Cardiac cycle.

  1. During a relaxation phase (diastole), blood returning from the large veins flows into the atria and ventricles.

  2. A brief period of atrial systole forces blood out of the atria into the ventricles when atrioventricular (AV) valves open.

  3. Then ventricular systole pumps blood into the large arteries when semilunar valves open.