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 only 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 and
in other alternating pumping systems in which during a first phase of the
pumping the fluid is moved in one direction and during the second phase
of operation the fluid is moved in the opposite direction.