The first heart sound, "lub," marks the start of . During this phase, the heart's lower chambers (ventricles) contract to pump blood out to the lungs and the rest of the body. How the Heart Works - How the Heart Beats | NHLBI, NIH
Between the and the Dub lies life. Between the Dub and the next Lub lies rest. Listen closely. That rhythm is the only conversation you never want to end. lub dub sounds systole diastole