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In orbit there is no force pulling fluid in the body toward the feet, so it tends to shift "upward" toward the head. That's what gives astronauts the puffy looking faces while they're in orbit. Some are bothered by this, and complain of stuffy sinuses or headaches; others don't notice it until they look at the pictures later.
There are some physiological effects that are not important on the short (week-long) Space Shuttle flights, but which could be important on longer Space Station or interplanetary flights. For example, scientists learned from earlier space projects that astronauts gradually lose calcium from their bones while they're in orbit. This is largely a result of the fact that there is no stress on the bones, and stress is an important stimulant to calcium production. This is not important for astronauts aboard the shuttle, because we're not in orbit long enough to lose a significant amount of calcium. Even on an extended flight, the bone loss would not be important to the astronaut while he or she is weightless. This is, after all, one of the body's ways of adapting to weightlessness. It only becomes a problem when the astronaut who has been weightless for a long time (many months of years) returns to Earth and once again needs strong bones. So far as we know, this condition is reversible--that is, upon return to Earth, the calcium in the bones returns to normal levels. But this problem requires considerable further study, and preferably also an