Most cells are freely permeable to water that diffuses through aquaporins (water channels) in the selectively permeable plasma membrane. This movement of water is termed osmosis and occurs in response to an osmotic pressure gradient across the cell membrane. The osmolality of a solution is the total concentration of particles of solute, measured by the number of osmotically active particles per kg-1 of water (Boron & Boulpaep, 2009). Osmolarity is the moles of osmotically active solute L-1. Osmolality and osmolarity are approximately equivalent for dilute solutions. For two solutions of the same osmolarity, the net movement of water across the semi-permeable membrane separating them is zero, and the solutions are said to be isosmotic.
             A solution with a higher osmolarity is hyperosmotic, and one with lower osmolarity is described as being hypoosmotic. The tonicity of a solution describes the effect of that solution on the volume of the cells suspended in it, and is sometimes referred to as the effective osmolality (Boron & Boulpaep, 2009). An isotonic solution does not alter the volume (or shape) of cells suspended in it because the osmolarity of the solution is the same as the cytosol of the cell. A hypertonic solution will cause the cells to decrease in volume, and a hypotonic solution will cause the cells to swell. The unique, biconcave, shape of RBCs in isotonic solutions enables them to change their volume over a much wider range of solution osmolarities than most other cells. They therefore simulate an osmometer in which the volume of the RBC is inversely related to the osmolarity of the solution.
             RBC's placed in a hypertonic solution will lose water, and shrink. This is termed crenation, where the shape of the RBC becomes shrivelled and spikey in appearance (Tortora and Derrickson, 2009). If the RBCs are placed in hypotonic solution, they swell until they are spherical in shape. Further swelling will cause rupture of th...