Which statement best defines an isotonic solution in the context of body fluid compartments?

Which statement best defines an isotonic solution in the context of body fluid compartments?

• A. Isotonic solutions have the same tonicity as normal blood, typically 0.9% NaCl.
• B. Isotonic solutions have a higher tonicity than blood, causing cells to shrink.
• C. Isotonic solutions have a lower tonicity than blood, causing cells to swell.
• D. Isotonic describes solutions that change tonicity with temperature.

Answer: (A)

Isotonic solutions have the same tonicity as body fluids, meaning their effective osmolality is about equal to plasma so water moves in and out of cells at the same rate. This keeps cells from shrinking or swelling. A classic example is normal saline, 0.9% NaCl, which is approximately iso-osmolar to blood. If a solution is more concentrated than plasma (hypertonic), water leaves cells and they shrink. If it is less concentrated (hypotonic), water enters cells and they swell. Tonicity is about relative osmolality to plasma and isn’t defined by temperature.

Isotonic solutions have the same tonicity as body fluids, meaning their effective osmolality is about equal to plasma so water moves in and out of cells at the same rate. This keeps cells from shrinking or swelling. A classic example is normal saline, 0.9% NaCl, which is approximately iso-osmolar to blood. If a solution is more concentrated than plasma (hypertonic), water leaves cells and they shrink. If it is less concentrated (hypotonic), water enters cells and they swell. Tonicity is about relative osmolality to plasma and isn’t defined by temperature.