process of dialysis in human beings
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Dialysis is a process by which small molecules in a solution are separated from large molecules. Dialysis has a number of important commercial and industrial applications and plays a crucial role in maintaining the health of humans. For some people, in fact, the term dialysis refers to a specific kind of medical treatment in which a machine (the dialysis machine) takes on the functions of a human kidney. Dialysis machines have made possible the survival of thousands of people who would otherwise have died as a result of kidney failure.
Dialysis is a specific example of a more general process known as diffusion. Diffusion was first described by Scottish chemist Thomas Graham (1805–1869) around 1861. Graham studied the movement of molecules of different sizes through a semipermeable membrane. (A semipermeable membrane is a thin sheet of material that allows some substances to pass through—or diffuse—but not others.) Many tissues in the human body are semipermeable membranes. Graham discovered that some substances, such as the sodium and chloride ions of which ordinary table salt is composed, diffuse through a semipermeable membrane up to 50 times as fast as other substances, such as ordinary table sugar.
Today we know the reason behind Graham's observation. Semipermeable membranes are not actually solid sheets of material. Instead, they contain tiny holes too small to be seen by the unaided eye. Those holes are just large enough to allow tiny particles like sodium and chloride ions to pass through, but they are too small to permit the passage of large molecules, such as those of sugar.
In a typical dialysis experiment, a bag made of a semipermeable membrane is filled with a solution to be dialyzed. The bag is then suspended in a stream of running water. Small particles in the solution within the bag gradually diffuse across the semipermeable membrane and are carried away by the running water. Larger molecules are essentially retained within the bag. By this process, a highly efficient separation of substances can be achieved.