What happens to your body when you are dehydrated?
When there is excessive loss of water from extracellular fluid then osmotic pressure in extracellular fluid rises.
Extracellular fluid means fluid present outside the cells. Osmotic pressure is defined as the pressure applied to solution to stop movement of solvent molecules through semipermeable membrane.
Osmosis is the movement of solvent (water) molecules from high solute concentration to low solute concentration.
In dehydration, body cells loses water to extracellular fluid by osmosis because there is low concentration of solute in cells and more solute concentration in extracellular fluid.
Cells shrink because water moves from low solute concentration to high solute concentration.