How Blood Clots
There are three stages in normal blood clotting (also called hemostasis or coagulation). First, the injured blood vessels constrict to reduce potential loss of blood. Platelets then stick to cells on the wall of the damaged blood vessel (endothelial cells) at the site of injury. Then the platelets begin to stick to each other (aggregate), promoting formation of a platelet plug (primary clot). Finally, clotting plasma proteins are activated in sequence in order to convert fibrinogen to fibrin to establish a mature clot that contains a stabilizing network (like a scab).
Formation of this network can be accomplished through the clotting proteins which are activated in order to trigger the final, common steps involved in the conversion of fibrinogen to fibrin - the final clot stabilizer. Plasma clotting factors circulate in an inactive form 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's disease, the first two steps function normally: the blood vessels constrict, an immature platelet plug is formed, and bleeding may stop or slow down. However, since one of the clotting factors is decreased or absent, a mature fibrin clot is not formed; 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 lot 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.