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Polycythemia (or polycythaemia or erythrocytosis) is a condition in which there is a net increase in the total number of blood cells, primarily red blood cells, in the body.[1] The overproduction of red blood cells may be due to a primary process in the bone marrow (a so-called myeloproliferative syndrome), or it may be a reaction to chronically low oxygen levels or, rarely, a malignancy. Secondary polycythemia is caused by either natural or artificial increases in the production of erythropoietin, hence an increased production of erythrocytes. In secondary polycythemia, there may be 6 to 8 million and occasionally 9 million erythrocytes per cubic millimeter (milliliter) of blood. Secondary polycythemia resolves when the underlying cause is treated. Secondary polycythemia in which the production of erythropoietin increases appropriately is called physiologic polycythemia. This physiologic (meaning normal) polycythemia is a normal adaptation to living at high altitudes (see altitude sickness). Many athletes train at high altitude to take advantage of this effect — a legal form of blood doping. Similarly, athletes with primary polycythemia may have a competitive advantage due to greater stamina.[citation needed] Other causes of secondary polycythemia include smoking, renal or liver tumors, hemangioblastomas in the central nervous system, heart or lung diseases that result in hypoxia, and endocrine abnormalities including pheochromocytoma and adrenal adenoma with Cushing's syndrome. Athletes and bodybuilders who use anabolic steroids or erythropoietin may develop secondary polycythemia.
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