Red blood cells are flexible and can change shape due to osmosis, which is the net movement of water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. Changes in the surrounding solution can affect the water balance and shape of red blood cells. Here are a few examples:

1. Isotonic Solution: In an isotonic solution, the concentration of solute particles is the same both inside and outside the red blood cell. There is no net movement of water across the cell membrane. The red blood cells remain in their normal shape, which is a biconcave disk.

2. Hypertonic Solution (Higher Solute Concentration Outside): When red blood cells are placed in a hypertonic solution, the concentration of solute particles outside the cell is higher compared to the inside. Water moves out of the cell to balance the concentration. As a result, red blood cells shrink and become crenated, taking on a wrinkled appearance.

3. Hypotonic Solution (Lower Solute Concentration Outside): In a hypotonic solution, the concentration of solute particles is lower outside the red blood cell compared to the inside. Water moves into the cell to equalize the concentration. The influx of water causes red blood cells to swell, becoming larger and more spherical in shape.

The shape changes in red blood cells in different solutions are driven by the osmotic pressure gradient across the cell membrane. The ability of red blood cells to change shape is essential for their function in transporting oxygen and carbon dioxide throughout the body.