Final answer:
The lub-dub sound heard with a stethoscope is caused by the closing of the atrioventricular valves (lub) and the semilunar valves (dub) in a normal, healthy heart.
Step-by-step explanation:
In a normal, healthy heart, there are only two audible heart sounds: S₁ and S2. S₁ is the sound created by the closing of the atrioventricular valves during ventricular contraction and is normally described as a "lub," or first heart sound. The second heart sound, S2, is the sound of the closing of the semilunar valves during ventricular diastole and is described as a "dub."
As the valves close, the openings within the atrioventricular septum guarded by the valves will become reduced, and blood flow through the opening will become more turbulent until the valves are fully closed.