The heartbeat

A heartbeat is characterized by a lub-dub sound. The closing of the valves in the heart causes this. The action of the heart consists of successive alternate contractions (systole) and relaxations (diastole) of the muscular walls of the auricles and ventricles.
In diastole, the heart muscles relax and blood is drawn from the body into the auricles or atria. Rising pressure in each auricle opens the tricuspid and bicuspid valves and blood flows into the ventricles; each auricle contracts filling the ventricle to capacity. During the systole stage the filled ventricles contract and the tricuspid and bicuspid valves close; the pulmonary and aortic valves open and blood is forced out of the ventricle into the arteries. The heart relaxes the pulmonary valves and aortic valves close and the whole process begins again.


If you really want to understand how the heart beats, read on the Mechanism of heartbeat

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