Isosmotic and Isotonic Are Not the Same

Abstract

Teaching the distinction between and isosmotic solutions is always challenging. It becomes even more challenging when one encounters the statement, Isosmotic solutions are also called isotonic (Anthony and Kolthoff 1975). misleading statement, found in one of the most popular textbooks on anatomy and physiology used for teaching allied health students, prompted me to survey other textbooks to see how they treat this important subject. The textbooks surveyed, which are listed in the references, use a variety of approaches to teaching anatomy and physiology and represent a cross-section of the major publishers of college textbooks. The treatment of isotonic-isosmotic solutions varies from completely omitting the subject to using the term without defining it, or using without a comparison to isosmotic. The best attempt at drawing the distinction between and isosmotic was made by Griffiths (1974): This (isotonic) implies that the solution has the same osmotic pressure as the cell and that the solute cannot penetrate the membrane rapidly. However this distinction is only partially correct. For example, it is not clearly stated or implied that an isosmotic solution may behave as a hypotonic solution. The comparison of solutions on the basis of osmotic pressure does not take into consideration the permeability properties of a membrane. Solutions that exhibit the same osmotic pressure as a reference solution are termed isosmotic solutions. Solutions that have a lesser osmotic pressure are termed hypoosmotic. Those with a greater osmotic pressure are hyperosmotic. In each situation, the sole criterion for determining the relative osmolarity of one solution with respect to another is the osmotic concentrations of the solutions, irrespective of their chemical composition. For example, a solution containing 300 milliosmoles per liter (mOsm/L) NaCl is isosmotic with a solution containing a total of 300 mOsm/L of any other substance or mixture of substances. Similarly, a solution containing 150 mOsm/L is isosmotic to any other solution containing a total of 150 mOsm/L of any other substance or mixture of substances. However, the 150 mOsm/L solution is hypo-osmotic with respect to the 300 mOsm/L solution. On the other hand, if the 150 mOsm/L is the standard for comparison, the 300 mOsm/L solution is hyperosmolar. In contrast to osmolarity, which is determined by the relative concentrations of dissolved materials, tonicity is determined by the relative concentrations of non-permeating dissolved materials. A solution is said to be with respect to a reference solution, when there is no net movement of liquid (water) across a specific membrane separating the two solutions. If there is a net movement of fluid out of the test solution into the reference solution, the test solution is said to be hypotonic. If there is a net movement of fluid out of the reference solution into the test solution, the test solution is said to be hypertonic. These principles, including the demonstration that the tonicity of a solution is not directly related to its osmolarity, can be demonstrated. The usual reference is the human red blood cell whose intracellular contents have an osmolarity of approximately 300 mOsm/L. For this experiment one could use outdated bloodbank blood or dog blood supplied by a veterinarian. Place one drop of red blood cells in each of a series of test tubes containing 5.0 ml of: (1) distilled water; (2) 0.22% NaCl (75 mOsm/L); (3) 0.88% NaCl (300 mOsm/L); (4) 1.8% urea (300 mOsm/L); (5) 5.4% glucose (300 mOsm/L); (6) 18.0% urea (3000 mOsm/L); or, (7) 8.8% NaCI (3000 mOs/L). Mix rapidly by inversion, and wait five minutes. It will be seen that solutions number 1, 2, 4, and 6 will have cleared, i.e., hemolyzed: the net movement of water entered the cell and stretched or burst the erythrocyte membrane thereby leaching the hemoglobin into the solution. Thus each of these solutions behaved as a hypotonic solution with respect to the erythrocyte reference, even though one solution, (4), was isosmotic and one solution, (6), was hyperosmotic with respect to the red blood cell. Other isosmotic solutions, (3) and (5), and the other hyperosmotic solution, (7), retained their opacity, i.e., the red cells remained intact and uniformly dispersed throughout the solution. The reason for the difference in behavior between urea and glucose or NaCl has to do with the differential permeability properties of the membrane. The erythrocyte membrane is readily permeable to urea. Glucose permeates the membrane slowly by facilitated diffusion. NaCl permeates