Since the discovery of the composition and structure of the mammalian cell membrane, biologists have had a clearer understanding of how substances enter and exit the cell’s interior. The selectively-permeable nature of the cell membrane allows the movement of some solutes and prevents the movement of others. This has important consequences for cell volume and the integrity of the cell and, as a result, is of utmost clinical importance, for example in the administration of isotonic intravenous infusions. The concepts of osmolarity and tonicity are often confused by students as impermeant isosmotic solutes such as NaCl are also isotonic; however, isosmotic solutes such as urea are actually hypotonic due to the permeant nature of the membrane. By placing red blood cells in solutions of differing osmolarities and tonicities, this experiment demonstrates the effects of osmosis and the resultant changes in cell volume. Using haemoglobin standard solutions, where known concentrations of haemoglobin are produced, the proportion of haemolysis and the effect of this on resultant haematocrit can be estimated. No change in cell volume occurs in isotonic NaCl, and by placing blood cells in hypotonic NaCl incomplete haemolysis occurs. By changing the bathing solution to either distilled water or isosmotic urea, complete haemolysis occurs due to their hypotonic effects. With the use of animal blood in this practical, students gain useful experience in handling tissue fluids and calculating dilutions and can appreciate the science behind clinical scenarios.
- handing tissue fluids