Remember that tonicity describes a solution based on whether cells stay the same size, shrink, or swell when placed in that solution. Red blood cell membranes are permeable to water. Thus if red blood cells are placed into solutions that are not isotonic, then water will cross the membrane. For each of the solutions below, determine if the solution is isotonic, hypotonic or hypertonic. To do this exercise, you need to know that Na and Cl- cannot permeate the red blood cell membrane, urea freely permeates the phospholipid bilayer of the red blood cell membrane, and glucose is actively transported across the red blood cell membrane. Red blood cell intracellular fluid is at 300 mOsm.