Blood Clotting Disorders How Does Blood Clot?
When a blood vessel is injured, the damaged cells in the vessel wall send out chemical signals. These signals cause clots that slow or stop bleeding.
A blood clot forms through several steps:
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The blood vessel narrows. First, chemical signals cause the injured vessels to narrow to prevent more blood from leaking out.
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Platelets travel to the site of the injury. The chemical signals travel through your blood to the spleen, where many platelets are stored. The signals tell your spleen to release the platelets into your blood. Back at the injury site, the vessel walls become sticky and capture the platelets as they float past.
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A platelet plug forms. The platelets change shape and become stickier. This allows them to attach to the vessel wall and clump together into a plug.
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The blood clot forms. Clotting factors in your blood are normally turned off so that you do not form abnormal blood clots. When there is an injury, platelets release molecules into the blood that help turn on clotting factors. One important clotting factor is fibrin, a long, thin, and sticky protein. When it is turned on, it forms a mesh to hold the platelet plug in place. This is called a fibrin clot. The mesh also traps red blood cells to form a blood clot. The platelets contract to pull the two sides of the damaged vessel closer together, so it is easier to repair.
Once the blood clot is formed, your body’s repairs the injury. At this point in the process, factors in your blood start to break down the blood clot.
If you do not have enough platelets or clotting factors in your blood, your blood will not be able to clot as well. Read more in our Bleeding Disorders topic.
In other cases, your blood may clot too easily. Some conditions cause overactive clotting so that blood clots form in blood vessels throughout your body. Eventually, the platelets in your body are used up, which can then lead to bleeding. These conditions include: