dialysis

/duy al"euh sis/, n., pl. dialyses /-seez'/.

1. Physical Chem. the separation of crystalloids from colloids in a solution by diffusion through a membrane.

2. Biochem. the separation of large molecules, as proteins, from small molecules and ions in a solution by allowing the latter to pass through a semipermeable membrane.

3. Med. (in kidney disease) the process by which uric acid and urea are removed from circulating blood by means of a dialyzer.

[1580-90; < LL < Gk diálysis a separation. See DIA-, -LYSIS]

* * *

I

In chemistry, separation of suspended colloidal (see colloid) particles from dissolved ions or small molecules via their unequal rates of diffusion through pores of semipermeable membranes (e.g., parchment, collodion, cellophane).

A slow process, dialysis may be accelerated by heating or by applying an electric field if the particles are charged.

II

or hemodialysis

Process of removing blood from a patient with kidney failure, purifying it with a hemodialyzer (artificial kidney), and returning it to the bloodstream.

Many substances (including urea and inorganic salts) in the blood pass through a porous membrane in the machine into a sterile solution; particles such as blood cells and proteins are too large to pass. This process controls the acid-base balance of the blood and its content of water and dissolved materials.

* * *

▪ chemical separation

      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.

▪ hemodialysis

also called  hemodialysis , renal dialysis , or  kidney dialysis 

 in medicine, the process of removing blood from a patient whose kidney functioning is faulty, purifying that blood by dialysis, and returning it to the patient's bloodstream (blood). The artificial kidney, or hemodialyzer, is a machine that provides a means for removing certain undesirable substances from the blood or of adding needed components to it. By these processes the apparatus can control the acid–base balance of the blood and its content of water and dissolved materials. Another known function of the natural kidney—secretion of hormones that influence the blood pressure—cannot be duplicated. Modern dialyzers rely on two physicochemical principles, dialysis and ultrafiltration.

      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.

* * *