A cardiac assist system having a pumping device for moving blood, wherein a pumping capacity of the pumping device is adjustable using an adjustment signal based on laser doppler with an optical fiber. A measuring device measures a flow rate of the body fluid, the measuring device comprising at least one light source for outputting a light beam and at least one sensor element for detecting a reflected partial beam of the light beam. The measuring device is adapted to measure the body fluid using the reflected partial beam of the light beam. A determination device is adapted to determine the adjustment signal using the measurement signal. The device may include a bore opening to a blood flow channel, with an optical fiber extending through the bore.