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The rate of platelet formation seems to be governed by the amount of oxygen in the blood and the presence of nucleic acid derivatives from injured tissue. At any given time about one-third of the total blood platelets can be found in the spleen; the remaining two-thirds are in the circulating blood. Their primary functions are related to coagulation of blood. Because of their adhesion and aggregation capabilities they can occlude small breaks in blood vessels and prevent escape of blood . They also are able to take up, store, transport, and release serotonin and platelet factor 3. Abnormally high numbers of platelets occur in the presence of malignancy, splenectomy, asphyxiation, polycythemia vera, and acute infections. A very low count can occur as a result of idiopathic thrombocytopenic purpura, pernicious anemia, and allergic conditions, and during cancer chemotherapy. Many drugs can cause a toxic decrease in the number of platelets.
See also: plate
Synonym(s): blood disc, elementary particle (1) , thrombocyte, thromboplastid (1) .
platelet(plat'let) [Gr. plate, flat]
Platelets contribute to chemical blood clotting and to other aspects of hemostasis. Platelet factors are the chemicals released by platelets to initiate the first stage of (intrinsic pathway) chemical clotting. When a capillary ruptures, platelets adhere to each other and to the cut edges of the vessel, forming a platelet plug. Blood clotting may be beneficial (e.g., in preventing blood loss from wounds) or may be harmful when it occurs within arteries or veins inside the body (e.g., during coronary thrombosis). Blood clotting is a positive feedback cascade that may continue and occlude an unbroken vessel.
Thrombocytopenia (reduced platelet count) occurs in acute infections, anaphylactic shock, and certain hemorrhagic diseases and anemias. Thrombocytosis (increased platelet count) occurs after operations, esp. splenectomy, and after violent exercise and tissue injury.