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How does spending prolonged time
in microgravity affect the bodies of
astronauts?
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Jeffrey Sutton, director of the National Space Biomedical Research Institute and
Nitza Cintrn, chief of NASA's Space Medicine and Health Care Systems Office,
explain.
Space is a harsh environment that affects the body in many ways. In microgravity,
bone loss occurs at a rate of 1 to 1.5 percent a month, leading to an acceleration of
age-related changes similar to osteoporosis. Decreases in bone density and
strength are more pronounced in some skeletal regions, such as the pelvis,
although much of the loss is reversible upon return to Earth. Prolonged exposure
to weightlessness also increases the risks of kidney stones and bone fractures,
which are both associated with bone demineralization. In addition, studies
suggest that microgravity alters the ability of bones to heal after fractures.
Long stays in space also impact muscles. There is loss of muscle mass, strength
and endurance, especially in the lower extremities. Changes in muscle
performance, coupled with the effects of microgravity on connective tissues and
the demands of activities of varying intensities, place astronauts at risk of fatigue
and injury.
The heart is a unique muscle, and diminished cardiac function and the possible
occurrence of heart rhythm disturbances are concerns faced during space flight.
The details of these cardiovascular changes and risks are not yet completely
known, however. In microgravity, body fluids are redistributed away from the
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extremities, which results in puffiness in the face during flight as well as changes
in cardiovascular physiology. Upon return to Earth, some astronauts experience
impaired orthostatic response, which means that their blood pressure drops
abnormally low when they move from lying down to a sitting or standing
position.
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Similarly, microgravity also impacts the
neurovestibular system--an integrated set of neural sensory, motor and brain
circuits that allows humans to maintain balance, stabilize vision and understand
body orientation in terms of location and direction. Exposure to microgravity
often leads to disorientation and decreased neuromuscular coordination upon
return from prolonged missions. Immediately after landing, astronauts may have
problems standing up, stabilizing their gaze, walking and turning.
In space, circadian rhythms are disrupted because the 24-hour day/night cycle is
absent. Sleep loss, stress related to workload, high performance expectations,
and psychosocial factors all affect the body on long-duration missions. The body
also suffers loss of blood volume, immunodeficiency, and transient post-flight
anemia (low red blood cell levels), despite adequate nutritional intake.
Space radiation is rich in heavy ions and poses one of the greatest risks to
humans on prolonged missions. Radiation can induce cataracts and cancer, as
well as adversely affect many physiological processes.
As space missions grow longer, much remains to be learned about just how the
space environment alters the body and how complex physiological and
psychological changes vary with mission length.
The reader also inquired as to what doctors recommend astronauts do to
counteract these effects.
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Astronauts and crews are carefully selected, trained, and monitored to minimize
risks and to promote health and safety. Improvements are constantly being made
to spacecrafts, in-flight medical care capabilities and countermeasure protocols,
as well as post-flight rehabilitation. During prolonged missions, exercise is
effective at minimizing large muscle atrophy, and astronauts use a cycle
ergometer and treadmill with downward applied pressure to maintain fitness.
Certain tasks, such as extravehicular activities, are not routinely performed until
bodily fluid redistribution stabilizes and astronauts have an opportunity to
acclimatize to space. Prior to re-entry to gravity, increasing fluid intake is helpful
in minimizing the effects of orthostatic hypotension.
Physicians and researchers for NASA and the National Space Biomedical
Research Institute are pursuing ways to develop and validate medical
technologies and risk management strategies for long-duration space flight.
These groups want to advance in-flight diagnostic and therapeutic capabilities for
crewmembers commensurate with current standards of health care delivery.
They are also pursuing ways to improve the ability to remain in space for long
periods by finding novel ways to decrease microgravity's ill effects.
Different lighting intensities and wavelengths are being implemented and studied
further to entrain the circadian cycle of astronauts and to shift the sleep-wake
schedules of crews in preparation for critical events, such as dockings between
spacecraft. Special shielding on spacecraft, including the International Space
Station, helps to protect against the harmful effects of space radiation. In
addition, missions are adapted when radiation exposure is particularly high, such
as during solar flares.
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Medications have proven
effective in treating space
motion sickness and
orthostatic hypotension, and some are potentially useful in reducing bone loss.
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Researchers continue to look for new ways to counteract the physical changes
associated with long-term space flight whether through diet, exercise, medication
or a combination of strategies.
Answer originally published October 6, 2003.
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