The multistep process of blood clot formation to stop bleeding is called coagulation. When the entire coagulation cascade works properly, blood holds together firmly at an injury site and bleeding stops. People who have a bleeding disorder, however, are unable to make strong clots quickly or at all.
How a blood clot is made
The coagulation cascade is a complex chemical process that uses as many as 10 different proteins (called blood clotting factors or coagulation factors) found in plasma in the blood. Put simply, the clotting process changes blood from a liquid to a solid at the site of an injury. Here’s how the process works:
A small tear in a blood vessel wall (for example, from a cut on the skin or an internal injury) causes bleeding.
- Vessel constriction
To control blood loss the blood vessel narrows (called constriction), thus limiting blood flow through the vessel.
- Platelet plug
In response to the injury, tiny cells in the blood called platelets are activated. The platelets stick to one another and to the wound site to form a plug. The protein von Willebrand factor (VWF) helps the platelets stick to each other and to the blood vessel wall.
- Fibrin clot
Next, clotting factor proteins trigger production of fibrin, a strong, strand-like substance that forms a fibrin clot, a mesh-like net that keeps the plug firm and stable. Over the next several days to weeks, the clot strengthens and then dissolves as the wounded blood vessel wall heals.
What happens when a person has a bleeding disorder?
People with von Willebrand disease either don’t have enough of the VWF protein or the VWF protein doesn’t work properly. When they have a bleed they’re not able to form a platelet plug. In addition, VWF acts as a carrier protein for factor VIII (FVIII), one of the clotting factors in plasma. VWF helps ensure enough FVIII is in the blood and that it gets to where it’s needed. Without VWF, FVIII will be broken down in the bloodstream and there may not be enough of it to stop bleeding.
When a person has hemophilia, the blood vessel narrows and the platelets form a plug, but one of the clotting factor proteins essential to a firm fibrin clot is missing or damaged, so the clot is not made or is not strong enough to stop the bleeding. This is why a person with hemophilia bleeds for a longer period of time.
A number of rare platelet disorders may also disrupt the blood clotting process.