Why does your heart beat in two thumps?
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Why 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.
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
Many people think the heart pushes blood around just by squeezing hard, and that the valves are minor extra parts. But if you hold one valve open while the heart squeezes just as hard, the blood simply squirts backward through the gap and sloshes in place instead of completing the loop. The slamming valves are what force blood to move one way; the squeeze alone would only push it back and forth.
The catch
Slamming one-way doors give you an efficient pump where every squeeze moves blood forward — that is the win. The catch is that valves are moving flaps that can wear out, stiffen, or stop sealing over a lifetime. A valve that leaks lets a little blood trickle backward, and that turbulent flow makes an extra hiss a doctor can hear, called a heart murmur.
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.
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.