Polycythemia (also known as polycythaemia or erythrocytosis) is a disease state in which the proportion of blood volume that is occupied by red blood cells increases. Blood volume proportions can be measured as hematocrit level.
It can be due to an increase in the mass of red blood cells[1] ("absolute polycythemia") or to a decrease in the volume of plasma ("relative polycythemia").[2]
source:  http://en.wikipedia.org/wiki/Polycythemia

What is polycythemia?

Polycythemia is a condition that results in an increased level of circulating red blood cells in the bloodstream. People with polycythemia have an increase in hematocrit, hemoglobin, or red blood cell count above the normal limits.
Polycythemia is normally reported in terms of increased hematocrit or hemoglobin.
  • Hematocrit (HCT): Polycythemia is considered when the hematocrit is greater than 48% in women and 52% in men.
  • Hemoglobin (HGB): Polycythemia is considered when a hemoglobin level of greater than 16.5g/dL in women or hemoglobin level greater than18.5 g/dL in men.
Polycythemia can be divided into two categories; primary and secondary.
  • Primary polycythemia: In primary polycythemia the increase in red blood cells is due to inherent problems in the process of red blood cell production.
  • Secondary polycythemia: Secondary polycythemia generally occurs as a response to other factors or underlying conditions that promote red blood cell production.
Red cell production (erythropoiesis) takes place in the bone marrow through a complex sequence of tightly regulated steps. The main regulator of the red cell production is the hormone erythropoietin (EPO). This hormone is largely secreted by the kidneys, although, about 10% may be produced and secreted by the liver.
Erythropoietin secretion is up-regulated in response to low oxygen levels (hypoxia) in the blood. More oxygen can be carried to tissues when erythropoietin stimulates red blood cell production in the bone marrow to compensate for the hypoxia.
Neonatal (newborn) polycythemia can be seen in 1% to 5% of newborns. The most common causes may be related to transfusion of blood, transfer of placental blood to the infant after delivery, or chronic inadequate oxygenation of the fetus (intrauterine hypoxia) due to placental insufficiency.

source: http://www.unm.edu/~mpachman/Blood/polycyt.htm

source: http://health.allrefer.com/pictures-images/polycythemia-red-blood-cells.html

Polycythemia vera (also known as erythremia, or primary polycythemia)[1] is a blood disorder in which the bone marrow makes too many red blood cells.[1] Polycythemia vera may also result in the overproduction of white blood cells and platelets. Most of the health concerns associated with polycythemia vera are caused by a blood-thickening effect that results from an overproduction of red blood cells.

source: http://www.youtube.com/watch?v=5x2VYu7HUXE&feature=BF&list=QL&index=1

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