It is disclosed an apparatus for extracorporeal blood treatment (1) having a control unit (15) connected to an ultrafiltration device (20) and to a fluid preparation section (30) of fresh dialysis liquid. The control unit (15) is configured to receive measured values of the change of blood volume, the amount of ultrafiltration volume, and conductivity or to the concentration for at least one substance in the blood (BV%mes(t)； UFmes(t), WLmes(t)； Cbmes(t))； the control unit (15) is also configured to receive prescription values for the same parameters and to control ultrafiltration and adjust conductivity in the fresh dialysis liquid based on the difference between said measured values and said prescription values.
