The "lub" isn't just the sound of the flaps touching; it is the vibration of the heart walls and the turbulent blood as these valves catch the backflow and lock tight. The "Dub": Closing of the Semilunar Valves
The "lub" occurs at the beginning of —the phase where the heart's lower chambers (ventricles) contract to push blood out to the body and lungs. As the ventricles squeeze, the pressure inside them rises sharply. To prevent blood from flowing backward into the upper chambers (atria), two valves snap shut: The Mitral Valve: Located on the left side. The Tricuspid Valve: Located on the right side. lub-dub sound of heart is caused by