Tonicity is the ability of the extracellular solution to change the shape of the cell by making the cell gain or lose water. Tonicity depends on the concentration of impermeable solutes outside a cell relative to the inside, which determines which way water moves. There can be three possible conditions. In isotonicity, the solute concentrations outside and inside the cell are equal, with no net water movement. So, the cell shape and volume remain unchanged. In hypertonicity, the solute concentration outside the cell is higher than inside, causing water to move out. As a result, the cell shrivels causing cell crenation. In hypotonicity, the solute concentration outside a cell is lower than inside. Thus, water moves in, causing the cell to swell and eventually rupture. For treating fluid loss in dehydrated patients, an isotonic solution of 0.9 percent sodium chloride is administered intravenously to prevent the crenation or lysis of red blood cells.