Dialysis is a treatment given to patients whose kidneys are not working at all- Kidney Failure. Basic principle on which a dialysis machine works is maintenance of concentration gradient. Blood of a patient is passed through a dialysis machine which has the fluid known as dialysis fluid. Blood of an individual and dialysis fluid are separated by partially permeable membrane.
The movement of substances is from a region of high concentration to the region of low concentration.This is why dialysis fluid has no urea. As blood has a great amount of urea so as per principle of diffusion urea moves from blood to dialysis fluid. Hence this is how urea is removed from blood.
However, we don't want useful substances to leave blood as well such as salts,glucose and minerals. In order to prevent them from leaving bood the concentrations are kept isotonic therefore there is no net movement of these important substances such as salts, glucose or minerals.