Kidney dialysis. Gloved hands of a nurse inserting a cannula into a blood vessel in the forearm of a patient with kidney failure, in preparation for haemodialysis. Blood will flow out of a tube attached to the cannula and into a dialysis machine. This acts as an 'artificial kidney' to remove waste products and excess fluid from the blood. The filtered blood is then returned through a separate tube into the patient's body. This cleaning of the blood is usually done by the kidneys. If they are damaged, waste products build up in the blood, which can be fatal. Haemodialysis takes between 2 and 6 hours and must be carried out several times a week.