The heart-lung preparation. Ventricular volume, and therefore SV, and CO, and arterial and venous pressures can be measured. The heart is isolated from the central nervous system so that there is no reflex control, heart rate therefore does not change significantly. The chest is open so that intra-thoracic pressure is constant at atmospheric pressure and does not vary with respiration. So the whole importance of the findings by Starling is that, within limits, the heart responds to the blood coming back to it, it pumps it out against whatever, reasonable, resistance it might face.
Cardiac muscle has a different troponin from skeletal muscle.
Steeply rising portion at normal right atrial pressures small changes in RAP cause large changes in output. This is because the heart is easily stretched at low volumes So increased pre-load leads to increased stretch leads to increased output. Plateau region at high right atrial pressures no further increase in output unless extrinsic influence can increase cardiac performance the heart is fully stretched, Increased output needs increased rate greater emptying.