting, in which proteins in the blood strengthen the platelet plug to repair the injured blood vessel. These proteins are called clotting factors. 


For more videos about hemophilia, please visit the Teens Taking Charge: Hemophilia playlist.

Clotting heals blood vessels, similar to the way roads are repaired in everyday life

One way to understand how clotting works is to compare it to the way construction workers repair roads. Here is how the analogy works:

Your bodyRoad repair
Blood vesselsRoads
Blood vessel wallRoad surface
Blood flowTraffic flow
PlateletsConstruction workers
Clotting factorsRoad patch
Chemical signalsRadio communication used by construction workers. They use radios signals to request for materials needed to fix the damaged road.
FibrinThe fence around the damaged part of the road.

Think of blood vessels in your body as roads, the vessel walls as the road surface, and blood flow as the flow of traffic on the road. When there is a crack in a part of a road, traffic flow slows down because the road narrows as cars cannot drive on the damaged site. Similarly, after a blood vessel breaks, the vessel shrinks (vasoconstricts) to decrease blood flow around the site of the damage.

Once the construction workers arrive, they use radios to request for materials they need to patch up the crack, such as concrete and sand. Like construction workers, platelets arrive at the broken site and release chemical signals to ‘call’ in the material they need to fix the damage. These materials are substances in the blood called clotting factors.

When the construction workers have all their material, the damaged area is fenced off. This way the workers can focus on repairing the road and allow the concrete patch to harden. A fence-like mesh also forms around the damaged vessel. In the bloodstream, this fence is made up of a protein called fibrin. This way the clotting cascade can continue to fix the damage, allowing time for the clot to harden - effectively healing the wound.

Last updated: March 13th 2019