ABSTRACT
The term ‘dialysis’ (derived from the Greek) refers to the transfer of substances across a semi-permeable membrane (one which allows through some, but not all, molecules) from an area of high to one of low concentration of the molecule concerned. The phenomenon was known to physicists long before its therapeutic application in treatment of renal failure and is best illustrated by a simple experiment. If a solution of sodium chloride (salt) in water enclosed within a bag composed of the semi-permeable colloidin is suspended in a beaker of water containing equivalent numbers of ions other than sodium and chloride, sodium and chloride ions will travel down their concentration gradient, across the membrane, so that eventually the concentration of salt in the beaker equals that within the bag. The rate of transfer is dependent upon the concentration gradient. Initially, when this is large, there is rapid transfer of salt across the membrane. As the concentration of salt on each side of the membrane becomes more nearly equal, the rate of transfer slows progressively. Dialysis occurs because atoms and molecules are in constant motion, and probability dictates that motion across the membrane from a high to low concentration will tend to occur. Transfer in the opposite direction occurs as the concentration gradients approach equivalence to a greater and greater degree.
