Hemostasis: Response to Tissue Injury

Hemostasis is the body’s response to tissue injury through the process of coagulation and growth factor release at the injury site. Coagulation is a complex process in which blood loss from a damaged blood vessel is blocked by a platelet-fibrin network. Coagulation involves more than 30 proteins and other body substances.

How it works:

The injured blood vessel immediately constricts itself to try to prevent further loss. Platelets provide the next step. They respond to chemicals released at the site of blood vessel damage. They gather at the injury site. Next, they form a plug to slow blood loss. As a plug is being formed, the platelets continue to release enzymes and factors that control the clotting process. Once the platelet plug is formed, the involved platelets then begin to contract, thereby reducing the size of the clot and pulling the damaged vessel walls together.

The wound itself releases substances that attract and begin to activate platelets so they can take part in the coagulation process. The platelets lose their discoid shape, become more spherical, “sticky,” and start to send out pseudopod extensions from their surfaces. They adhere to the wound and granules within the platelets then emit more substances to attract additional platelets and continue the coagulation process.

Platelets are formed by fragmentation from very large bone marrow cells called megakaryocytes.  Once formed in the bone marrow, platelets can be stored in the spleen until needed. After they enter the circulation, platelets will remain in the blood for 10-12 days before being removed by the spleen.

Role of Growth Factors

When platelets become activated after adhering to the growing platelet plug, they begin to release factors in addition to the clotting factors previously described. The chemicals released in this second group are called polypeptide growth factors. Growth factors are secreted by activated platelets, and other cells, such as macrophages, fibroblasts, and endothelial cells, all of which are involved in the clotting and tissue healing process.

These released growth factors are chemotactic (the migration of additional White Blood Cells to an area of inflammation in response to the release of chemical mediators by neutrophils, monocytes and injured tissue). The presence of these newly migrated cells also creates an antimicrobial environment in the wound site. The growth factors and cells continue to work together to repair the injured tissue. In addition to soft tissue repair, many of these growth factors also play a role in bone healing and regenerative applications.