I think I have got an answer for why end diastolic volume is determined by end diastolic pressure . If you have a copy of Guyton and Hall medical physiology 12th edition , turn to page 108 , you will see a graph of left ventricular pressure (y axis) vs left ventricular volume (x axis) ,or just search for a pressure vs volume graph . During diastole as blood flows into the ventricles from the atria , the pressure in the ventricle slowly rises for the first 150ml of blood , then the gradient of the graph rises steeply and the pressure sharply increase for small increment of volume . This is because of the compliance of the cardiac muscle and that of the pericardium . The heart is to a certain extend stretchable and it stores potential energy in its wall the more its stretches and it tend to recoil back to its original state .
the more stretched the heart is the higher pressure it's wall exerts on the blood inside . This pressure opposes blood coming in from the atrium so as the more filled the ventricles , the harder it is for blood to move in from the atria . The pressure build in the ventricle determines the volume of blood entering it .