An apparatus for extracorporeal blood treatment, comprising: - a blood removal line having an end destined to be connected to a cardiovascular system of a subject; - at least a blood treatment unit comprising a semipermeable membrane which divides the unit itself into a first and a second chamber (11, 12), the first chamber being connected to a second end of the blood removal line; - a return line of the treated blood to the patient having a first end connected to the first chamber of the blood treatment unit and a second end destined to be connected to the cardiovascular system of the patient; - at least a first pump operating in correspondence of the blood removal line; - at least a second pump operating in correspondence of the blood return line to the patient; - a discharge line connected to the second chamber of said treatment unit for receiving waste fluid in outlet from the unit itself; - at least a first blood reservoir operating on the blood return line upstream of the second blood pump; - at least a sensor predisposed to detect at least a parameter selected from among a group comprising: ▪ a pressure in said first reservoir, ▪ a blood level present in said first reservoir, ▪ a blood volume present in said first reservoir, ▪ a pressure in a tract of the return line upstream the second pump, and predisposed to emit a signal corresponding to a measured value of the parameter; - a control unit connected to the first pump, to the second pump and to said sensor, wherein said control unit is configured such as to operate in a first operating mode, corresponding to a single-access configuration of the extracorporeal circuit, and in a second operating mode, corresponding to a double-access configuration of the extracorporeal circuit, the control unit, in said second operating mode, being configured to carry out the following steps: ∘ receiving the signal emitted by said sensor, ∘ controlling said first pump such as to promote the removal of blood from said patient along said blood removal line; characterised in that the control unit, in the second operating mode, is further configured to carry out the following step: ∘ controlling said second pump such as to promote return of the blood to the patient along the return line, the control of said second pump being carried out on the basis of the signal emitted by said sensor acting on the first reservoir. - a blood removal line having an end destined to be connected to a cardiovascular system of a subject; - at least a blood treatment unit comprising a semipermeable membrane which divides the unit itself into a first and a second chamber (11, 12), the first chamber being connected to a second end of the blood removal line; - a return line of the treated blood to the patient having a first end connected to the first chamber of the blood treatment unit and a second end destined to be connected to the cardiovascular system of the patient; - at least a first pump operating in correspondence of the blood removal line; - at least a second pump operating in correspondence of the blood return line to the patient; - a discharge line connected to the second chamber of said treatment unit for receiving waste fluid in outlet from the unit itself; - at least a first blood reservoir operating on the blood return line upstream of the second blood pump; - at least a sensor predisposed to detect at least a parameter selected from among a group comprising: ▪ a pressure in said first reservoir, ▪ a blood level present in said first reservoir, ▪ a blood volume present in said first reservoir, ▪ a pressure in a tract of the return line upstream the second pump, and predisposed to emit a signal corresponding to a measured value of the parameter; ▪ a pressure in said first reservoir, ▪ a blood level present in said first reservoir, ▪ a blood volume present in said first reservoir, ▪ a pressure in a tract of the return line upstream the second pump, - a control unit connected to the first pump, to the second pump and to said sensor, wherein said control unit is configured such as to operate in a first operating mode, corresponding to a single-access configuration of the extracorporeal circuit, and in a second operating mode, corresponding to a double-access configuration of the extracorporeal circuit, the control unit, in said second operating mode, being configured to carry out the following steps: ∘ receiving the signal emitted by said sensor, ∘ controlling said first pump such as to promote the removal of blood from said patient along said blood removal line; characterised in that the control unit, in the second operating mode, is further configured to carry out the following step: ∘ controlling said second pump such as to promote return of the blood to the patient along the return line, the control of said second pump being carried out on the basis of the signal emitted by said sensor acting on the first reservoir. ∘ receiving the signal emitted by said sensor, ∘ controlling said first pump such as to promote the removal of blood from said patient along said blood removal line; ∘ controlling said second pump such as to promote return of the blood to the patient along the return line, the control of said second pump being carried out on the basis of the signal emitted by said sensor acting on the first reservoir.