Why does your heart beat in two thumps?
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After you watchWhy does your heart beat in two thumps?
The short answer
Your heart beats in two thumps because two different sets of one-way valves slam shut, one just after the other. The 'lub' is the valves between the heart's upper and lower rooms closing, and the 'dub' is the valves at the exits to the big arteries closing. The thump is the sound of the doors slamming, not the muscle squeezing.
Try this next
- What changes when the outlet can close again? After the first model run, let the left outlet close and run the same cycle. Compare the arrows at each phase.
- Does faster playback change the order? Change the model playback speed. Predict whether the two closure pairs change order, then replay. Playback speed is not a clinical heart-rate simulation.
Now you — bend it
- What if Let the left outlet close and replay the cycle.Compare the forward and backward arrows at matching phases.
- What if Change the model playback speed.Check whether the inlet and outlet pairs still close in the same order.
Can you prove it?Four valves form two closing pairs during a normal cycle. — Run the model in calm mode and name the valves that close at each sound.
Design your own test:Predict how the same squeeze-and-relax cycle changes when the outlet is held open.
Explain it to a 6-year-old: Two pairs of little valves close at different times, making the main lub-dub heart sounds.
The whole story
How it works
The heart is a muscle pump with four rooms, and between the rooms and exits are flap-like valves that only open one way. When the lower chambers squeeze, the valves behind the blood snap shut so blood can only go forward — that slam is the 'lub' (called S1). A moment later, when the chambers relax, the valves at the artery exits snap shut so blood can't fall back in — that second slam is the 'dub' (called S2). Two sets of doors closing in turn make the two thumps you hear and feel.
What people get wrong
A single drop does not travel through all four valves during one heartbeat. The right and left ventricles pump together into different circulations. Two inlet valves close near the start of contraction, followed by two outlet valves near the start of relaxation. A valve leak can reduce effective forward flow without stopping all circulation.
The catch
Valves guide efficient forward flow. A valve that does not seal can allow backflow. Turbulent flow can produce a murmur, but not every murmur means a valve is damaged.
Questions kids ask
What exactly makes the lub-dub sound?
It is heart valves slamming shut, not the muscle squeezing. The 'lub' is the valves between the heart's rooms closing, and the 'dub' is the valves at the artery exits closing a moment later. The blood suddenly stops against the closed flaps, and that makes the thump.
Why can't the heart just squeeze the blood around without valves?
A squeeze pushes blood in every direction it can go. Without a valve slamming shut behind it, some blood squirts straight back into the room it came from. The one-way valves block the backward path, so each squeeze can only send blood forward.
What is a heart murmur?
A murmur is an extra whooshing sound a doctor hears between the normal thumps. It often happens when a valve does not seal perfectly and lets a little blood leak backward, making the flow turbulent. Many murmurs are harmless, but some need a doctor to check the valve.
Why is the gap between 'lub' and 'dub' shorter than between beats?
The 'lub' and 'dub' happen during one beat: lub when the chambers start squeezing and dub a fraction of a second later when they relax. The longer pause is the resting gap before the next beat begins, so the two thumps come close together and then there is a wait.
Talk about it
- Put your hand on your chest — guess how many times you think it thumps in one minute, then we'll count and see.
- If a heart valve is a one-way door, what do you think happens to the blood when the door won't shut all the way?
- Why do you think your heart beats faster after running but you never decided to make it?
For grown-ups
The heart sounds are valves closing, not the myocardium contracting. 'Lub' (S1) is the atrioventricular valves (mitral and tricuspid) snapping shut at the start of ventricular systole; 'dub' (S2) is the semilunar valves (aortic and pulmonary) snapping shut at the start of diastole. The sound comes from the abrupt deceleration of the blood column against the closed leaflets. A valve that fails to seal allows regurgitation, and that turbulent backflow is the whoosh of a heart murmur.
Keep going
What else makes you wonder?
- Why does your heart speed up when you run, but you never have to tell it to?
- If valves can wear out, how does a heart keep slamming its doors billions of times without breaking?
- Animals come in all sizes — does a tiny mouse heart thump faster or slower than a big elephant's?
