Polycythaemia is the high concentration of red blood cells in the blood. This is caused by the over-stimulation of red blood cell production or a relative increase in red blood cells in the circulating blood. The haematocrit value can be up to 60-80%, increasing the blood's viscosity. There will be extra load upon the heart and it may lead to heart hypertrophies.
an abnormally increased concentration of hemoglobin in the blood
An irregularly high red blood cell count.