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What is Abnormal increase in the number of RBCs is known as: and explain in detail?
Abnormal increase in the number of RBC is known as Polycythemia.
In severe polycythemia (excessive production of red blood cells), the increase in resistance does increase the work of the heart. 
Reference: Ganong’s Review of Medical Physiology T W E N T Y - F I F T H   E D I T I O N page no 574