Final answer:
The heart's filling relies on venous pressure, which influences the preload or end diastolic volume, crucial for cardiac output.
Step-by-step explanation:
The filling of the heart relies critically on the venous pressure. Venous pressure helps to return blood to the heart, which contributes to ventricular filling, and the end diastolic volume (EDV) or preload. Preload is the amount of blood present in the ventricles at the end of atrial systole just before ventricular contraction, when the ventricles have filled to approximately 70-80% of their capacity due to the inflow during diastole, plus the additional 20-30% contributed by the atrial kick during atrial contraction. The end systolic volume (ESV) is the amount of blood remaining in each ventricle following systole. Enhanced venous return can be influenced by any sympathetic stimulation to the venous system and is crucial for maintaining adequate EDV and thus cardiac output.