Spending time in zero gravity can reverse the flow of a person’s blood and lead to clots, as seen in astronauts who spent months on the International Space Station
Source:
https://www.newscientist.com/article/2223705-zero-gravity-made-some-astronauts-blood-flow-backwards/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home