The heart valves- the bicuspid and tricuspid valves- are there to prevent blood from back-welling into the atria after atrial systole (contraction). Once the atria have filled… the ventricles of the heart with blood, the valves close- making the "heartbeat sound." The ventricles can then contract. If these valves didn't exist, then the blood would flow back into the atria, and you'd probably have a colossal heart attack and die. Isn't biology a cheery subject?! (MORE)

  The rise in pressure inside the ventricles, when the walls of the ventricles contract.