Shuttle mission yields surprising results.

Biologists may have to rethink traditional theories about how the human body functions in light of new results from experiments conducted on shuttle astronauts during last June's Spacelab Life Sciences-1 mission, scientists announced last week.

"It is now clear that we are obtaining a significant number of surprising results from this mission," says Ronald J. White, chief scientist in NASA's life science division in Washington, D.C. "The ideas the investigators had prior to the space flight about how the body actually would work in space were either incomplete or incorrect."

Some of the most important findings relate to the detailed workings of the cardiovascular system. "One of the major concerns regarding the body's adaptation to space flight is that the adjustments the cardiovascular system makes during flight to maintain blood pressure in weightlessness may compromise its ability to readjust to gravity upon return,' says David R. Pendergast, a physiologist at the State University of New York at Buffalo.

In a study presented July 24 at an American Heart Association meeting in Chicago, Pendergast and his colleagues found that cardiac output -- the volume of blood pumped by the heart -- increased 50 percent and remained elevated throughout the shuttle flight, even though heart rate decreased and mean blood pressure remained constant. "This type of regulation of blood pressure was completely unexpected," Pendergast says.

On the ground, cardiac output rises temporarily every time a person lies down. The physical effects of weightlessness resemble those of prolonged bed rest, so astronauts may feel weak or have difficulty standing for several days after landing. According to Pendergast, models for weightlessness based on bed rest studies redicted an initial increase in cardiac output, followed by a decrease to normal levels within one to three days. Calculations of other cardiovascular variables showed that in space, unlike on Earth, the body maintains blood pressure by engorging organs with blood -- a condition called hyperfusion.

While this condition does not affect people's ability to perform in space, it may contribute to their difficulties upon return, says Pendergast. The researchers quantified the physical extent of these difficulties by comparing three astronauts' ability to exercise before, during, and after the flight. They found that the cardiovascular system remained compromised for up to seven days after landing.

Unexpected results from a related experiment were announced at a NASA briefing the same day. Researchers from the University of Texas Southwestern Medical Center at Dallas monitored an astronaut's central venous pressure, a measurement of the filling pressure of the heart (SN: 10/5/91, p.220). They had expected the pressure to increase slowly as weightlessness released blood formerly pooled in the lower body but instead they observed a sudden, dramatic decrease in pressure that lasted for the entire nine-day flight. "This was an enormous surprise for us," says Lynda D. Lane, senior research scientist on the project. She says the study underscores how much scientists still need to learn about the cardiovascular system.

Earthbound studies of the cardiovascular system are complicated by gravity, which exerts different pressures depending on whether a person is standing up or sitting down, says Lane. She looks forward to more shuttle-based studies "free from the aggravation of gravity"

Both research groups will continue their investigations in the next Spacelab Life Sciences mission, scheduled to fly next summer.