Which statement best describes the relationship between isotonic and hypertonic IV fluids in terms of their effect on cells?

• A. Isotonic fluids move water into cells; hypertonic fluids stay mainly in extracellular space.
• B. Isotonic fluids move water into cells; hypertonic fluids stay extracellular.
• C. Isotonic fluids stay mainly in the extracellular space; hypertonic fluids draw water out of cells.
• D. Hypertonic fluids move water into cells; isotonic fluids stay in the extracellular space.

Answer: (C)

Understanding how IV fluids affect cell size through osmosis helps explain this relationship. Isotonic fluids have about the same osmolality as intracellular and plasma fluids, so there is no net movement of water into or out of cells. They stay mainly in the extracellular space, expanding that compartment without changing cell size. Hypertonic solutions have a higher osmolality than the inside of cells, so water moves out of cells into the extracellular space, causing cells to shrink. So the statement that isotonic fluids stay mainly in the extracellular space while hypertonic fluids draw water out of cells best captures their effects on cells.