What happens in polycythemia vera?
Genetic mutations cause uncontrolled division of red cell precursors, as well as white blood cells and platelets. This is called trilineage proliferation with erythrocytic dominance, which means that the greatest difference in cell count is that of red cells.
This increased count causes an increased viscosity of the blood, which is responsible for most symptoms. Other symptoms result from the imbalance in the blood clotting mechanisms either due to increased platelet and red cell count or platelet dysfunction. Red cells are responsible for oxygen delivery through acting as carriers using the iron in hemoglobin, but an increase in their count can paradoxically impair such function due to the sluggish circulation of the viscid blood.