An extracorporeal blood treatment apparatus comprises a sensor (10) for emitting a signal indicating a change of hematic volume of an individual (7) subjected to a treatment and a weight loss system for actuating the individual's weight loss. A control unit (20) receives an effective weight value of the individual and a desired weight loss value and from these values determines a desired value of a change in hematic volume at end of treatment. The weight loss system is controlled on a basis of the hematic volume change signal and the desired value of the hematic volume change. The apparatus enables automatic control of a dialysis operation while preventing some complications arising from hypotension.