A dialysis system is described. The system comprises a dialyzer a blood pump configured to pump blood to the dialyzer a dialysate inlet pump configured to pump dialysate to the dialyzer a dialysate outlet pump configured to pump dialysate from the dialyzer a fluid velocity sensor including an emitter and a receiver and a disposable cassette including a to-dialyzer dialysate pathway carrying dialysate pumped by the dialysate inlet pump and a from-dialyzer dialysate pathway carrying used dialysate pumped by the dialysate outlet pump, one of the to-dialyzer dialysate pathway and the from-dialyzer dialysate pathway including a sensing area that is coupled operably to both the emitter and the receiver of the fluid velocity sensor.
