So basically your circulatory system is evolved to work in a gravity field. Without gravity, blood doesn’t circulate properly, and the further a part of your body is from your heart the worse the problem is. Your heart doesn’t have to work as hard to pump against gravity… so it doesn’t.
As the human body consists mostly of fluids, gravity tends to force them into the lower half of the body, and our bodies have many systems to balance this situation. When released from the pull of gravity, these systems continue to work, causing a general redistribution of fluids into the upper half of the body.
Without gravity pulling down on your body, your legs don’t have to work to keep you upright. Your muscles atrophy, and after extended periods in low gravity the tough tissue on the bottom of your foot falls off.
In a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up. Those muscles then start to weaken and eventually get smaller. Consequently, some muscles atrophy rapidly, and without regular exercise astronauts can lose up to 20% of their muscle mass in just 5 to 11 days.
You lose bone mass.
Due to microgravity and the decreased load on the bones, there is a rapid increase in bone loss, from 3% cortical bone loss per decade to about 1% every month the body is exposed to microgravity, for an otherwise healthy adult. The rapid change in bone density is dramatic, making bones frail and resulting in symptoms that resemble those of osteoporosis.
And… the shape of your eyeball changes… because it’s supposed to be in gravity.
[a] NASA survey of 300 male and female astronauts, about 23 percent of short-flight and 49 percent of long-flight astronauts said they had experienced problems with both near and distance vision during their missions. Again, for some people vision problems persisted for years afterward.
And all of that doesn’t start to address the problems created by radiation exposure due to being outside the protection of the magnetic field and the atmosphere - the increased cancer rates, the weakened immune system…
There’s more detail in this Wikipedia article: https://en.wikipedia.org/wiki/Effect_of_spaceflight_on_the_human_body
Long-term life in space is functionally a fantasy right now. Astronauts who spend months in space sacrifice their health. To spend years in space would shorten your lifespan substantially.
You can leave the gravity well, but… you can’t escape your evolution.
On paper. No one’s actually built a rotating space habitat to simulate gravity with centripetal force (yet).
We are so, so far away from this… biology is one of those fields where the more we learn, the more questions come up. We barely have a surface grasp of some of the mechanisms of the brain. Blood gets more complicated with every study. We’ve mapped the genome but we have little understanding of what most of it actually does. And the link between gut bacteria and neurological health… we kind of just know that it exists, and not much beyond that.