|MadSci Network: Medicine|
Dear Ivan, Thanks for your question. What happens has to do with a blood cell's membrane. There are tiny pumps made of protein in it whose job it is to try to balance the concentration of salt inside and outside the cell. The normal salt concentration in blood is about 0.9%, so if you put blood cells into that, they are just fine. But if you put them into water, they begin to pump water in to dilute their internal salt concentration to match that of the water. They swell up like little balloons until they burst and release the cells' contents. Not good. If you do the opposite, put them into very high salt, they pump water out and shrivel up. This property is taken advantage of in freezing blood for blood banks. The cells are protected from freezing by adding glycerol to them. When they are thawed out, the glycerol has to be removed before they can be transfused into a patient. This is done by adding high salt (6%), so the cells pump out the glycerol. The glycerol-containing liquid is removed and then the salt is reduced back down to 0.9% so they can be used safely.
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