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ARRoGANT CoURiERS WiTH ESSaYS
Grade Level: Type of Work Subject/Topic is on:
[ ]6-8 [ ]Class Notes [General Info on what ]
[ ]9-10 [ ]Cliff Notes [happens to your bones ]
[ ]11-12 [ ]Essay/Report [in space. ]
[x]College [x]Misc [ ]
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File BONES-SP.TXT has 1433 words, and 9417 bytes.
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Your Bones in Space ASTRONOMY AND SPACE SCIENCE
SIG
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Hypogravitational Osteoporosis: A review of literature.
By Lambert Titus Parker. May 19 1987. (GEnie Spaceport)
Osteoporosis: a condition characterized by an absolute decrease in the
amount of bone present to a level below which it is capable of maintaining
the structural integrity of the skeleton.
To state the obvious, Human beings have evolved under Earth's gravity
"1G". Our musculoskeleton system have developed to help us navigate in this
gravitational field, endowed with ability to adapt as needed under various
stress, strains and available energy requirement. The system consists of
Bone a highly specialized and dynamic supporting tissue which provides the
vertebrates its rigid infrastructure. It consists of specialized connective
tissue cells called osteocytes and a matrix consisting of organic fibers
held together by an organic cement which gives bone its tenacity,
elasticity and its resilience. It also has an inorganic component located
in the cement between the fibers consisting of calcium phosphate [85%];
Calcium carbonate [10%] ; others [5%] which give it the hardness and
rigidity. Other than providing the rigid infrastructure, it protects vital
organs like the brain], serves as a complex lever system, acts as a storage
area for calcium which is vital for human metabolism, houses the bone
marrow within its mid cavity and to top it all it is capable of changing
its architecture and mass in response to outside and inner stress. It is
this dynamic remodeling of bone which is of primary interest in
microgravity. To feel the impact of this dynamicity it should be noted that
a bone remodeling unit [a coupled phenomena of bone reabsorption and bone
formation] is initiated and another finished about every ten seconds in a
healthy adult. This dynamic system responds to mechanical stress or lack of
it by increasing the bone mass/density or decreasing it as per the demand
on the system. -eg; a person dealing with increased mechanical stress will
respond with increased mass / density of the bone and a person who leads a
sedentary life will have decreased mass/density of bone but the right
amount to support his structure against the mechanical stresses she/she
exists in. Hormones also play a major role as seen in postmenopausal
females osteoporosis (lack of estrogens) in which the rate of bone
reformation is usually normal with the rate of bone re-absorption
increased.
In Skeletal system whose mass represent a dynamic homeostasis in 1g
weight- bearing,when placed in microgravity for any extended period of time
requiring practically no weight bearing, the regulatory system of
bone/calcium reacts by decreasing its mass. After all, why carry all that
extra mass and use all that energy to maintain what is not needed?
Logically the greatest loss -demineralization- occurs in the weight bearing
bones of the leg [Os Calcis] and spine. Bone loss has been estimated by
calcium-balance studies and excretion studies. An increased urinary
excretion of calcium , hydroxyproline & phosphorus has been noted in the
first 8 to 10 days of microgravity suggestive of increased bone
re-absorption. Rapid increase of urinary calcium has been noted after
takeoff with a plateau reached by day 30. In contrast, there was a steady
increase off mean fecal calcium throughout the stay in microgravity and was
not reduced until day 20 of return to 1 G while urinary calcium content
usually returned to preflight level by day 10 of return to 1G.
There is also significant evidence derived primarily from rodent
studies that seem to suggest decreased bone formation as a factor in
hypogravitational osteoporosis. Boy Frame,M.D a member of NASA's
LifeScience Advisory Committee [LSAC] postulated that "the initial
pathologic event after the astronauts enter zero gravity occurs in the bone
itself, and that changes in mineral homeostasis and the calcitropic
hormones are secondary to this. It appears that zero gravity in some ways
stimulate bone re-absorption, possibly through altered bioelectrical fields
or altered distribution of tension and pressure on bone cells themselves.
