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Polycythemia is a condition in which there is a net increase in the total circulating erythrocyte mass of the body. There are several types of polycythemia.
PRIMARY POLYCYTHEMIA: Primary polycythemia (also called polycythemia vera or erythremia) occurs when excess erythrocytes are produced as a result of tumorous abnormalities of the tissues that produce blood cells. Often, excess white blood cells and platelets are also produced. In primary polycythemia there may be 8 to 9 million and occasionally 11 million erythrocytes per cubic millimeter of blood, and the hematocrit may be as high as 70 to 80%. In addition, the total blood volume sometimes increases to as much as twice normal. The entire vascular system can become markedly engorged with blood, and circulation times for blood throughout the body can increase up to twice the normal value. The increased numbers of erythrocytes can increase the viscosity of the blood to as much as five times normal. Capillaries can become plugged by the very viscous blood, and the flow of blood through the vessels tends to be extremely sluggish.
SECONDARY POLYCYTHEMIA: Secondary polycythemia is caused by either appropriate or inappropriate increases in the production of erythropoietin that result in an increased production of erythrocytes. In secondary polycythemia their may be 6 to 8 million and occasionally 9 million erythrocytes per cubic millimeter of blood. A type of secondary polycythemia in which the production of erythropoietin increases appropriately is called physiologic polycythemia. Physiologic polycythemia occurs in individuals living at high altitudes (4275 to 5200 meters), where oxygen availability is less than at sea level. Such people may have 6 to 8 million erythrocytes per cubic millimeter of blood.