An apparatus for extracorporeal blood treatment, comprising: - at least one treatment unit 5 (2) having at least a first chamber (3) and at least a second chamber (4) separated from each other by a semipermeable membrane ( 5)； - at least one blood withdrawal line (6) connected to an inlet opening of the first chamber and configured to take blood from a patient, - at least one blood return line (7) connected to an outlet port of the first chamber and configured to return the treated blood to the patient, forming the blood withdrawal line (6), the blood return line (7) and the first chamber part of an extracorporeal blood circuit (8)； - a dialysate circuit comprising: at least one dialysis line (11) connected to the inlet port of the second chamber (4) and configured to transport new dialysis fluid to the second chamber (4), at least one line of fluid evacuation (10) connected to an outlet orifice of the second chamber (4) and configured to discharge the spent dialysis fluid exiting the second chamber (4), and a fluid preparation section connected to the line of dialysis (11) and configured to adjust the conductivity, or concentration for at least one substance, in the new dialysis fluid； - at least one ultrafiltration device (20) connected to the dialysate circuit and configured to determine ultrafiltration through the membrane from the first to the second chamber； - a control unit (15) connected to the ultrafiltration device (20) and the fluid preparation section and configured to perform a first control procedure comprising: receiving measured values of: a first parameter (BV% med (t) ) related to the change in blood volume in the blood circulating in the extracorporeal blood circuit between the start of treatment and a respective time of treatment time (t), and a second parameter related to the amount of ultrafiltration volume (UFmed ( t)； WLmed (t)) accumulated up to the time of treatment time (t) from the beginning of the treatment, and receive prescription valu
