Osmosis is the flow of water from lower concentration to higher concentration through a semipermeable membrane. This flow brings the solutions to the same (or at least closer to the same) concentration of solutes. If there is more dissolved solute in the blood than in the cells, water will flow from the cells into the blood resulting in crenation of the cells (i.e. shrinking of the cell). Neither crenation nor hemolysis (water flowing into the cell) are desirable so IV fluids given to patients are designed to closely match the concentration of the bodily fluids to avoid either of this situations happening.