Soon after entry into space, physical changes occur within the human body, and these changes become more severe and diverse as flight duration increases. When in space, humans experience early signs of a decrease in blood volume and red cell mass, aerobic capacity, endurance, strength, and muscle mass. Moreover, there is a reduction of bone density in the lower limbs, hips, and spine, and in the absorption of calcium through the gut. Visual-spatial orientation and eye-hand coordination are also affected.
When humans return to Earth's gravity, this reduction in physical fitness manifests itself through the body's inability to maintain the blood pressure control necessary to prevent fainting. This inability occurs because the heart and blood vessels are less responsive. Balance, gait, and motor coordination are also severely affected. Similar but less-intense symptoms occur during and after complete bed rest.
Bed Rest Studies
Researchers study bed rest to understand the mechanisms that bring about these symptoms, and to develop preventive treatments. Both astronauts and men and women volunteers for these bed rest studies recover, with the speed of recovery being proportional to the duration of the flight or the bed rest. Research into the mechanisms that contribute to these symptoms has pointed to the variety of ways humans use Earth's gravity to promote stimuli the body needs to maintain normal physiology.
In space, where the influence of gravity is negligible, the load we normally feel on Earth from our weight is absent. Exercise, such as walking, is ineffective, because we are not working against the force of gravity. Signals to the parts of our nervous system that control blood pressure from changing position (such as standing or lying down) are also absent, and we can no longer sense what is up and what is down.
Bed rest studies have been conducted to determine the minimum daily exposure to gravity's stimuli needed to maintain normal physiology. This research indicated that a change in posture from lying down to upright—a total of two to four hours of being upright so gravity pulled maximally in the head-to-toe direction—at least eight to sixteen times a day could prevent the decline in blood pressure control, aerobic capacity, blood volume, and muscle strength. As little as thirty minutes a day of walking at a pace of three miles per hour prevented the increased loss of calcium produced by bed rest. These results suggested that intermittent exposure to gravity in space, as provided by a centrifuge , may be an effective way to keep astronauts healthy on long trips.
Are the Effects of Aging Irreversible?
The similarity of the set of symptoms resulting from going into space to those associated with aging is striking. In the elderly, these symptoms have been assumed to be part of the normal course of aging and therefore irreversible. Space and bed rest research argues against this assumption. Research indicates that the symptoms of aging are due to an increasingly sedentary lifestyle rather than aging and are thus also reversible in the elderly. In the mid-1990s, Maria Fiatarone and her coworkers reported that weight training and nutritional supplements reversed muscle and bone atrophy of aging in people aged seventy-three to ninety-eight.*
It remains to be seen if space will also help us understand the more fundamental mechanisms of aging. Preliminary research suggests cell cycle and cell death may be affected. But it will only be through the conducting of experiments long enough to explore life span and chromosomal and genetic mechanisms that these questions will be answered.
see also Careers in Space Medicine (volume 1); Glenn, John (volume 3).
Fiatarone, Maria Antoinette, et al. "Exercise Training and Nutritional Supplementation for Physical Frailty in Very Elderly People." New England Journal of Medicine 330 (1994):1,769-1,775.
Vernikos, Joan. "Human Physiology in Space." Bioessays 18 (1996):1,029-1,037.
*John Glenn's second trip to space in 1998 at the age of seventy-seven on shuttle mission STS-95 helped increase understanding of the effects of a healthy lifestyle on aging.