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. Valve closure sets the blood and surrounding structures vibrating, making a heart sound.
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
Watch the two sides pump together
Run one cycle. The right side sends blood to the lungs; the left sends blood to the body. The two inlet valves close together, then the two outlet valves close together: two sounds from four valves. This is a simplified pump diagram, not the shape of a real heart.
Ready to fill, squeeze, then relax.
4Now test a leaky outlet
When would the blood flow backward?
The left pump's outlet valve will stay open. The muscle still squeezes and relaxes. Predict when the backward flow will appear, then follow the arrows.
Guess before you find out
The outlet to the body cannot close. During which phase will blood leak back from the artery into the left pump?
Follow the left outlet. Left outlet held open
Run the same filling, squeezing and relaxing cycle.