We evolved to fight gravity: if we want to avoid flopping onto the ground like beached jellyfish, we need our strong bones, our muscles, our sense of balance, and a whole variety of systems in our body that depend on always knowing which way is down.
NASA already knows that muscles weaken in space — but what does lack of gravity do the our hearts? Read more at: NASA – The Beating Heart, Minus Gravity.
I’ve been a NASA fan since I was a wee tiny geek, using a flashlight after bedtime to study the solar-system facts printed on my trashcan. So I was thrilled at a con recently, when retired NASA astronaut Storey Musgrave bought a book from me! Let me see if I can find the photo.