Overview

Blood Clotting Disorders

There are three stages in the normal blood clotting process (also called hemostasis or coagulation):

1. First, the injured blood vessels constrict to reduce potential loss of blood.
2. Platelets then stick to cells on the wall of the damaged blood vessel (endothelial cells) at the site of the injury. The platelets begin to stick to each other (aggregate), promoting formation of a platelet plug (primary clot).
3. Finally, clotting plasma proteins are activated in sequence to convert fibrinogen to fibrin—the final clot stabilizer—which is necessary to establish a mature clot that contains a stabilizing network (like a scab).

In healthy individuals, an adequate amount of plasma clotting factors circulate in an inactive form in the blood until they are stimulated to convert to fully activated enzymes that can activate the next factor in the sequence.

In a person with a bleeding disorder, one of these steps is not working properly. For example, in hemophilia and von Willebrand disease, the first two steps function normally: the blood vessels constrict, an immature platelet plug is formed, and bleeding may stop or slow down. Since one of the clotting factors is decreased or absent, however, a mature fibrin clot is not formed. Instead, a soft, ineffective clot results. In this case, the platelet plug breaks down, the bleeding resumes, and the process begins again, leading to a repeat cycle of bleeding and stopping.

In a person with a low platelet count (thrombocytopenia) or a platelet disorder where they do not function properly, the first step of blood vessel constriction is intact. A platelet plug does not form and the fibrin plug is fully formed.