The second heart sounds are produced as a result of the closure of the aortic (A2) and the pulmonary (P2) valves as ventricular pressures fall below those in the aorta and the pulmonary artery.
A2 and P2 separate on inspiration (P2 following A2). This is because of the increased right ventricular stroke volume that occurs as the result of increased venous return. In the expiratory phase of continuous respiration (in normal subjects reclining at 30-40 degrees to reduce venous return) A2 and P2 are almost fused because the systoles of the left ventricle and the right ventricle occur almost simultaneously.
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