ELI5: starling's law

Starling's Law means that the more your heart fills up with blood and stretches, the harder it snaps back to pump that blood out to your body.

Think of your heart like a simple rubber band.

If you pull a rubber band back just a tiny bit and let go, it snaps forward with very little power. But if you stretch that rubber band way back, it snaps forward with a huge, powerful smack. Your heart muscle works the exact same way.

Here is how you can see this happening in everyday life:

• Playing outside: When you sit on the couch watching cartoons, your body is resting. Your heart only needs a little bit of blood, so it barely stretches and beats gently. But when you jump up to play tag, your running legs push lots of blood back to your heart.
• The water balloon effect: Imagine filling a water balloon under a faucet. When your heart gets flooded with all that extra blood from running, its walls stretch out wide, just like the rubber of the balloon filling with water.
• The big squeeze: Because your heart is now stretched like that pulled rubber band, Starling's Law kicks in. The heart snaps shut with extra force, blasting a huge wave of blood to your tired legs so you can keep running without fainting.