Why does zero gravity cause problems for astronauts?

Answer 1

Astronauts on long spaceflights in zero-gravity might experience problems
with loss of calcium in their bones, loss of muscle mass and muscle tone all
over the body including the heart. There has been a lot experience with
this already with astronauts living on the Mir space station for many
months. Some things can be done to reduce these effects including exercise.

Answer 2

Almost everything they have ever become accustomed to is no longer true. Nothing falls, it just floats. There is no down nor up. If you want to walk up the wall or across the ceiling you can just do it. If you jump up there is nothing to pull you back down. Drinking through a straw would be a problem because without the gravity when you create a vacuum on the straw all of the liquid would try to come at once since there is no gravity holding it back. We are not even going to get into the personal problems they have when they go to the bathroom.

Answer 3

Gravity provides a constant pull on the body. It is this pull that constantly makes our muscles work against it. Our bodies are accustomed to this and when it is not there the muscles relax. The muscles don't pull on bones and this causes a reduction in the blood flow withing the bones where blood is made.. Thats as simple as I can explain it.