Middle-aged woman undergoing kidney dialysis, reading a magazine to help pass away the time. A kidney machine is used to replace the function of the kidneys in removing waste materials from the bloodstream. Inside the machine, a solution of comparable electrolytic concentration circulates on one side of a semi-permeable membrane while a stream of blood from an artery is circulated on the other side. While blood cells and proteins are too large to pass across the membrane, water and waste products are easily filtered. The resulting purified blood is returned to the patient via a vein.