If extracellular fluid solute concentration decreases due to excessive water intake, which statement about osmosis is most accurate?

• A. The extracellular fluid will become hypotonic to the cytosol, resulting in a net movement of water out of the cell by osmosis
• B. The extracellular fluid will become hypotonic to the cytosol, resulting in a net movement of water into the cell by osmosis
• C. The extracellular fluid will become hypertonic to the cytosol, resulting in a net movement of water into the cell by osmosis
• D. The extracellular fluid will become hypertonic to the cytosol, resulting in a net movement of water out of the cell by osmosis

Answer: (B)

When osmosis is at work, water moves toward the side with the higher solute concentration. If extracellular fluid becomes diluted by excessive water intake, its solute concentration drops relative to the cytosol. The cytosol now has a higher solute concentration, so water will move from the extracellular fluid into the cell to try to equalize solute levels. That means water enters the cell by osmosis, which is why the statement describing a hypotonic extracellular environment relative to the cytosol and water moving into the cell is the correct one. If the outside were hypertonic, water would leave the cell instead, which would be described by the other options.