The bony skeleton is a remarkable organ that serves both a structural
function—providing mobility, support, and protection for the body—and a
reservoir function, as the storehouse for essential minerals. It is not a
static organ, but is constantly changing to better carry out its functions. The
development of the bony skeleton likely began many eons ago, when animals left
the calcium-rich ocean, first to live in fresh water where calcium was in short
supply, and then on dry land where weight bearing put much greater stress on
the skeleton. The architecture of the skeleton is remarkably adapted to provide
adequate strength and mobility so that bones do not break when subjected to
substantial impact, even the loads placed on bone during vigorous physical
activity. The shape or structure of bone is at least as important as its mass
in providing this strength.
The skeleton is also a storehouse for two minerals, calcium and phosphorus,
that are essential for the functioning of other body systems, and this
storehouse must be called upon in times of need. The maintenance of a constant
level of calcium in the blood as well as an adequate supply of calcium and
phosphorus in cells is critical for the function of all body organs, but
particularly for the nerves and muscle. Therefore, a complex system of
regulatory hormones has developed that helps to maintain adequate supplies of
these minerals in a variety of situations. These hormones act not only on bone
but on other tissues, such as the intestine and the kidney, to regulate the
supply of these elements. Thus one reason that bone health is difficult to
maintain is that the skeleton is simultaneously serving two different functions
that are in competition with each other. First, bone must be responsive to
changes in mechanical loading or weight bearing, both of which require strong
bones that have ample supplies of calcium and phosphorus. When these elements
are in short supply the regulating hormones take them out of the bone to serve
vital functions in other systems of the body. Thus the skeleton can be likened
to a bank where we can deposit calcium or phosphorus and then withdraw them
later in times of need. However, too many withdrawals weaken the bone and can
lead to the most common bone disorder, fractures.
For many people, hearing "You have osteoporosis" is startling.
Some hear it in the hospital after breaking a hip. Others get the news after getting a bone density test.
Osteoporosis is most common in women after menopause, people with osteoporosis in their family, and people with a small frame. But others can also get it, raising their risk of bone fractures.
Cutting that risk is crucial. About half of women and a quarter of men over age 50 will have an osteoporosis-related fracture, notes the...
Both the amount of bone and its architecture or shape are determined by the
mechanical forces that act on the skeleton. Much of this is determined
genetically so that each species, including humans, has a skeleton that is
adapted to its functions. However, there can be great variation within a
species, so that some individuals will have strong bones and others will have
weak bones, largely because of differences in their genes (Huang et al. 2003).
Moreover, bone mass and architecture are further modified throughout life as
these functions and the mechanical forces required to fulfill them change. In
other words, bones will weaken if they are not subjected to adequate amounts of
loading and weight bearing for sufficient periods of time. If they are not
(such as in the weightless condition of space travel), rapid bone loss can
occur. In other words, as with muscle, it is “use it or lose it” with bone as
well. Conversely, the amount and architecture of the bones can be improved by
mechanical loading. However, as described in Chapter 6, some types of exercise
may be better than others in strengthening the skeleton.
To respond to its dual roles of support and regulation of calcium and
phosphorus, as well as to repair any damage to the skeleton, bone is constantly
changing. Old bone breaks down and new bone is formed on a continuous basis. In
fact, the tissue of the skeleton is replaced many times during life. This
requires an exquisitely controlled regulatory system that involves specialized
cells that communicate with each other. These cells must respond to many
different signals, both internal and external, mechanical and hormonal, and
systemic (affecting the whole skeleton) and local (affecting only a small
region of the skeleton). It is not surprising that with so many different tasks
to perform and so many different factors regulating how the skeleton grows,
adapts, and responds to changing demands, there are many ways that these
processes can go astray.
WebMD Public Information from the U.S. Department of Health and Human Services