E. How to overcome weightlessness

　　F. Factors that are not so sure

　　During weightlessness, the forces within the body undergo dramatic change. Because the spine is no longer compressed, people grow taller. The lungs, heart and other organs within the chest have no weight, and as a result, the rib cage and chest relax and expand. Similarly, the weights of the liver, kidneys, stomach and bowels disappear. One astronaut said after his flight: “You feel your guts floating up. I found myself tightening my belly, sort of pushing things back。”

　　41.

　　Meanwhile muscles and bones come to be used in different ways. Our muscles are designed to support us when stand or sit 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。

　　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。

　　42.

　　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。

　　43.

　　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. 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。

　　44.

　　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。