Our heart has four chambers. The walls of these chambers are made of special muscles. There is a node called the sinoatrial node. This node sends an electrical signal at regular intervals to the atrioventricular node. The sinoatrial node is at the right atrium and the atrioventricular node is at the lower right. This node is split into two branch on the left and the right and these are further split into tiny fibers which spread across the ventricles.
Blood is pumped through contraction into the arteries. After the blood is pumped, the heart is in a diastole stage (rest stage). During this period blood from the veins fill up the atria again. The left atria have veins from the lungs and the right atria has veins from the rest of the body. After the diastole stage the pumping starts again. This cycle is repeated. The sound of the heart that the doctor hears with the stethoscope is due to the closing of the valves in the heart.
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