A pumping apparatus comprises a rotatable shaft coupled to a motor. At least one control structure is mounted on the shaft, and at least one impeller is carried by the shaft. The impeller is adapted to rotate independently of the shaft so as to pump a fluid. The apparatus also comprises a magnetic coupling between the control structure and the impeller. Further, the impeller is adapted to translate axially along the shaft in response to a change in downstream pressure to alter the strength of the magnetic coupling.