Final answer:
The first heart sound, S1, is heard when the atrioventricular valve closes during ventricular contraction. The second heart sound, S2, is heard when the semilunar valves close during ventricular diastole.
Step-by-step explanation:
In a normal, healthy heart, the first heart sound, S1, is heard when the atrioventricular valve closes during ventricular contraction, and is described as a 'lub'. This occurs when the ventricles are contracting to pump blood out of the heart. The second heart sound, S2, is heard when the semilunar valves close during ventricular diastole, and is described as a 'dub'. This occurs when the ventricles are relaxing and filling with blood.