A good illustration about normal physiology is always refreshing. Fee things to remember: Heart muscle needs to strech in order to contract, so it requires a lot of blood to return to the ventricle from both sup and inf vena cava. This returning blood dilates helps the ventricle to dilate (it does it partially alone) causing a strong sistole and increasing heart rate. The net effect is an increase in cardiac output (more blood pumped per minute). This is known as the Frank Starling Law:
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-The Plague Doctor