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It's because most of the blood return to your heart is passive. The heart pumps OUT through the arteries pretty hard, but the blood finds it's way back through the veins whenever it feels like it, it just can only go one way because the veins have valves along the way that prevent backflow. A lot of the pressure that actually gets the blood back comes from the contraction of the nearby muscles squeezing the blood up past the valves. So especially if you're standing with your legs all the way down there from your heart, most of the work to get the blood back up is going to come from your calves and thighs flexing.
My A&P teacher very cheerfully illustrated the point by telling us there's actually one animal that does have valves in it's arteries.
spoiler
It's the giraffe! It has valves in the artery going up it's neck to help keep up enough pressure to get the blood all the way up to it's head!We had a guy at one of my old jobs who was trying to get a not guilty by reason of insanity charge because he was facing a life sentence for something he didn't want to spend that time in prison for (the only time it's worth it). Unlike most guys however, instead of faking, he actually drove himself insane! He was actually fine coming in, just had some (dubious) suicidal ideation but perfectly cognitively intact. But the doctor wasn't buying the suicidal thing after a week or two so he started staying awake for weeks on end then slept for weeks on end, soiled himself constantly, refused to eat then binge ate. Just went absolutely feral until he really was.
One of the things he did for a while was refused to lie down at all. Just stood completely still in the hallway staring at the wall. Did it for weeks. Started to look like a candle melting down into his feet until they began to split open and weep interstitial fluid. Anyway the point is we started having to chase him around the unit a little. You'd go just stand next to him and bug him a bit until you got him to walk a little because we needed to start stimulating some blood return. I forget how that story ended he may have still been there when I finally had it with that place and left.
Bro, I thought you were describing your co-worker!