Space travelers experience many other changes in their bodies. One of the first and most noticeable is shrinking of the legs and swelling of the face, as fluids--freed from gravity's pull--redistribute more evenly throughout the body tissues. Each leg loses about a liter of water in the first day, and the legs remain smaller throughout the space flight. The collection of fluid in the head produces a perpetual case of 'space sniffles' that abates only during strenuous exercise. The redistribution of fluid has effects that are more serious, one of which is a form of anemia unique to space travelers. The loss of fluid from the bloodstream to the tissues creates an overabundance of red blood cells. In response, the body produces fewer and destroys more. Astronauts feel the loss when they return to Earth and must work against gravity again.
Exercise is essential to the health and well-being of women and men working in space. Research has shown that astronauts lose bone and muscle mass during their flights. The loss of bone raises calcium levels in the blood, which may lead to kidney stones. NASA planners think resistance exercise using elastic bands should reduce such effects, but whether they can be prevented entirely is unknown. Our experience so far in space shows that all body systems except the skeletal and muscular return to normal after astronauts return to Earth. So far, evidence suggests that human beings can live and work safely in space for long periods, but so far 'long' had meant 'months.' How might humans cope with trips to the other planets that require several years? No one knows.
'If I have seen a little further it is by standing on the shoulders of Giants.'