Hypotonic crystalloids have what relative tonicity compared to the extracellular fluid?

• A. Higher
• B. Lower
• C. Equal
• D. Not defined

Answer: (B)

Tonicity compares a solution’s osmolality to that of the extracellular fluid. If a crystalloid is hypotonic, its osmolality is lower than the ECF, so infusion lowers the ECF’s osmolality and water moves into cells. This causes cellular swelling. That’s why hypotonic crystalloids have lower tonicity than the extracellular fluid. Hypertonic would have higher tonicity, isotonic would be equal, and not defined isn’t correct.