ANATOMY

Detailed explanation-1: -The first sound LUB is produced when the atrioventricular valves i.e. tricuspid and bicuspid valves close at the start of ventricular systole. The second sound DUB is produced at the beginning of ventricular diastole when the pulmonary and aortic semilunar valves close.

Detailed explanation-2: -The closure of semilunar valves produces the heart sound “dub”. This occurs at the end of ventricular systole.

Detailed explanation-3: -The familiar ‘lub-dub’ sound of the heartbeat is caused by the rhythmic closing of the heart valves as blood is pumped in and out of the chambers.

Detailed explanation-4: -The second heart sound (S2) represents closure of the semilunar (aortic and pulmonary) valves (point d). S2 is normally split because the aortic valve (A2) closes before the pulmonary valve (P2).

Detailed explanation-5: -When the ventricles relax, blood fills the cusps and causes their free edges to meet in the middle of the vessel, closing the valve and preventing any backflow. This selection is the only option that accurately explains why the aortic and pulmonic valves close after the ventricles contract.