Preload to the hea depends upon

When the muscle is stretched, the developed tension increases to a maximum and then declines as stretch becomes more extreme. Starling pointed this out when he stated that the "energy of contraction is propoional to the initial length of the cardiac muscle fiber" (Starling's law of the hea or the Frank-Starling law). For the hea, the length of the muscle fibers (ie, the extent of the preload) is propoional to the end-diastolic volume.Ref: Ganong&;s Review of medical physiology 25th edition. Page: 546