A peristaltic roller pump includes a rotor carrying a pumping roller on one end and an occluding roller on the other end. Occluding roller has a number of protrusions designed to occlude the tubing section that is located under that roller as it moves over it in order to prevent any back flow through that section of the tube. Occluding roller is designed to not cause any pumping of fluid through the section of the tubing under that roller. Such design is useful in a single-needle dialysis system in which during a first phase of pumping blood is moved in one direction from the patient towards the dialysis unit and during the second phase of operation blood is returned to the patient in the opposite direction.