Which mechanism explains the movement of water when a hypertonic IV solution is infused?

• A. Osmosis
• B. Diffusion
• C. Active transport
• D. Facilitated diffusion

Answer: (A)

Osmosis is the movement of water across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration, driven by the need to equalize osmolarity on both sides.

When a hypertonic IV solution is infused, the extracellular fluid becomes more concentrated with solutes than the intracellular fluid. Because water moves toward the higher solute concentration to balance levels, water flows out of cells and into the extracellular space and bloodstream. This water movement occurs through water channels in the cell membranes (aquaporins) and does not require energy.

Other mechanisms involve solute movement rather than water. Diffusion moves solutes down their concentration gradients; active transport uses energy to move substances against their gradients; facilitated diffusion assists solutes across membranes down their gradients via transport proteins. None of these describe the solvent (water) moving in response to osmotic pressure, which is osmosis.