Unit D

Module 7 ~ Lesson 3

Blood Clotting

Platelets are cell fragments of larger cells called megakaryocytes. You normally have about 150,000 to 400,000 platelets/µl of blood. The number of platelets in your bloodstream is controlled by a negative feedback system. If there are too many platelets, their production is inhibited. If there are not enough platelets in the blood, their production will be stimulated. If this value should drop much below 50,000/µl, there is a danger of uncontrolled bleeding because of the essential role that platelets have in blood clotting.

When blood vessels are cut or damaged, the loss of blood from the system must be stopped to avoid fatal blood loss. This is accomplished by solidification of the blood, a process called clotting. A cascade of events results in the formation of a network of fibrin threads blocking the cut. The blood clot stimulates the growth of the structural framework and smooth muscle cells within the vessel wall. The repair process begins, and the blood clot is dissolved.

CDC/ Janice Haney Carr CC0

Hemophilia is a disease where the body does not have the ability to stimulate the production of a mesh-like network of fibrin molecules. As a result, an injured blood vessel in a hemophiliac will continue to lose blood for a longer period of time, and no blood clot will form. Healing also takes a longer time, which often increases the likelihood of infection or a reopening of the wound. Hemophiliacs can receive injections of a chemical that will stimulate the production of fibrin.