A dialysis system includes a dialysis instrument including a blood pump actuator, a dialyzer, a heparin pump, and a disposable cassette including a blood pumping portion operable with the blood pump actuator of the dialysis instrument, the blood pumping portion including an inlet and an outlet, the inlet of the blood pumping portion of the disposable cassette positioned and arranged to receive blood from an arterial line of an extracorporeal circuit, the outlet of the blood pumping portion of the disposable cassette positioned and arranged to deliver the blood to the dialyzer and to receive heparin from the heparin pump.