What is an isotonic solution?

Answer: Isotonic solutions are those that have the same concentration of water and solutes as the cytoplasm of a cell. Because there is no net gain or loss of water in an isotonic solution, cells will neither shrink nor swell. 

The concentration of solutes in cell cytoplasm, bodily fluids (including blood, sweat, and tears), and 0.9% wt percent NaCl solution is similar. As a result, they’re all isotonic solutions, and blood freezes at a lower temperature than water at -52 degrees Celsius.