What is the role of hypotonic, isotonic, and hypertonic IV solutions in fluid balance?

• A. Hypotonic fluids move water into cells, isotonic fluids stay mainly in extracellular space, and hypertonic fluids draw water out of cells.
• B. Hypotonic fluids draw water out of cells; isotonic fluids move water between compartments; hypertonic stays extracellular.
• C. Isotonic fluids move water into cells; hypotonic fluids stay extracellular; hypertonic move water into cells.
• D. Hypertonic fluids move water into cells; isotonic fluids draw water from cells; hypotonic stay in extracellular.

Answer: (A)

Water movement between compartments is driven by osmolarity relative to plasma. Hypotonic IV solutions have lower osmolarity than plasma, so they cause water to enter cells, making cells swell. Isotonic solutions have similar osmolarity to plasma, so they mainly stay in the extracellular space and expand intravascular and interstitial fluid without shifting water into cells. Hypertonic solutions have higher osmolarity than plasma, so they pull water out of cells into the extracellular space, reducing cellular volume and increasing extracellular fluid.

This alignment with how osmosis works explains why those roles are correct: hypotonic fluids move water into cells, isotonic fluids remain largely outside cells, and hypertonic fluids draw water out of cells. For memory anchors, think of hypotonic as “water into cells,” isotonic as “stays in extracellular space,” and hypertonic as “water out of cells.”