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While a person is standing erect in 1-G
environment, gravity forces blood and body fluids toward the feet.
Natural muscular tension in the legs tends to pump body fluids to the upper body.
After moving from 1-G to 0-G, the gravity force is absent but the muscular
tension force remains continuing to exert a force which moves the fluids toward
the head. This effect is called the "fluid shift".
A graphic example of the physical change
resulting from the fluid shift is astronaut Jack Lousma's face before and during
flight. Eyes become bloodshot and the face swells as fluid is forced to his
The swelling, or edema, decreases somewhat after three days but is
noticeable during the entire period of weightlessness.