in chemistry, separation of suspended colloidal particles from dissolved ions or molecules of small dimensions (crystalloids) by means of their unequal rates of diffusion through the pores of semipermeable membranes. This process was first employed in 1861 by a British chemist, Thomas Graham (Graham, Thomas). If such a mixture is placed in a sack made of parchment, collodion, or cellophane and suspended in water, the ions and small molecules pass through the membrane, leaving the colloidal particles in the sack. Separation by dialysis is a slow process, depending for its speed on the differences in particle size and diffusion rates between the colloidal and the crystalloidal constituents, and may be accelerated by heating or, if the crystalloids are charged, by applying an electric field (electrodialysis). For medical applications, see also artificial organ.

In dialysis two liquids separated by a porous membrane exchange those components that exist as particles small enough to diffuse through the pores. When the blood is brought into contact with one side of such a membrane, dissolved substances (including urea and inorganic salts) pass through into a sterile solution placed on the other side of the membrane. The red and white cells, platelets, and proteins cannot penetrate the membrane because the particles are too large. To prevent or limit the loss of diffusible substances required by the body, such as sugars, amino acids, and necessary amounts of salts, those compounds are added to the sterile solution; thus their diffusion from the blood is offset by equal movement in the opposite direction. The lack of diffusible materials in the blood can be corrected by incorporating them in the solution, from which they enter the circulation.

Although water passes easily through the membrane, it is not removed by dialysis because its concentration in the blood is lower than in the solution; indeed, water tends to pass from the solution into the blood. The dilution of the blood that would result from this process is prevented by ultrafiltration, by which some of the water, along with some dissolved materials, is forced through the membrane by maintaining the blood at a higher pressure than the solution.

The membranes first used in dialysis were obtained from animals or prepared from collodion; cellophane has been found to be more suitable, and tubes or sheets of it are used in many dialyzers. In the late 1960s hollow filaments of cellulosic or synthetic materials were introduced for dialysis; bundles of such filaments provide a large membrane surface in a small volume, a combination advantageous in devising compact dialyzers.

Dialysis—which was first used to treat human patients in 1945—replaces or supplements the action of the kidneys in a person suffering from acute or chronic renal failure or from poisoning by diffusible substances, such as aspirin, bromides, or barbiturates. Blood is diverted from an artery, usually one in the wrist, into the dialyzer, where it flows—either by its own impetus or with the aid of a mechanical pump—along one surface of the membrane. Finally the blood passes through a trap that removes clots and bubbles and returns to a vein in the patient's forearm. In persons with chronic kidney failure, who require frequent dialysis, repeated surgical access to the blood vessels used in the treatments is obviated by provision of an external plastic shunt between them.