Device and method for controlling the balance of fluids in an extracorporeal blood circuit

A blood treatment apparatus, comprising:

• a treatment device;
• an inlet line connected to the treatment device for conveying blood from the vascular system of a patient into the treatment device;
• an outlet line connected to the treatment device for conveying blood away from the treatment device back to the patient;
• a bypass line interconnecting the inlet line and the outlet line;
• the inlet line, the outlet line, and the bypass line defining a recirculation loop;
• means for selectively preventing flow between the recirculation loop and the patient to isolate the patient therefrom;
• means connected to one of said inlet line and said outlet line for injecting a substitute liquid into the recirculation loop;
• means connected to the treatment device for withdrawing liquid from blood in the treatment device;
• and
• pressure measuring means connected to the recirculation loop for generating a signal indicative of a difference between a flow rate of the substitute liquid and a flow rate of the liquid withdrawn from blood in the treatment device when the recirculation loop is isolated from the patient.