Hello!
When a person becomes dehydrated, giving them pure water directly into the bloodstream would result in the saline concentration outside the cell to be hypotonic (i.e. the saline concentration would be lower outside the cell) and the cell to achieve equilibrium will absorb water until it explodes. The saline solution is isotonic (i.e. the saline concentration is equal to the concentration inside the cell) and giving that would keep the cells stable.
Have a nice day!
