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* The atria relax
* The thick muscular wall of the ventricles contract forcing blood out of the heart into the pulmonary artery and the aorta
* The pressure of blood against the atrio-ventricular valves causes them to shut , preventing blood going back into the atria . This produces the first part of the heart sound "lub"
* The pressure of blood against the semi-lunar valves opens them.
* The pulmonary artery then carries deoxygenated blood to the lungs , and the aorta carries oxygenated blood to the different parts of the body.
