What characteristic defines isotonic solutions?

Isotonic solutions are characterized by having the same concentration of solutes, such as sodium, as the intracellular fluid of the cells. This equivalence in solute concentration allows for the maintenance of cell shape and function because there is no net movement of water into or out of the cells. As a result, cells neither swell nor shrink when placed in an isotonic solution.

The correct understanding of isotonic solutions is vital in medical settings, particularly when administering intravenous fluids. An isotonic solution can effectively maintain fluid balance without disrupting the homeostasis of the cells. The other options reflect conditions that either describe hypertonic or hypotonic solutions or incorrectly generalize the makeup of isotonic solutions.