Final answer:
A blood cell crenates when placed into a hypertonic solution, which means the solution has a higher concentration of solutes outside the cell than inside, causing water to move out and the cell to shrivel.
Step-by-step explanation:
If a blood cell crenates, this means the blood cell was placed into a hypertonic solution, where the osmotically active solute particles were more concentrated outside of the cell. In a hypertonic solution, the concentration of solutes outside the cell is greater than that inside the cell. This creates an osmotic pressure that causes water to move out of the cell, leading to crenation, where the cell shrivels up as it loses water.
In contrast, in an isotonic solution, the extracellular fluid has the same osmolarity as the cell, resulting in no net movement of water. Conversely, in a hypotonic solution, the extracellular solute concentration is lower than that inside the cell, leading to water moving into the cell, potentially causing it to swell and even lyse (burst).