|MadSci Network: Cell Biology|
There have already been several posts discussing osmosis, so I will simply summarize it as the movement of solvent (e.g. water) across a semi-permeable membrane (i.e. permeable to solvent but not solute) from the side with less solute (e.g. salt) to the side with more solute, in order to equilibrate the concentrations on either side. In other words, if you separate a water tank in two with a filter that allows water through but not salt, and then add salt to one side, the water will flow through the filter from the unsalted to the salted side until the water level is higher on the salted side and lower on the unsalted side. The force generated from the movement of the water that changes the levels in the sides of the tank is refered to as "Osmotic Pressure". So, blood is about 0.9% salt, and the contents of all of the blood cells are also around 0.9% salt. When you place these blood cells in distilled water, the amount of salt inside the cells is greater than that outside the cells, so water flows through their membranes into the cells. This influx of water generates osmotic pressure in the cells which becomes more than the membranes can handle, and the cells burst open. Conversely, if the cells are placed in really salty water, the concentration inside the cells is less than outside the cells, and the water flows through the membranes out of the cells, causing them to shrink and "crenate". In medicine, solutions are often categorized by their effects on blood cells: saline solutions that don't effect blood cells are "isotonic" (same pressure); high salt solutions that make blood cells crenate are "hypertonic" (high pressure); and solutions that cause blood cells to explode are "hypotonic" (low pressure).
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