It's a disorganized sentence containing two separate notions. One is that having fewer red cells can reduce the viscosity of blood so it doesn't clot as readil

It's a disorganized sentence containing two separate notions. One is that having fewer red cells can reduce the viscosity of blood so it doesn't clot as readily, which is good for sedentary people. The other is that bleeding will reduce the overall age of red cells and thus reduce the amount of unusable methemoglobin, containing oxidized iron. Iron (II) is in hemoglobin, Iron (III) is in methemoglobin.