A hemodialysis system (110) is described. The system comprises: an on-line dialysate generation source (112)； a dialyzer (20, 30)； a first dialysate pump (22) in fluid communication with the on-line dialysate generation source (112) for pumping dialysate to the dialyzer (20, 30)； a second dialysate pump (24) in fluid communication with the on-line dialysate generation source (112) for pumping dialysate to the dialyzer (20, 30)； a third dialysate pump (26) for pumping dialysate from the dialyzer (20, 30)； and a fourth dialysate pump (28) for pumping dialysate from the dialyzer (20, 30). Each dialysate pump is associated with an inlet valve (56, V2, V3, V16, V18) and an outlet valve (56, V4, V5, V15, V17), and the system (110) is configured using the dialysate pumps and the inlet and outlet valves to alternately pump dialysate from the first and second dialysate pumps to the dialyzer (20, 30) and to alternately pull dialysate from the dialyzer to the third and fourth dialysate pumps.
