A blood processing apparatus includes a measurement device having at least one chamber element extending along a longitudinal axis and including a circumferential wall extending about the longitudinal axis, a bottom wall and a top wall together defining a flow chamber. The blood processing apparatus further includes a holder for the measurement device, the holder including a base having a reception opening for receiving the measurement device and a closure element movably arranged on the base for locking the measurement device in an inserted position in the reception opening. An ultrasonic sensor of the holder is arranged on the base. The ultrasonic sensor, in the inserted position of the measurement device, faces the bottom wall of the at least one chamber element, wherein the bottom wall extends transversely with respect to the longitudinal axis.