Answer:
The sound "lub" is produced by the closure of the atrioventricular valves (mitral and tricuspid valves) while the sound "dub" is produced by the closure of the semilunar valves (aortic and pulmonic valves).
Step-by-step explanation:
The question above is related to the body's circulatory system. The heart is pumping non-stop in order to deliver oxygenated blood to the body. Blood follows a particular flow once it enters and leaves the heart. Part of that flow produces the "lub" and "dub" sound. These sounds are produced by the two valves that ensure the blood follows a correct path. These are the atrioventricular valves and the semilunar valves.
The atrioventricular valves are located in the middle of the atria and the ventricles while the semilunar valves are located at the bottom of the vessels. Once the blood flows towards the atrioventricular valves and they close, it produces the "lub" sound, also called the first heart sound. When the blood flows towards the semilunar valves and they close, it produces the "dub" sound, also called the second heart sound.