Behaviour of a red blood cell in an isotonic medium. When a cell is placed in an isotonic solution, the solute concentration outside the cell is equal to that inside the cell. As a result, there is no net flow of water into or out of the cell by osmosis, and the cell's volume remains stable. The intracellular and extracellular fluids are in osmotic equilibrium across the cell membrane and the shape of red blood cells is normal. Red blood cells retain their normal shape in isotonic conditions, which are ideal for them. The body has homeostatic systems in place to maintain these conditions.