Cardiac cycle.
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<Li> During  a  relaxation  phase  (diastole), blood returning from the
large veins flows into the atria and ventricles.
<Li> A  brief  period  of  atrial systole forces blood out of the atria
into the ventricles when atrioventricular (AV) valves open.
<Li> Then  ventricular systole pumps blood into the large arteries when
semilunar valves open.