Answer
The concentration of of NaCl in the blood is about 0.9%. This is an isotonic fluid for red blood cells (RBC's) for when RBCs are placed in a fluid with this tonicity, just as much water enters the cell as leaves the cell. Consequently, the shape of the RBC does not change. If however, RBCs are placed in a hypertonic fluid--say 2% NaCl--more water will leave the cell per unit time than enter it, and the cell will shrink. This effect is called crenation.
Work Step by Step
In a hypotonic fluid--say 0.02% NaCl-- the RBC would hydrolyze or burst.
