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What Happens When a Person Bleeds

When the body is injured and starts to bleed, the bleeding stops when a clot forms. The multistep process of clot formation is called coagulation. When blood clot formation occurs properly, the blood holds together firmly at the site of the injury.

People who have a bleeding disorder are unable to make strong clots quickly or at all.

This section of The Basics of Bleeding Disorders covers:

What Is a Bleeding Disorder?

A bleeding disorder is a condition in which a person tends to bleed longer (not faster) before a blood clot forms to stop the bleeding. A bleeding disorder can be caused by a defect in the blood vessels or from an abnormality in the blood itself. The abnormality may be in blood clotting factors (proteins called coagulation factors) or in platelets.

How Is a Blood Clot Made?

Blood clotting (called coagulation) is the complex chemical process that controls bleeding. This process uses as many as 10 different proteins (called blood clotting factors or coagulation factors) found in plasma in the blood. Simply stated: this multistep process changes blood from a liquid to a solid at the site of an injury.

Blood is made up of many different cells—most notably: red blood cells, white blood cells, and platelets—as well as clotting factors. These and other elements float through blood vessels in a straw-like liquid called plasma. Normally, when a blood vessel is injured (for example, from a cut on the skin or an internal injury), blood starts to flow. The first step in controlling blood loss is for the blood vessel to narrow (called constriction) so blood flow lessens. Within seconds, the tiny platelets rush to the site of the injury and bunch together around the wound. They attract other platelets and help form a plug to close up the break at the site.

When a blood vessel is injured, other elements called tissue factors come into play to help stop bleeding. They start a chemical reaction that causes clotting factors to produce fibrin, a strong, strand-like substance that surrounds the platelet plug. Ultimately, the fibrin clot takes on the look of mesh, which keeps the plug firm and stable. Over the next several days to weeks, the clot strengthens and then dissolves as the wound heals.

Watch the video below to learn how blood clots form.

What Happens When a Person Has a Bleeding Disorder?

People with hemophilia can’t make a firm fibrin clot. The first two steps (see Figure below) to stop bleeding usually work normally in a person with hemophilia: the blood vessel narrows and the platelets make a plug. A person with hemophilia has a problem when a fibrin clot is needed to stop the bleeding. They don’t have enough of certain clotting factors to do the job. Because of this, the fibrin clot is not made or is so thin that it’s not strong enough to stop the bleeding.

What happens when a person has a bleeding disorder



No bleeding disorder

Bleeding starts


Vessels Constrict


Platelet plug


Fibrin clot


bleeding disorder

Bleeding starts


Vessels Constrict


Incomplete platelet plug; continued bleeding


Incomplete and/or delayed formation of fibrin clot; continued bleeding