A dialyser has very thin fibres of cellulose. The fibres work as a semi-permeable membrane (SPM). There are about 10,000 of them. Impurities such as urea keratin leave through the SPM. Blood cells cannot go through because the blood cells are bigger than the impurities. The whole process of purification is supported by a liquid called dialysate. Dialysate is sent into the dialyser by the pump situated in the dialysis machine. The pure liquid goes in and the impure dialysate comes out. Dialysers are also called artificial kidneys.