U.S. researchers say they\'ve identified low or zero gravity as the cause of low blood pressure experienced by astronauts returning to Earth from space. The condition, known as orthostatic hypotension, occurs in up to half of those astronauts on short-term missions of two weeks or less and in nearly all astronauts after long-term missions of four to six months, researchers writing in the journal of the Federation of American Societies for Experimental Biology report. The scientists conducting the study say low gravity compromises the ability of arteries and veins to constrict normally, inhibiting the proper flow of blood. Michael D. Delp of the University of Florida and colleagues examined and compared arteries and veins from mice housed at Kennedy Space Center in Florida with blood vessels from groups of mice flown on three of the last five space shuttle missions. The mice that returned from space experienced the equivalent of orthostatic hypotension in humans, researchers said, noting it takes as many as four days in normal gravity before the condition is reversed. \"There has been considerable interest in sending humans to the moon, asteroids, and Mars,\" Gerald Weissmann, editor in chief of the FASEB Journal, said, \"but what we\'re finding is that extended space missions have their own inherent risks above and beyond the obvious. \"If we ever hope to visit distant worlds for extended periods of time -- or colonize them permanently -- we\'ve got to figure out how to mitigate the effects that low and no gravity has on the body.\"