Each of the cells in your body -- blood cells included -- is surrounded by a cell membrane that selectively allows certain chemicals to pass through. Because of this, cells can swell and burst or shrink and desiccate in the wrong type of fluid. The proper balance of water, salts, sugar and other chemicals keeps blood cells from either swelling or shrinking. Blood Cells You have several different kinds of cells in your bloodstream, but the cells most commonly referred to as "blood cells" in colloquial language are the red blood cells, which represent the largest proportion in your blood by both volume and quantity. Your red blood cells carry oxygen to your body cells, explains Dr. Lauralee Sherwood in her book, "Human Physiology." They also assist in the process of carrying carbon dioxide waste back to the lungs for exhalation. Tonicity of Solution Your red blood cells have a mixture of water, salts, proteins, sugar and many other chemicals inside them. Some of these chemicals help the blood cell function, while others are building blocks for various cellular materials. The salts and other chemicals inside a blood cell give it a certain "tonicity," which refers to the amount of dissolved material in a sample of water. Compared to the mixture inside a blood cell, pure water is "hypotonic," meaning it has less dissolved material, while very salty water would be "hypertonic." Swelling If you were to put red blood cells in a hypotonic solution, such as a glass of pure water, they'd swell and burst. This is because water moves across a cell membrane in whatever direction it needs to in an attempt to equalize the tonicities of the solutions. In the case of a blood cell in water, water would move into the cell to try to dilute the solution inside the cell. This would continue until the cell burst, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." Preventing Bursting If you needed to put a red blood cell in water and didn't want it to burst, you'd need to add salts, sugar or other chemicals to the water so that there was no "osmotic pressure" on the cell. Osmotic pressure refers to the force that causes water to move through the membrane. If you put a red blood cell in isotonic solution, meaning solution that is of equal tonicity to that inside the cell, it won't swell.