Because there’s no gravity to pull the blood back down from their head. On earth, your heart works hard to pump blood up to your head, but in space to doesn’t have to fight against gravity, so blood that gets pumped to your head stays there. Which is why your face goes red and puffy if you’re upside down for extended periods of time, gravity is keeping the blood in your head.
It’s also why astronauts often have constantly blocked noses and sinus pressure, since gravity isn’t there to drain the mucus like it usually would.
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