Magnetic field dynamics recorded with NMR field sensors located close to beating heart provide detailed access to cardio-vascular action. We present a realistic model that examines the physical pathways which translate mechanical heart activity into observable magnetic field dynamics. It is based on a 4D tissue model obtained from cardiac cine MR images. We confirmed that the observed signals are mainly caused by displacements of blood and muscle tissue. The presented modelling approach is expected to advance the physiological interpretation of the observed signals.