After the heart squeezes blood out to the lungs and body, the ventricles relax. The blood in the major arteries (the Aorta and Pulmonary Artery) wants to rush back into the heart, but two different valves—the Aortic and Pulmonary valves—snap shut to stop it. Their closure creates the "dub."
Spoiler alert: It’s not the heart muscle contracting, and it’s not the blood whooshing around. lub-dub sounds
"The squeeze is starting. No blood is allowed back into the top rooms." Breaking Down the "Dub" (S2) The second sound, "Dub" (clinically known as S2 ), marks the end of systole and the beginning of rest (diastole). After the heart squeezes blood out to the