Hemodialysis ultrafiltration apparatus comprises a dialyzer. A closed circuit is provided for supplying fresh dialysate to and for removing spent dialysate from the dialyzer. Blood is supplied to and removed from the dialyzer. The closed circuit for supplying fresh dialysate and removing spent dialysate is apportioned into below mean dialyzer blood pressure and above atmospheric pressure sections, with the dialyzer being positioned in the below mean dialyzer blood pressure section. A mechanism is provided for controlling the pressure of fresh dialysate introduced into the below mean dialyzer blood pressure section to substantially match the pressure on the dialysate side of the dialyzer. A mechanism is provided for controlling the above atmospheric pressure portion of the spent dialysate circuit. A pump removes spent dialysate of equivalent ultrafiltrate volume from the closed circuit to control the rate of ultrafiltrate removal from blood in the dialyzer, thus affecting the pressure on the dialysate side of the dialyzer required to sustain that rate.
A method of hemodialytic ultrafiltration is carried out substantially in accordance with the above-described apparatus.