WEDNESDAY, Jan. 30, 2002 (HealthDayNews) -- When astronauts return to terra firma after weeks in space, they don't need a throat-clogging pretzel to feel a little woozy. Many faint on the spot when they try to stand up for the first time back on Earth.
The same thing happens to bedridden hospital patients who rise after being horizontal for days.
Now, after an ambitious experiment aboard the Space Shuttle, researchers think they have a better idea of why astronauts -- and the sick -- hit the deck.
They suspect some astronauts' circulatory systems have trouble regulating the flow of blood after weightlessness. Under the stress of standing, their systems can't prevent blood from heading south due to gravity, and the brain has no choice but to shut off, says Dr. Benjamin Levine, professor of internal medicine at the University of Texas Southwestern Medical Center in Dallas.
The research is far from trivial. For men and women to travel to other planets, like Mars, "we have to understand what happens to the body," Levine says.
The problem of fainting faces many astronauts after they return to Earth. As many as 60 percent can't stand for more than 10 minutes during the first few weeks following their return, Levine says.
Smelling salts aren't needed to revive the astronauts, however. They regain consciousness immediately upon hitting the ground.
"When they fall, the blood pressure is restored and they wake up," says Dr. Nosratola Vaziri, professor of medicine at the University of California at Irvine.
Fainting "may be a defense response," he says, "because the brain doesn't get sufficient blood supply. You don't want to have that condition for long because it could damage the brain."
The experience is similar to that of a person waking up in the morning and getting out of bed, Levine says.
"Many people feel a little dizzy. That's what happens [after] space flight, but it's so much worse," he says.
Researchers think the bedridden suffer from the same fainting condition.
Usually an astronaut's fainting spells vanish after a few weeks or months; the recovery process takes more time if an astronaut has been in space longer.
Researchers tried to study fainting after flight by making human subjects lie in a bed for weeks at a time. But, in 1998, a new opportunity arose during the so-called Neurolab Space Shuttle mission, whose goal was to study how space affects the body.
University of Texas scientists studied six male astronauts who spent 16 days in space. Their findings appear in the January issue of the Journal of Physiology.
When a person stands, his or her blood vessels should constrict and raise blood pressure, says Levine, study co-author. The process is much like putting a finger over the end of a garden hose to increase the pressure of the water coming out, he says.
However, the circulatory systems of some astronauts studied appeared to be unable to constrict blood vessels properly after prolonged weightlessness, Levine says. The space travelers' blood pressure dropped, and they fainted. Their bodies may have individual limits, he says.
According to Levine, astronauts who suffer from fainting problems upon standing need to get up slowly, and perhaps drink salt solutions to replenish bodily fluids.
Vaziri has also studied the astronaut fainting problem. He found that weightlessness increases the level of nitric oxide in the body, and that lowers the volume of blood and sends blood pressure downward.
Vaziri says he agrees with the general findings of the University of Texas study, but adds it's important for astronauts to take medication to prevent problems from the buildup of nitric oxide.
What To Do: Why do people faint? Learn the facts from the National Library of Medicine. Learn about how space flight affects the heart in this NASA fact sheet.
