An extracorporeal blood treatment apparatus, comprising: a blood treatment device having a blood chamber and a fluid chamber separated from one another by a semipermeable membrane; an extracorporeal blood circuit connected to the blood chamber such as to transport blood from an individual to the blood chamber and to return the blood from the blood chambter to the individual; a sensor predisposed in the extracorporeal blood circuit for emitting at least a signal indicating the variation of blood volume (ΔBV) of the individual subjected to the extracorporeal blood treatment; a sensor configured such as to emit at least a signal indicating the weight loss of the individual subjected to the extracorporeal blood treatment; a weight loss system for actuating the weight loss of the individual during the extracorporeal blood treatment; a control unit configured for performing following operations: - receiving the signals indicating a relative variation of blood volume (ΔBV) and weight loss during the extracorporeal blood treatment; - acquire a desired value (IR des) of a parameter of the individual, wherein the parameter of the individual comprises an index which is a function of the ability of the patient to re-absorb, from the cells of her or his tissues, the mass of liquids subtracted from the plasmatic part of the blood, or an index which is a function of the quantity of liquid that displaces from the interstitial space of the patient's body to the intravascular space thereof; characterized in that the control unit is further configured for performing the following operations: - determining a calculated value of the parameter of the individual as a function of the relative variation of the blood volume (ΔBV%) and a variation of weight loss (ΔWL%) of the individual; - controlling the weight loss system on a basis of the desired value (IR des) of the individual and on the calculated value of the individual. a blood treatment device having a blood chamber and a fluid chamber separated from one another by a semipermeable membrane; an extracorporeal blood circuit connected to the blood chamber such as to transport blood from an individual to the blood chamber and to return the blood from the blood chambter to the individual; a sensor predisposed in the extracorporeal blood circuit for emitting at least a signal indicating the variation of blood volume (ΔBV) of the individual subjected to the extracorporeal blood treatment; a sensor configured such as to emit at least a signal indicating the weight loss of the individual subjected to the extracorporeal blood treatment; a weight loss system for actuating the weight loss of the individual during the extracorporeal blood treatment; a control unit configured for performing following operations: - receiving the signals indicating a relative variation of blood volume (ΔBV) and weight loss during the extracorporeal blood treatment; - acquire a desired value (IR des) of a parameter of the individual, wherein the parameter of the individual comprises an index which is a function of the ability of the patient to re-absorb, from the cells of her or his tissues, the mass of liquids subtracted from the plasmatic part of the blood, or an index which is a function of the quantity of liquid that displaces from the interstitial space of the patient's body to the intravascular space thereof; - receiving the signals indicating a relative variation of blood volume (ΔBV) and weight loss during the extracorporeal blood treatment; - acquire a desired value (IR des) of a parameter of the individual, wherein the parameter of the individual comprises an index which is a function of the ability of the patient to re-absorb, from the cells of her or his tissues, the mass of liquids subtracted from the plasmatic part of the blood, or an index which is a function of the quantity of liquid that displaces from the interstitial space of the patient's body to the intravascular space thereof; characterized in that the control unit is further configured for performing the following operations: - determining a calculated value of the parameter of the individual as a function of the relative variation of the blood volume (ΔBV%) and a variation of weight loss (ΔWL%) of the individual; - controlling the weight loss system on a basis of the desired value (IR des) of the individual and on the calculated value of the individual. - determining a calculated value of the parameter of the individual as a function of the relative variation of the blood volume (ΔBV%) and a variation of weight loss (ΔWL%) of the individual; - controlling the weight loss system on a basis of the desired value (IR des) of the individual and on the calculated value of the individual.