Red blood cells maintain normal morphology and chemical exchange rates in isotonic solutions. A cell is in an isotonic solution if the osmotic pressure inside the cell is equivalent to the osmotic pressure of the solution surrounding the cell. Plasma is the primary isotonic solution for red blood cells.
The morphology of the cell, specifically the surface area-to-volume ratio, is a critical factor for diffusion of oxygen and carbon dioxide across the cell membrane. The disc shape of a red blood cell in plasma is unique because it has a large surface area-to-volume ratio while maintaining a high level of agility; the cells remain small and retain the ability to travel appropriate speeds within veins of small diameter.
Hypotonic solutions have lower osmotic pressure than red blood cells, causing the cells to take in additional water. Subsequently, the cells bulge. This does not impact the surface area-to-volume ratio, but it does impact the turbidity of the cells within the veins. If the red blood cells take in too much water, cytolysis can occur. Hypertonic solutions also have higher osmotic pressure than red blood cells, causing the cells to shed water. Crenation of red blood cells occurs in hypertonic solutions, leading to a decrease in the ability to carry oxygen.