National League for Nursing (NLN) Science Practice Exam

What happens to a human red blood cell when it is placed in distilled water?

1. It shrivels up

2. It neither shrinks nor swells

3. It takes up more salts to balance concentrations

4. It swells to a larger size

The correct answer is: It swells to a larger size

When a human red blood cell is placed in distilled water, it undergoes a process called osmosis. Osmosis is the movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. In the case of distilled water, the concentration of solutes (such as salts) outside the red blood cell is lower than that inside the cell. As a result, water moves into the red blood cell to equalize the solute concentrations on both sides of the cell membrane. This influx of water causes the cell to swell. If enough water enters, the cell can ultimately burst, a phenomenon known as hemolysis. Thus, the correct answer reflects that the red blood cell swells to a larger size due to the absorption of water in an environment where it is exposed to a dilute solution. The other choices describe different scenarios. Shriveled cells would indicate the loss of water, which occurs in hypertonic solutions, while neither shrinking nor swelling would be the case in isotonic solutions. Taking up salts to balance concentrations does not accurately describe what is happening; instead, the focus is on the movement of water rather than salt.