A system, method and apparatus for performing a renal replacement therapy is provided. In one embodiment, two small high flux dialyzers are connected in series. A restriction is placed between the two dialyzers in the dialysate flow path. The restriction is variable and adjustable in one preferred embodiment. The restriction builds a positive pressure in the venous dialyzer, causing a high degree of intentional backfiltration. That backfiltration causes a significant flow of dialysate through the high flux venous membrane directly into the patient's blood. That backfiltered solution is subsequently ultrafiltered from the patient from the arterial dialyzer. The diffusion of dialysate into the venous filter and removal of dialysate from the arterial dialyzer causes a convective transport of toxins from the patient. Additionally, the dialysate that does not diffuse directly into the patient but instead flows across the membranes of both dialyzers provides a diffusive clearance of waste products.