Meanwhile muscles and bones come to be used in different ways. Our muscles are designed to support us when standing or sitting upright and to move body parts. But in space, muscles used for support on the ground are no longer needed for that purpose; moreover, the muscles used for movement around a capsule differ from those used for walking down a hall. Consequently, some muscles rapidly weaken. This doesn't present a problem to space travelers as long as they perform only light work. But preventing the loss of muscle tissue required for heavy work during space walks and preserving muscle for safe return to Earth are the subject of many current experiments. Bone physiology, too, changes substantially. One of the strongest known biological materials, bone is a dynamic tissue. Some cells have the job of producing it, whereas others destroy it. Both types usually work together to maintain bones throughout life.
Bone contains both organic materials, which contribute strength and stability, and inorganic materials, which make the bones stiff and serve as a reservoir of minerals within the body. For example, 99 percent of the calcium in the body is in the skeleton. Stable levels of calcium in the body's fluids are necessary for all types of cells to function normally.Studies have shown that astronauts lose bone mass from the lower spine, hips, and upper leg at a rate of about 1 percent per month for the entire duration of their time in space. Some sites, such as the heel, lose calcium faster than others. Studies of animals taken into space suggest that bone formation also declines.
Needless to say, these data are indeed cause for concern. During space flight, the loss of bone elevates calcium levels in the body, potentially causing kidney stones and calcium crystals to form in other tissues. Back on the ground, the loss of bone calcium stops within one month, but scientists do not yet know whether the bone recovers completely: too few people have flown in space for long periods. Some bone loss may be permanent, in which case ex-astronauts will always be more prone to broken bones.These questions mirror those in our understanding of how the body works here on Earth. For example, elderly women are prone to a loss of bone mass（骨质疏松症）. Scientists understand that many different factors can be involved in this loss, but they do not yet know how the factors act and interact; this makes it difficult to develop an appropriate treatment. So it is with bone loss in space, where the right prescription still awaits discovery.
Many other body systems are affected directly and indirectly. One example is the lung. Scientists have studied the lung in space and learned much they could not have learned in laboratories on Earth. On the ground the top and bottom parts of the lung have different patterns of air flow and blood flow. But are these patterns the result only of gravity, or also of the nature of the lung itself? Only recently have studies in space provided clear evidence for the latter. Even in the absence of gravity, different parts of the lung have different levels of air flow and blood flow. Not everything that affects the body during space flight is related solely to weightlessness. Also affected, for example, are the immune system (the various physical and psychological stresses of space flight probably play roles in weakening the immune system in astronauts) and the multiple systems responsible for the amount and quality of sleep (light levels and work schedules disrupt the body's normal rhythms). Looking out the spacecraft window just before going to sleep (an action difficult to resist, considering the view) can let enough bright light into the eye to trigger just the wrong brain response, leading to poor sleep. As time goes on, the sleep debt accumulates.
For long space voyages, travelers must also face being confined in a tight volume, unable to escape, isolated from the normal life of Earth, living with a small, fixed group of companions who often come from different cultures. These challenges can lead to anxiety, depression, crew tension and other social issues, which affect astronauts just as much as weightlessness — perhaps even more. Because these factors operate at the same time the body is adapting to other environmental changes, it may not be clear which physiological changes result from which factors. Much work remains to be done.Finally, space flight involves high levels of radiation. An astronaut spending one year in a low-Earth orbit would receive a radiation dose 10 times greater than the average dose received on the ground. A year's stay on the moon would result in a dose seven times higher still, whereas a flight to Mars would be even worse. A sudden surge in radiation from the sun, as occurred in August 1972, can deliver a dose more than 1,000 times the annual ground dose in less than a day. Fortunately, such events are rare, and spacecraft designers can guard against them by providing special shielded rooms to which astronauts can retreat.
Obviously, the radiation hazard to long-duration space travelers — and the consequent cancer risk — is a major problem. The problems of space radiation are difficult to study because it is nearly impossible to duplicate on Earth the radiation environment of space, with its low but steady flow of high-energy particles. Even so, researchers generally believe that with proper radiation shields built into the spacecraft and protective drugs, the risks can be brought within satisfactory limits.(Words: 1,006)
