answered Sep 23, 2009 at 11:01AM
Tonicity refers to the amount of dissolved substances in a fluid, and here I'm guessing that "RBC fluid" means the plasma, rather than the fluid inside the red cell itself. Fluids that have the same tonicity as one another are called "isotonic" and a very common fluid to give intravenously is 0.9% saline, also known as "normal saline" or "isotonic saline." You can make isotonic saline by dissolving 9 g of salt in a liter of water.