Tonicity

"Cells maintain water volume by actively controlling electrolyte concentrations. Human erythrocytes, or red blood cells, are shown under hypertonic, isotonic, and hypotonic conditions. In hypertonic conditions, the erythrocytes contract and appear spiky. In isotonic conditions, the erythrocytes appear normal. In hypotonic conditions, the erythrocytes expand and become more round. If a cell is placed in a solution that contains fewer dissolved particles, a hypotonic solution, than the cell itself, water moves into the more concentrated cell, causing it to swell. Alternatively, if a cell is placed in a solution that is more concentrated, known as a hypertonic solution, water moves from inside the cell to the outside, causing it to shrink. Cells keep their water volume constant by pumping electrolytes in and out in an effort to balance the concentrations of dissolved particles on either side of their membranes. When a solution contains an equal concentration of dissolved particles on either side of the membrane, it is known as an isotonic solution."