Which is an example of a hypotonic IV solution?

• A. Lactated Ringer's
• B. 0.9% NaCl
• C. 3% NaCl
• D. 0.45% NaCl

Answer: (D)

Hypotonic IV solutions have a lower osmolality than blood plasma, so water moves from the outside of cells into the cells, causing them to swell. An IV fluid with about 154 mEq/L of sodium and chloride and an osmolarity around 150 mOsm/L is lower than plasma (~275–295 mOsm/L), which makes it hypotonic. That’s why 0.45% NaCl is the example: it provides free water with some salt but less solute than plasma, prompting intracellular fluid gain.

In contrast, isotonic solutions like Lactated Ringer’s and 0.9% NaCl have osmolarities similar to plasma and mainly expand extracellular fluid without causing cells to swell. Hypertonic solutions like 3% NaCl have higher osmolarity than plasma and pull water out of cells, shrinking them.