The failure of both kidneys to function due to disease or injury is fatal: the accumulation of excess toxic substances and potassium in the blood. In such cases the condition can be relieved by the use of an "artificial kidney" i.e. by filtration of blood, called hemodialysis. Dialysis entails using a semipermeable membrane to separate large nondiffusible particles from smaller ones.

In an artificial kidney, commonly reffered to as a dialysis machine, blood is diverted from the radial artery in the patient’s arm through a coiled cellophane tube and then returned to a vein in the same arm. The cellophane tube is immersed in a dialyzing solution, containing a regulated amount of dissolved salts sugar, pH, etc. Since the dialyzing solution contains no wastes and the cellophane tubing is differentially permeable, substances like urea, salts, sugar, hydrogen ions etc. can diffuse out from patients blood into the solution. The diffusion gradient for any substance (salts, glucose, etc.) can be controlled by dissolving more or less of the substance in the dialyzing solution. Excess water is removed by artificially raising the blood pressure in the cellophane tubing. The external solution is changed regularly. Thus, the kidney machine performs the function of the nephron in an efficient manner. The dialysis is done for about 12 hours, twice a week, to remove toxins and excess salts from blood.