Spaceflight and long-distance swimming shrink the heart

What do extreme long-distance swimming and spaceflight have in common? They can both shrink the heart, according to a new study.

Both activities reduce the pressure of gravity on the heart, making it so that it doesn’t have to work as hard to pump blood upwards through the body. The heart is a muscle, and just like any other muscle in the body, if it’s not used as much as it used to be, it will shrink.