Measuring the change of intravascular blood volume during blood filtration

To determine in simple manner the change of the intravascular blood volume during filtration, in the extracorporeal blood circuit (2) of a blood purification apparatus at least one ultrasonic sensor (15) is arranged which is connected to an evaluating unit (18) which is configured in such a manner that at the start of the filtration a first ultrasonic signal is stored and during the filtration the change of the ultrasonic signals is determined. From the change of the ultrasonic signals the change of the hematocrit is determined and from this the change of the intravascular blood volume is deduced.