What defines an isotonic solution?

What defines an isotonic solution?

• A. Osmolality similar to blood plasma
• B. Osmolality higher than blood plasma
• C. Osmolality lower than blood plasma
• D. Contains no solutes

Answer: (A)

Isotonic solutions have the same osmolality as body fluids, especially blood plasma, so there is no net movement of water across cell membranes and cells stay the same size. In other words, their osmotic pressure matches that of plasma, roughly around 275–299 mOsm/kg. If a solution is hypertonic (higher osmolality than plasma), water moves out of cells and they shrink; if it is hypotonic (lower osmolality), water moves into cells and they may swell. A solution with no solutes would be extremely hypotonic relative to plasma, causing rapid water influx into cells. This is why isotonic solutions, like 0.9% saline, are used to maintain fluid balance without causing cellular shrinking or swelling.

Isotonic solutions have the same osmolality as body fluids, especially blood plasma, so there is no net movement of water across cell membranes and cells stay the same size. In other words, their osmotic pressure matches that of plasma, roughly around 275–299 mOsm/kg. If a solution is hypertonic (higher osmolality than plasma), water moves out of cells and they shrink; if it is hypotonic (lower osmolality), water moves into cells and they may swell. A solution with no solutes would be extremely hypotonic relative to plasma, causing rapid water influx into cells. This is why isotonic solutions, like 0.9% saline, are used to maintain fluid balance without causing cellular shrinking or swelling.