Final answer:
In every heartbeat or cycle, there are two distinct heart sounds known as "lub-dub," corresponding to the closing of the atrioventricular valves (S1) and the closing of the semilunar valves (S2).
Step-by-step explanation:
There are two distinct heart sounds in every heartbeat or cycle, described as "lub-dub." The first heart sound, S1, is created by the closing of the atrioventricular valves during ventricular contraction and is described as a "lub", or first heart sound. The second heart sound, S2, occurs when the semilunar valves close during ventricular diastole and is described as a "dub".
There is also a third heart sound, S3, which is rarely heard in healthy individuals, indicating the presence of blood flowing into the atria, or blood sloshing back and forth in the ventricle. The fourth heart sound, S4, can indicate the presence of a stiff or hypertrophic ventricle. When both S3 and S4 are present, this is referred to as S7. Cardiologists sometimes use the terms Kentucky gallop and Tennessee gallop to describe the rhythms of these sounds.