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Your Bones In Space

Term Paper Title: Your Bones In Space
Word Count: 1336
Page Count: 5.34 (250 words per page double spaced)

Your Bones in Space

ASTRONOMY AND SPACE SCIENCE

Hypogravitational Osteoporosis: A review of literature.
By Lambert Titus Parker. May 19 1987.

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, possi...

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