Semipermeable membranes play important roles in the normal functioning of many living systems.  In addition, they are used in a wide variety of industrial and medical applications.   Membranes with different permeability characteristics have been developed for many different purposes.  One of these, water purification, was discussed in " Desalinizing Seawater," another of these Practical Chemistry applications.   Semipermeable membranes are crucial, though, in the treatment of patients with kidney failure.

Human kidneys carry out many important functions.   One of the most crucial is the removal of metabolic waste products (such as creatinine, urea, and uric acid) from the blood without removal of substances needed by the body (such as glucose, electrolytes, and amino acids).  The process by which this is accomplished in the kidneys involves dialysis, a phenomenon in which the membrane allows transfer of both solvent molecules and certain solute molecules and ions, usually small ones.  A patient whose kidneys have failed can often have this dialysis performed by an artificial kidney machine.  In this mechanical procedure, called hemodialysis, the blood is withdrawn from the body and passed in contact with a semipermeable membrane.

A schematic diagram of the hollow fiber (or capillary) dialyzer, the most commonly used artificial kidney.  The blood flows through many small tubes constructed of semipermeable membrane; these tubes are bathed in the dialyzing solution.

The membrane separates the blood from a dialyzing solution, or dialysate, that is similar to blood plasma in its concentration of needed substances (such as electrolytes and amino acids) but contains none of the waste products.  Because the concentrations of undesirable substances are thus higher in the blood than in the dialysate, they flow preferentially out of the blood and are washed away.  The concentrations of needed substances are the same on both sides of the membrane, so these substances are maintained at the proper concentrations in the blood.  The small pore size of the membrane prevents passage of the blood cells.  However, Na⁺ and Cl^- ions and some small molecules do pass through the membrane.

A patient with total kidney failure may require up to four hemodialysis sessions per week, at three to four hours per session.  To help hold down the cost of such treatment, the dialysate solution is later purified by a combination of filtration, distillation, and reverse osmosis and is then re-used.

Whitten/Davis/Peck:  General Chemistry with Qualitative Analysis,  5/e,  pp. 530–531