MODEL RELEASED. Kidney dialysis. Nurse inserting a needle into a patient's wrist before starting renal dialysis (haemodialysis). The dialysis machine (artificial kidney) is behind the nurse. Haemodialysis is required when the kidneys are no longer able to filter waste products from the blood properly. During the procedure, a steady flow of blood passes over an artificial semi- permeable membrane in the dialysis machine. The small waste products in the blood pass through the membrane, but the larger blood cells and proteins cannot. The purified blood is then returned to the patient via a vein.