How do changes in plasma osmolality affect the physiology of erythrocytes?

Hi there.   I am happy to help you.

When red blood cells are in plasma that has a higher osmolarity than they do, then they lose water to the plasma and they shrivel up.   When the plasma has a lower osmolarity then the red blood cells gain water and they may burst.   Osmolarity is how much solute is dissolved in a watery solution.  The saltier solution will pull water out of a solution that has less water.   This isn't an active process, it has to do with diffusion.

So when the blood cell is saltier than the plasma, water moves into the cell and when the plasma is saltier, water moves out of the cell.   I'm using "saltier" as a substitute for "more solutes".

Hope this helps you.