When a red blood cell is placed in a hypertonic solution, it shrinks as water is drawn out of the cell and into the surrounding solution. If the same blood cell is placed in a hypotonic solution, the blood cell grows in size. Blood cells in isotonic solutions do not shrink or swell.
The reason that blood cells change size when placed in solution with different salt concentration is due to the process of osmosis. Osmosis causes solutions with high salt concentrations to draw the water out of areas with low salt concentration.
There are a few exceptions to this phenomenon. Blood cells may draw water and burst when placed in a hypertonic solution under a few special occasions. Some diseases effect the structural integrity of blood cells. Additionally, when human blood cells are exposed to near freezing temperatures, they may draw water and burst.
Osmosis is an important phenomenon for living systems. The amount of salt in a given solution exhibits a tendency to diffuse through the environment, eventually resulting in equilibrium. In addition to blood cells, kidneys work by using osmotic principles. Kidneys filter an animal’s blood to remove excess salt and balance the amount of water in the animal.