The blood volume in the human body varies with a person’s age, body size, and sex. On average, this volume is about 5.0 L. A typical value of red blood cells (erythrocytes) per volume is 5 000 000

(5.0 × 106) cells per cubic millimeter. Estimate the number of red blood cells in an average human body.

THINKING IT THROUGH. The red blood cell count in cells per cubic millimeter is sort of a red blood cell “number density.”

Multiplying this figure by the total volume of blood

[(cells/ volume) × total volume]

will give the total number of cells.

But note that the volumes must have the same units. First let’s start by converting 5.0 L to cubic meters (m3): 1 L = 10−3 m3. (See inside front cover.)