Isotonic crystalloids maintain cell size when surrounded by electrolyte solutions that mimic extracellular fluid. What is the effect on cell size?

• A. Increases
• B. Decreases
• C. No change
• D. Varies

Answer: (C)

Osmosis and tonicity determine whether cells gain or lose water. When the external fluid is isotonic with the cell’s interior, the osmolarities are equal, so water moves in and out at the same rate and the cell’s size remains unchanged. Isotonic crystalloids, like 0.9% saline, are designed to match plasma osmolarity, expanding extracellular fluid without causing water to shift into or out of cells. If the solution were hypertonic, water would leave the cell and it would shrink; if hypotonic, water would enter the cell and it would swell. With a surrounding electrolyte solution that mimics extracellular fluid, there is no net change in cell size.