What best describes isotonic fluid and when is it typically used?

• A. Fluid with higher osmolality than plasma used to draw water from cells.
• B. Fluid with the same osmolality as plasma (e.g., normal saline) used to expand extracellular fluid volume.
• C. Fluid with lower osmolality than plasma used to hydrate cells intravenously.
• D. Fluid that contains no electrolytes and is hypotonic.

Answer: (B)

Isotonic fluids have osmolality similar to that of plasma, so they do not cause water to move into or out of cells. Because of this balance, they stay mainly in the extracellular space, especially the intravascular compartment, and effectively expand extracellular fluid volume. Normal saline is the classic example. This makes isotonic fluids ideal for volume resuscitation and for maintaining circulating volume in dehydration, blood loss, or shock without causing cellular swelling or shrinking. In contrast, fluids with higher osmolality draw water out of cells, while those with lower osmolality draw water into cells; a hypotonic solution with no electrolytes is not isotonic and does not reliably expand the intravascular volume.