Final answer:
The 'dub' sound (S2) heard during a heart auscultation corresponds to the closure of the semilunar valves at the beginning of ventricular diastole to prevent blood backflow.
Step-by-step explanation:
The Second Heart Sound (S2) During Diastole
The second heart sound, known as S2 or dub, begins at the onset of ventricular diastole and is caused by the closing of the semilunar valves. During the cardiac cycle, ventricular systole is followed by ventricular diastole, when the ventricular muscle relaxes and the pressure within the ventricles falls. As the pressure in the ventricles drops below that in the aorta and pulmonary artery, blood starts to flow back toward the heart but is stopped by the closure of the semilunar valves. This closure results in the 'dub' sound and prevents backflow of blood into the ventricles. This event also signifies the beginning of the isovolumic ventricular relaxation phase where there is no change in the volume of blood in the ventricle due to both sets of valves (atrioventricular and semilunar) being closed.