Polycythemia is a condition where the volume of red blood cells as a proportion of blood is abnormally high.  It is characterized by a high hematocrit result.  No matter what its cause, the usual response is to bleed the patient and replace the blood volume with fluids.  

Polycythemia vera or primary polycythemia is caused when bone marrow produces a larger than required number of red blood cells.  However, it can be caused by other conditions, usually when erythropoietin levels are high either due to natural or artificial means.  

Polycythemia is normal in individuals who live at very high altitudes, typical of countries such as Tibet or Ecuador.  However, it can also occur when any individual is having trouble getting oxygen, such as smokers.  It can also occur when the blood loses plasma but not red blood cells, such as is common with dehydration and burns.

Polycythemia at Wikipedia

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