A. The consequence of losing bones B. A better
lab than on earth C. Two different cases D.
Multiple effects form weightlessness 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 he liver, kidneys, stomach
and bowels disappear. One astronaut said after his flight: "You feel our 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. 45.______ Not everything that affects the
body during space flight is related solely to weightlessness. Also affected, for
example, are the immune system 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.