One heart beat consists of **contraction, relaxation and a pulse.** The *beating** of the heart itself and it can only be controlled by the nervous system. The heart beat or cardiac cycle startd with the contraction of the atrium (atrial contraction). The stimulus for the rhythmic beat of the heart cones from a small part of the right atrium called the **sino-atrial**. The action is wavelike and so it spreads to the left atrium (auricle). The blood is pumped to all parts of the body enters the **right atrium** from the superior and inferior venae cavae while oxygenated blood enters the left atrium from the lungs through the **pulmonary veins**. When the two atria contract, the blood presses open the **tricuspid** and **bicusoid valves** leading into thr ventricles and fills the ventricles. 
    When the ventricles contract, the tricuspid and the bicuspid valves are closed in such a way that no blood flows back into the atria. The blood from the righy ventricle is forced into the pulmonary artery which supplies the lungs. The blood from left ventricle eterd the aorta which supplies all parts of the body. The entrance of the pulmonary artery is guarded by semi-lunar valves which prevent the blood from flowing into the ventricles.
          All these constitute one cardiac cycle. The cardiac output is the total volume of blood pumped out by one ventricle in minute.


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