It is possible that gravitational and muscular strains on the skeletal
system cause friction between bone crystals which creates bioelectrical
fields. This bioelectrical effect in some way may stimulate bone cells and
affect bone remodeling." In the early missions, X-ray densitometry was
used to measure the weight-bearing bones pre & post flight. In the later
Apollo, Skylab and Spacelab missions Photon absorptiometry (a more
sensitive indicator of bone mineral content) was utilized. The results of
these studies indicated that bone mass [mineral content] was in the range
of 3.2% to 8% on flight longer than two weeks and varying directly with the
length of the stay in microgravity. The accuracy of these measurements
have been questioned since the margin of error for these measurements is 3
to 7% a range being close to the estimated bone loss.
Whatever the mechanism of Hypogravitational Osteoporosis, it is one of
the more serious biomedical hazard of prolonged stay in microgravity. Many
forms of weight loading exercises have been tried by the astronauts &
cosmonauts to reduce the space related osteoporosis. Although isometric
exercises have not been effective, use of Bungee space suit have shown some
results. However use of Bungee space suit [made in such a way that
everybody motion is resisted by springs and elastic bands inducing stress
and strain on muscles and skeletal system] for 6 to 8 hrs a day necessary
to achieve the desired effect are cumbersome and require significant
workload and reduces efficiency thereby impractical for long term use other
than proving a theoretical principle in preventing hypogravitational
osteoporosis.
Skylab experience has shown us that in spite of space related
osteoporosis humans can function in microgravity for six to nine months and
return to earth's gravity. However since adults may rebuild only two-third
of the skeletal mass lost, even 0.3 % of calcium loss per month though
small in relation to the total skeletal mass becomes significant when Mars
mission of 18 months is contemplated. Since adults may rebuild only
two-thirds of the skeletal mass lost in microgravity, even short durations
can cause additive effects. This problem becomes even greater in females
who are already prone to hormonal osteoporosis on Earth.
So far several studies are under way with no significant results. Much
study has yet to be done and multiple experiments were scheduled on the
Spacelab Life Science [SLS] shuttle missions prior to the Challenger
tragedy. Members of LSAC had recommended that bone biopsies need to be
performed for essential studies of bone histomorphometric changes to
understand hypogravitational osteoporosis. In the past, astronauts with
the Right Stuff had been resistant and distrustful of medical experiments
but with scientific personnel with life science training we should be able
to obtain valid hard data. [It is of interest that in the SLS mission, two
of the mission specialists were to have been physicians, one physiologist
and one veterinarian.]
After all is said, the problem is easily resolved by creation of
artificial gravity in rotating structures. However if the structure is not
large enough the problem of Coriolis effect must be faced. To put the
problem of space related osteoporosis in perspective we should review our
definition of Osteoporosis: a condition characterized by an absolute
decrease in the amount of bone present to a level below which it is capable
of maintaining the structural integrity of the skeleton. In microgravity
where locomotion consists mostly of swimming actions with stress being
exerted on upper extremities than lower limbs resulting in reduction of
weight bearing bones of lower extremities and spine which are NOT needed
for maintaining the structural integrity of the skeleton. So in
microgravity the skeletal system adapts in a marvelous manner and problem
arises only when this microgravity adapted person need to return to higher
gravitational field. So the problem is really a problem of re-adaptation to
Earth's gravity.
To the groups wanting to justify space related research: Medical
expense due to osteoporosis in elderly women is close to 4 billion dollars
a year and significant work in this field alone could justify all space
life science work. It is the opinion of many the problem of osteoporosis
on earth and hypogravity will be solved or contained, and once large
rotating structures are built the problem will become academic. For
completeness sake: Dr. Graveline, at the School of Aerospace Medicine,
raised a litter of mice on a animal centrifuge simulating 2G and compared
them with a litter mates raised in 1G. "They were Herculean in their build,
and unusually strong...." reported Dr.Graveline. Also X-ray studies showed
the 2G mice to have a skeletal density to be far greater than their 1G
litter mates.