332 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. Platelets play an important role in hemostasis, the sequence of events that causes blood clots to form and bleeding to stop (Figure 11.9). Hemostasis involves four key steps: 1. Vessel wall injury and constriction. When the endothelium, or inner lining of a blood vessel, is injured, it releases endothelin (ehn-doh-THEE-lin), a hormone that causes the blood vessel to constrict and spasm for several minutes. This constriction helps reduce blood loss at the site of the vessel wall injury. Under normal circumstances, platelets do not adhere to the blood vessel wall because the wall is coated with a platelet repellent called prostacyclin. When the blood vessel wall is injured, collagen fibers along the wall are exposed. Thrombocytes Thrombocytes, also called platelets, are part of the formed elements of the blood. They are small, irregularly shaped cell fragments and have a lifetime of 8–9 days. As cell fragments, platelets do not have a nucleus. Old platelets are removed from the body by phagocytosis in the spleen and liver. Platelets are derived from multinucleated mega- karyocytes, which are specialized bone marrow cells. Like red and white blood cells, megakaryocytes develop from a hematopoietic stem cell. The hormone thrombopoietin (thrahm-boh-POY-eh-tin) regulates platelet production from megakaryocytes. Thrombo- poietin is produced by the liver and kidneys. 1 1 2 2 3 3 4 4 1 1 2 2 3 3 1 1 2 2 1 1 2 2 1 Vessel wall injury and constriction 3 Platelet plug formation and coagulation 2 Platelet aggregation 4 Blood clot formation Endothelium Fibrin mesh secures platelet plug Endothelium Collagen fibers Endothelin release causes constriction Constriction Collagen fibers exposed Site of injury Smooth muscle of artery Clotting factors released Tissue factor released Release of coagulation inhibitors and other chemicals Red and white blood cells are trapped in mesh Chemicals released by platelets Platelets gather Platelet adhesion Platelets cluster to repair wall © Body Scientifi c International Figure 11.9 The four steps in the process of hemostasis.