1. Why Every Heartbeat Makes Two Sounds, Not One
Medical textbooks call the two main sounds of a healthy heartbeat the first heart sound (S1) and the second heart sound (S2) [3]. "Lub" is S1 and "dub" is S2. Doctors write the sounds in different ways, and what you hear depends on the person and the room, so "lub-dub" is just one common way to put it into words.
These sounds are very short. The Healio heart-sounds review describes them as brief, separate events with a specific cause, such as a valve closing. That separates them from a "murmur," which comes from turbulent blood flow and can last through much of the time between beats [3]. The MSD Manual describes heart sounds as brief, transient sounds produced by valve opening and closure [1]. So the question is which valves are involved, and when they close.
2. The Heart's Four Valves Work Like One-Way Doors
The heart has four chambers: two small upper chambers (atria) and two larger lower chambers (ventricles). Between each atrium and ventricle sits a valve, and each ventricle has an exit valve that leads out to an artery [4]. On the left side these are the mitral valve (between the chambers) and the aortic valve (the exit to the body). On the right side they are the tricuspid valve (between the chambers) and the pulmonary valve (the exit to the lungs).
A simple way to picture them is as one-way doors. Blood pushes a door open as it moves forward, and when the pressure behind the door drops, the door swings shut so blood cannot flow backward. That picture is only an aid, since real valves are flexible flaps rather than hinged doors.
3. Which Valves Close for "Lub" and Which Close for "Dub"?
A physiology teaching page from Loyola University Chicago gives a clear rule: blood flowing through a valve is silent, valves open silently, and they make sound when they shut [2]. The same page matches the two sounds to two pairs of valves:
- Lub (S1): the mitral and tricuspid valves close. This happens right after the electrical signal that starts the heartbeat, as the lower chambers begin to squeeze.
- Dub (S2): the aortic and pulmonary valves close, after the lower chambers have finished pushing blood out [2].
The Healio review agrees that S1 is the closing of the mitral and tricuspid valves [3]. So one heartbeat contains two door-closing moments: first the doors between the chambers, then the doors at the exits. That is where the two-part rhythm comes from.
4. Why Breathing In Can Split the "Dub" Into Two Sounds
The two exit valves, the aortic and the pulmonary, do not close at exactly the same instant. According to the Loyola page, the pulmonary valve closes about 60 milliseconds (0.06 seconds) after the aortic valve [2]. That gap is so small that the two closures usually blend into a single "dub."
When a person breathes in, the gap can grow just enough for a trained listener to hear two separate parts. Healio's review calls this a "physiologic split" of S2, using "physiologic" in the sense of a normal body process rather than a sign of illness [3]. Doctors learn to listen for this split with a stethoscope, because other kinds of split sounds can point to problems. This article only introduces the normal kind and cannot be used to judge anyone's health.
5. Do the Valves Themselves Make the Sound? Explanations Differ
"The sound of valves closing" is how many textbooks put it, but it does not settle exactly what vibrates. The StatPearls overview from the US National Library of Medicine says heart sounds are created by blood flowing through the heart chambers as the valves open and close, and that vibrations of these structures from the blood flow create audible sound [4]. In that wording, the valves, the surrounding heart structures, and the blood are all part of the picture.
What the sources agree on is the timing. Lub lines up with the closing of the valves between the chambers, and dub with the closing of the exit valves [2][3]. Which part of the system vibrates most is a more detailed question, and the sources used here do not resolve it. This article leaves it open.
6. How a Rolled-Up Piece of Paper Led to the Stethoscope
For centuries, the only way to hear inside a person's chest was to press an ear against it. In 1816 the young French doctor René Laennec rolled a piece of paper into a tube and found that the sound came through easily and clearly. Soon afterward he carved a tube from wood. This was the first stethoscope [5].
In 1819 he published a book, De l'auscultation médiate ("On mediated auscultation"), describing the tool and what he heard. At first the book and the device were received coolly, but by the time the second edition came out in 1826, use of the stethoscope had become accepted [5]. Because a stethoscope let doctors compare sounds in a more regular way, descriptions like "lub" and "dub" became easier to tie to what was happening inside the heart.
7. Try Listening for Lub-Dub With a Paper Tube
You can copy the idea behind Laennec's paper tube. Roll a sheet of stiff paper into a tube, sit somewhere quiet, and ask a family member to sit still. Rest one end of the tube gently on their chest, on the left side, and put your ear to the other end. Do not press hard. See whether you can count the sounds in pairs, and compare with listening through the shirt with your bare ear. If you hear nothing clear, that is fine, since results change from person to person and place to place. This is just a game. It cannot tell you anything about someone's health.
If you want to read more, search for the MSD Manual's article on cardiac auscultation, or look up Laennec's 1819 book, which is held in libraries and described by the University of Pittsburgh page below.