What is an isotonic solution?

• A. Higher concentration than blood
• B. Equal concentration solution
• C. Lower concentration than blood
• D. No solute

Answer: (B)

Isotonic means the solution has the same osmotic pressure as blood plasma. In practical terms, its osmolality is about the same as that of plasma (roughly 285–295 mOsm/kg). Because the solute concentration matches that of cells in the bloodstream, there’s no net movement of water into or out of cells when the solution is in contact with them, so cells stay the same size. An example often used in medicine is normal saline (0.9% NaCl), which is isotonic to plasma and commonly used for IV fluids.

If a solution had a higher solute concentration than plasma (hypertonic), water would move out of cells and they would shrink. If it had a lower solute concentration (hypotonic), water would move into cells and they could swell or burst. A solution with no solute at all would be hypotonic relative to plasma and would drive water into cells.