system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so comfortable to put ones feet up?
gpt
Normally your heart must work to pump your blood back up your legs all the way from the bottoms of your feet, lifting it as much as four or five feet upwards if you are standing up. When you put your feet up, and lean back in your chair, your body's circulatory system is much closer to all being on one level plane, and your heart only has to pump the blood back and forth rather than up and down, which is much much easier. Try lifting a large cup of water off of the floor up to your mouth and then putting it down again and then lifting it back up again over and over continuously and see how fast your arms and legs get tired. Basically, when you put your feet up you're reducing the amount of work your heart, the hardest working muscle in your body, has to do, and giving it a nice rest.