Final answer:
The 'lub-dup' sounds of the heartbeat, heard through auscultation, correspond to the closing of the atrioventricular and semilunar valves during the cardiac cycle.
Step-by-step explanation:
The characteristic lub-dup sounds heard with a stethoscope during auscultation of the heart are caused by the closing of heart valves. The 'lub' sound, also known as S₁, is the result of the atrioventricular (AV) valves closing during ventricular contraction. The 'dup' sound, or S₂, is caused by the closure of the semilunar (SL) valves during ventricular diastole. When the valves shut, the openings in the heart become narrowed, leading to turbulent blood flow until the valves are completely closed. These sounds can indicate the healthy function of the heart, whereas additional sounds like S3 and S4 may suggest heart abnormalities such as hypertrophic cardiomyopathy or congestive heart failure.