A typical human erythrocyte has a volume of about 90 fL (femtoliters), and red blood cells make up approx. 40% of blood volume. How many red blood cells are present in 1 mL of blood?
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Seriously, simple math. The question is how many femtoliters are in a milliliter. Since a femtoliter is 10^-15 liters and a milliliter is 10^-3 liters, there are obviously 10^12 femtoliters in a milliliter. If red blood cells represent 40% of that total then the red blood cells are .4(10^12) or 4(10^11) femtoliters. As to how many blood cells there are, just divide. 400,000,000,000/90 = your answer.