Though strictly speaking polycythaemia means an increase in all three formed elements in the peripheral blood the term is usually used to describe an increase above normal in the number of circulating red cells per unit volume of blood. The polycythaemia may be relative, due to a fall in the plasma volume, or true or absolute when the total number of red cells in the body, the red cell mass, is increased. Such an increase in the mass of circulating red cells could in theory be produced by a prolongation of the average life span of red cells beyond the normal value of about no days, or by an increased output of red cells by the haemopoietic system. Present evidence indicates that in the majority of true polvcythacinic syndromes, it is the latter which occurs (Pike 1958).
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