system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is astronaut Scott Kelly’s body suffering severe health issues after 300+ days in space?

He is suffering joint pain, swelling, rashes, vision problems, etc.
gpt
Low gravity causes problems with our bodies since they were built around an environment where there are certain pressures places on our body from living on Earth. Since he is in space with none of these pressures (namely gravity) his body becomes weaker, his blood flow is effected, pressure on the eyes is effected, etc.

ETA:  We just aren't built to be in zero g for long periods of time.  Its one of the problems that prevent us from going to Mars right now is that it would take a year for a round trip.  Thats really hard on our bodies and we haven't come up with a way to counter the effects yet.