The Brainliest Answer!
During ventricular systole, the aperture between the artria and the ventricles is closed by valves when  the valves are closed forcibly.we can listen to the Lubb.
During ventricular  diastole, the valves which are present in the blood vessels are closed to prevent backward flow of blood into the ventricles.then we can listen to a dull hert sound, Dubb........

hope its help u....

2 5 2
  • Brainly User
The "lub" sound is generated by the atrioventricular valves closing. Once the atria – have finished emptying blood into the ventricles, the valves between the atria and the ventricles clamp shut in order to prevent blood leaking in the wrong direction. The“dub” sound occurs after the ventricles have pumped oxygenated blood into the aorta, to be delivered to the body, and deoxygenated blood into the pulmonary artery, to be returned to the lungs. The flaps of the aortic and pulmonary valves are more rigid than those of the atrioventricular valves, so the sound they make when they close is slightly shorter and crisper.
1 3 1