Do you care about the sound of Sen. Jake Garn's gut as he hurtles weightless through space?

NASA does.

Eight and a half minutes after liftoff Friday, a tape recorder and two stethoscopes strapped to Garn's belly began recording his bowel sounds for what NASA's press kit calls a "gastric motility" experiment.

What goes on in Garn's weightless intestines aboard the space shuttle Discovery, NASA scientists say, may provide clues to the causes of space motion sickness -- the most common health complaint of the nearly 200 humans who have ventured into space.

The bowel-sounds recording is only one of a battery of tests on Garn, the first senator to escape gravity. The Utah Republican was listed officially by NASA as "payload specialist" -- the payload consisting of instruments to test his adaptation to weightlessness.

They measured his heart rate, his brain waves, his facial color, his height and girth, his fluid loss and his blood pressure. They evaluated his eye-hand coordination, tested his saliva, photographed the pupils of his eyes.

"He's wired for sound for most of the flight, especially during takeoff and landing," said a NASA spokesman last week on the eve of the shuttle's mission.

It's easy to make fun of experiments that record, for example, a senator's bowel sounds in outer space, but to NASA scientists the tests make perfect sense.

"It's a unique laboratory," says Dr. Arnauld Nicogossian, director of NASA's life sciences division. "By studying the physiology of adaptation to weightlessness in space, we can learn how the body works on the ground."

The shuttle mission's tests on Garn, Nicogossian says, could lead to a better understanding of earthly afflictions from heart disease to varicose veins, from osteoporosis (the weakening of bones with age) to motion sickness.

"We understand so little about space sickness that we don't know how to prevent it," says Joseph Sharp, deputy director of life sciences at NASA's Ames Research Center in Mountain View, Calif. Information is limited because so few humans have spent time without gravity.

"We take every opportunity to study these things," Sharp says, "and Sen. Garn is an opportunity."

Weightlessness -- or zero-G, as it's called by scientists -- is a condition that cannot be duplicated on Earth. Zero-G has profound effects on the human body, which over the eons has evolved ingenious ways of countering the force of gravity.

Weightlessness disrupts every bodily system and makes nearly half the astronauts sick to their stomachs. But NASA experiments since the beginning of the space program have shown that the human body makes heroic attempts to adapt to weightlessness.

In the early days of space travel, says Dr. Charles E. Berry, former medical director for NASA, "There was lots of concern, lots of fear about what would happen to a man in space: He was never going to be able to sleep. He was always going to be asleep. He was never going to be able to urinate. He was always going to be urinating."

Twenty-five years after the first crude experiments, Berry says his main reaction is one of awe at the human body's "tremendous ability to adapt to zero-G.

"It does that very, very well," says Berry, who now heads a medical consulting firm in Houston. "Then the difficulty is, you've got to come back and adapt to gravity again."

The human cardiovascular system has developed an extraordinary ability to circulate blood throughout the body against the force of gravity, which tends to pull blood continuously down to the legs and feet.

But in the weightlessness of space, the cardiovascular system keeps working as if gravity were in force. More blood than necessary is pumped up toward the chest, neck and head. That's why astronauts in flight usually look puffy in the face.

"The first sensation an astronaut feels when he becomes weightless," Berry says, "is he feels like he has extra blood in his head."

With the blood shifting toward the upper body, the brain senses an oversupply and sends a hormone to signal the kidneys get rid of more fluids. Most astronauts tend to lose body fluid during a space flight.

"The body is acting 'normally' in a very abnormal environment," Sharp says.

Though the body adapts remarkably well to weightlessness, usually within a couple of days, an even more difficult adjustment comes with reentry.

"You're in a state of cardiovascular deconditioning," Sharp says. "The heart hasn't had to work very hard or fight gravity for several days."

"When you come back to Earth again, suddenly you're in a gravity environment," Berry says. "If you stand up, all the blood goes back down to your legs."

He recalls astronaut Gordon Cooper's landing after a 36-hour orbital flight in 1963.

"Gordo, when he stood up on the carrier deck, nearly fainted."

In an attempt to study on Earth what happens to the heart in space, NASA sometimes simulates a gravity-free environment by putting test subjects to bed for a week or longer. That minimizes the heart's pumping duties and causes a cardiovascular deconditioning similar to that in space travelers.

Such experiments, say Berry and Sharp, have altered treatment for heart attack.

Heart attack patients used to spend six weeks in the hospital flat on their backs. Today, with more sophisticated monitoring, such patients are encouraged to begin physical rehabilitation much sooner. Most surgical patients also sit up and walk around within a day after their operation.

"We've learned that bedrest is not always a good thing," Berry says.

Space experiments also may offer some insight into osteoporosis, the mineral loss that makes the bones brittle with normal aging, particularly in women. That process appears to be accelerated at zero-G.

"Take gravity away, and the body senses it doesn't need a skeletal system anymore," Sharp says. "It starts to get rid of calcium and potassium in the bones."

Animal studies suggest that the effect of weightlessness on bones is similar to the process of aging. But it is not known how long the process continues, or whether the body can learn to adapt and curb it.

"We suspect it goes on forever, but we just don't know," Sharp says.

On short flights such as Garn's shuttle mission, the bone-weakening is a trivial problem, but it could be serious on, say, a 600-day voyage to Mars and back.

Exercise during flight is vital to help offset the atrophy of the muscles and bones and to keep the heart from relaxing too much at zero-G.

But why monitor the sound of Sen. Garn's bowel?

There's some evidence that the same phenomenon that upsets the body's sense of balance also affects the gut, Sharp says.

"One thing we noticed was that for reasons nobody understands, whenever an astronaut got sick, the bowel peristalsis seemed to stop," he says.

Peristalsis is the wave-like motion of the intestines that helps move material through the digestive system. It makes a rumbling sound audible with a stethoscope, but some NASA studies suggest that it slows or stops during the worst nausea of space sickness.

"We want to understand this so we can prevent it," Sharp says.

Letting a 52-year-old senator, even a physicially fit former Navy pilot, up in space is part of NASA's effort to move beyond the "Right Stuff" super-specimens of the original seven astronauts and broaden the eligibility for space travel.

"Our ultimate goal is to open it up like airline travel," says Sharp. "To do that we need to understand more" about the body.

He noted that in the early days of commercial airline flight, there was so much concern about health effects that the stewardesses were all registered nurses.

The next shuttle flight, scheduled for April 29, will prepare for more detailed experiments on rats and monkeys that for ethical reasons can't be done on humans.

One of the challenges of that mission, Sharp says, is to work out ways of taking care of animals during space flight.

"How do you take care of the biological waste from an animal?" he says. "It's tricky at zero-G."