Final Answer:
The blood cell remains unchanged in a 400 mOsm solution because the solution is isotonic, with the same osmotic pressure as the cell, preventing any net movement of water.
Step-by-step explanation:
When a blood cell is placed in a 400 mOsm solution and does not change volume over time, it suggests that the solution is isotonic to the cell. Isotonic solutions have the same osmotic pressure as the cell's cytoplasm, resulting in no net movement of water across the cell membrane. In this case, the osmolarity inside and outside the cell is balanced, maintaining cellular integrity.
Osmosis, the movement of water across a semipermeable membrane, occurs when there is a difference in solute concentration between the intracellular and extracellular environments. In an isotonic solution, the concentration of solutes inside the cell is equal to that in the surrounding solution. As a result, water molecules move in and out of the cell at the same rate, preventing any significant change in cell volume. This balance ensures the stability of the cell and its normal physiological functions.