During diastole, the ventricles are relaxing and filling with blood from the atria. No major valves are snapping shut, so there is no loud sound. This silence is just as important as the beats, because it is the time when the heart refills for the next pump.
The "lub-dub" sound of a heartbeat is the sound of your heart's four valves snapping shut in a timed sequence to keep blood flowing in one direction . The Story of the "Lub" and the "Dub" Imagine your heart is a busy house with four main rooms and two pairs of doors that must never be open at the same time to prevent a "leak" (backflow). The First Sound: "Lub" (S1) As the lower chambers of the heart (the ventricles) fill up with blood from the upper rooms, they prepare for a big push to send that blood to the rest of the body. To make sure no blood accidentally splashes backward into the upper rooms, the what is lub dub sound
: This is the first heart sound. It occurs when the atrioventricular (AV) valves —the mitral and tricuspid valves—close. This happens at the start of systole , when the heart's lower chambers (ventricles) contract to pump blood out of the heart. During diastole, the ventricles are relaxing and filling