The crew members may feel disoriented or nauseous during and after the flights. The astronauts really use it to train. This astronaut is working out on the Space Shuttle. Astronauts have to exercise 2 hours a day in space. They lose muscle mass and their organs change shape.
Astronauts have to exercise at least two hours every day in space.
Their bones demineralize. This means not only do they lose calcium, they lose other vital components. They are exposed to severe radiation from the sun, because they are not protected from ultraviolet rays by the earth's ozone layer. Their red blood cell count drops, but then stabilizes and actually rises after a certain amount of time, which is different for each individual. They lose about one liter of fluids in space, but they do not feel dehydrated until they return to earth, because they do not sweat as much in space.