Which statement best defines an isotonic solution in terms of osmolality?

Which statement best defines an isotonic solution in terms of osmolality?

• A. It has the same osmolality as blood, about 300 mOsm/kg.
• B. It has higher osmolality than blood, around 550 mOsm/kg.
• C. It has very low osmolality, near 0 mOsm/kg.
• D. It varies widely and is not comparable to blood.

Answer: (A)

Isotonic solutions keep cell size stable by matching the osmolality of body fluids. Osmolality measures solute concentration per kilogram of solvent, reported as mOsm/kg. When a solution has about the same osmolality as blood plasma (around 300 mOsm/kg), there is no net movement of water into or out of cells, so cells maintain their volume. This is why such solutions are described as isotonic and are used to expand extracellular fluid without causing cells to swell or shrink. If the osmolality were higher than plasma, water would move out of cells and cause them to shrink (hypertonic effect); if it were lower, water would move into cells and cause them to swell (hypotonic effect). Therefore, having the same osmolality as blood, about 300 mOsm/kg, best defines an isotonic solution.

Isotonic solutions keep cell size stable by matching the osmolality of body fluids. Osmolality measures solute concentration per kilogram of solvent, reported as mOsm/kg. When a solution has about the same osmolality as blood plasma (around 300 mOsm/kg), there is no net movement of water into or out of cells, so cells maintain their volume. This is why such solutions are described as isotonic and are used to expand extracellular fluid without causing cells to swell or shrink. If the osmolality were higher than plasma, water would move out of cells and cause them to shrink (hypertonic effect); if it were lower, water would move into cells and cause them to swell (hypotonic effect). Therefore, having the same osmolality as blood, about 300 mOsm/kg, best defines an isotonic solution.