1Two things to know first
A squishy pump, and doors that only open one way
Your heart is a muscle that squeezes to push blood. And between its rooms are little flaps called valves — doors that swing open one way, then slam shut so nothing can sneak back.
The squeeze
The heart tightens and lets go, over and over, shoving blood ahead of it like a fist around a water balloon.
The one-way door
A valve flops open when blood pushes forward, then snaps shut when blood tries to slide back. That snap is a sound you can hear.
2So a door can do two things
Snaps shut, or stuck open
Here are the two cases at the heart of the whole story. Same blood, same squeeze — the only difference is whether the door behind it slams.
Blood goes forward
The door slams behind the blood, so the only way left is ahead.
Blood sloshes back
No slam means the door's still open — blood squirts back where it came from.
3Your turn — pump the heart
Send one drop of blood around the loop
Tap Squeeze and watch a drop travel through all four doors and out to the body. Each time a door slams behind it, you get a thump. Count them.
Ready. Give it a squeeze!
4Now hold a door open
What if a valve never slams?
The heart still squeezes just as hard. But this time you'll jam one door open so it can't snap shut. Then pump — and watch the drop.
Guess before you find out
You hold one heart valve open instead of letting it snap shut. The squeeze is just as strong. Does the blood still go all the way around to the body?
Tap a glowing door to jam it open or let it snap shut. Right now: all doors snap shut
Pick a door, then squeeze.
5So why two thumps, not one?
Two sets of doors, slamming in turn
The lub is the doors between your heart's rooms slamming. A moment later the dub is the doors to the big arteries slamming. Two slams = two thumps.
Valves are moving flaps. They can get stiff or stop sealing, and a leaky one lets a little blood sneak back.
The two thumps are two sets of one-way doors slamming shut, one after the other — that's what keeps your blood always moving forward instead of sloshing back.
Psst, grown-ups: the heartbeat sounds are valves closing, not the muscle contracting. "Lub" (S1) is the atrioventricular valves (mitral and tricuspid) snapping shut as the ventricles contract; "dub" (S2) is the semilunar valves (aortic and pulmonary) snapping shut as the ventricles relax. The sound is the abrupt stop of the blood column against the closed leaflets. A valve that fails to seal lets blood regurgitate backward, and that turbulent flow is the whoosh of a heart murmur.