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Osmosis is a natural process of balancing the concentration of two water-based solutions. Osmosis happens at the molecular level.
There osmosis processes are classified as it follows: forward osmosis, reverse osmosis, pressure enhanced osmosis and pressure retarded osmosis.
Forward osmosis can be explained by the following example consisting of two tanks that communicate with each other by a semipermeable membrane. If in one tank is pure water while in the other one is saline solution for a certain concentration, the pure water molecules will pass to the saline solution in an attempt to reduce the concentration of saline solution, until the balance is reached. In this case, the semipermeable membrane is built in such a way as to allow only the passage of water molecules.
The most advanced water filtration systems are based on the physical phenomenon of reverse osmosis. To obtain pure water from the saline solution, the process must be reversed, the process being called reverse osmosis. Thus, in order to force the water molecules in the saline solution to pass to the other side of the membrane, the so-called osmotic pressure must be applied to the saline solution. In reverse osmosis, the solvent (water) runs much faster than the dissolved solids (salt). The osmotic pressure, and thus the amount of energy required for the reverse osmosis process is directly proportional to the concentration of the saline solution. The higher the concentration is, the lower the chemical potential is. In this case, the necessary osmotic pressure that maintains the balance must be greater.
The previous post gives a good example of osmosis's most common industrial role, desalination, but osmosis is also a critical mechanism in biology and medicine. Osmosis is the dominant method by which fluid balances are maintained across any membrane in the body. At the cellular scale, the flow of water into or out of a cell is determined by the osmolarity difference across the cellular membrane, mostly determined by sodium and potassium concentrations. If you drop a cell into distilled water, an extremely hypotonic solution, the cell will pop, as the water rushes into the much saltier cell.
At the level of the body, control over osmolarity is how you move water across membranes like the intestines, blood vessels, and certain parts of the kidneys. Generally speaking, if your body wants to establish a flow of water across a membrane, it does so by actively transporting salt. Water then follows by osmosis. This is best represented by what happens when you eat a salty meal. The salt is transported into your body tissues, water follows, and the next day you may find yourself weighing five pounds more due to water weight. Conversely, diarrhea is caused by active transport of ions into your intestine. Water follows, and in the most extreme cases such as the disease cholera, a person can lose twenty pounds of water in a day.
That brings up medical uses of osmosis, oral rehydration therapy and dialysis. Oral rehydration therapy (ORT) is a simple mixture of salt, sugar, and water which a dehydrated patient can drink. The salt and sugar is rapidly transported across the membrane of the intestine, water follows, and the effects of the diarrheal disease is reversed. ORT, as simple as it is, is credited with saving millions of lives since the World Health Organization began distributing it. Dialysis is even more broadly used, mostly by people with some form of kidney damage, often a byproduct of diabetes. Dialysis allows removal of toxins from the blood through diffusion, but it also removes excess water by osmosis. A fluid known as the dialysate is maintained at a given osmolarity, slightly hypertonic compared to the blood, and the body's excess water flows across a membrane into the dialysate. A patient on dialysis can visibly shrink over several hours, growing less puffy and reducing the risk of tissue damage from the excess water.
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