Renal dialysis. Male nurse assisting a male patient who is undergoing renal dialysis (haemo- dialysis). The dialysis machine (upper left) receives blood drawn from an artery in the arm. This procedure of cleansing the blood is necessary when the patient's kidneys fail to remove waste products from the blood. The dialyser takes over the role of the kidneys in this respect. A steady flow of blood is passed over an artificial, semi- permeable membrane; waste products in the blood are filtered across the membrane, but blood cells and proteins are too large to pass across. Purified blood is then returned to the patient through a vein.