Anatomy & Physiology

Anatomy & Physiology Study Guide Platelets, disc-shaped structures formed from megakaryocytes, function in the clotting process: • Platelets are flattened discs that appear round from above and spindle-shaped in section. They move around for 9–12 days before being removed by phagocytes. • The functions of platelets include transporting and releasing chemicals important to the clotting process, forming a temporary patch in the walls of damaged blood vessels, and contracting after a clot has formed to reduce the size of the break in the vessel wall. • During thrombocytopoiesis, megakaryocytes in the bone marrow furnish packets of cytoplasm (platelets) to the circulating blood. The rate of platelet formation is stimulated by thrombopoietin or thrombocyte-inducing factor, interleukin-6, and multi-CSF. Hemostasis includes vascular spasm, platelet plug formation, and blood coagulation: • Hemostasis stops blood loss through the walls of damaged vessels. It contains three phases: the vascular phase, the platelet phase, and the coagulation phase. • During fibrinolysis, the clot gradually dissolves through the action of plasmin, which is the activated form of circulating plasminogen. • The platelet phase follows as platelets are activated, aggregate at the site, and adhere to the damaged surfaces. • The coagulation phase occurs as factors released by platelets and endothelial cells interact with clotting factors (through either the extrinsic pathway, the intrinsic pathway, or the common pathway) to form a blood clot. In this reaction sequence, suspended fibrinogen is converted to large, insoluble fibers of fibrin. • During clot retraction, platelets contract and pull the torn edges of the damaged vessel closer together. • The vascular phase is a time of local blood vessel constriction, or vascular spasm, at the site of the injury.

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