A blood flow measuring apparatus has a measuring electrode and a counter-electrode adapted to be brought into contact with the blood of a patient. A measuring unit measures a voltage or a current between the measuring electrode and the counter-electrode and determines the flow of blood therefrom. The measuring electrode is ring-shaped and sized to fit within a blood vessel in the body of the patient. An inner surface of the ring-shaped electrode forms an active measuring electrode surface. The blood flow measuring apparatus is suitable for use in a cardiac pacemaker.