Model released. A young girl undergoing renal dialysis. Dialysis is used in cases where the kidneys are unable to filter waste products from the blood naturally. A steady flow of blood is taken from an artery and passed into the dialysis machine (behind nurse). There the blood passes over a semi-permeable membrane, on the other side of which is a suitable electrolyte. Water and waste products cross the membrane, but blood cells and proteins are unable to cross. The 'purified' blood is then returned to the body into a vein. This photograph was taken at the National Pediatric Institute in Mexico City.