Osmosis (Cellular)

Mammalian red blood cells have a biconcave (doughnut-like) shape. If red blood cells are placed in a 0.3 M NaCl solution, there is little net osmotic movement of water, the size and shape of the cells stay the same; the NaCl solution is isotonic to the cell. If red blood cells are placed in a solution with a lower solute concentration than is found in the cells, water moves into the cells by osmosis, causing the cells to swell; such a solution is hypotonic to the cells. When red blood cells are placed in pure water, water rapidly enters the cells by osmosis and causes the cells to burst, a phenomenon known as hemolysis. If the red blood cells are placed in a solution with a higher solute concentration, water moves out of the cell by osmosis, the cell becomes smaller and crenated in shape; such a solution is hypertonic to the cells.

These observations have several important practical implications. First, hospitals must store red blood cells in a plasma solution which has the correct proportions of salts and proteins. The plasma solution is made to be slightly hypertonic to the red cells so that the integrity of the cells is preserved and hemolysis is prevented. Second, when doctors inject a drug intravenously into a patient, the drug is suspended in a saline solution which is slightly hypertonic to red blood cells. Intravenous injection of a drug in pure water will cause some of the patient's red blood cells to hemolyze because water is hypotonic to the red blood cells.