Functions of the Skeletal System
Alicia Scott
The skeleton is a magnificent organ. While many view bones as inactive and lifeless,
these resilient objects are actually alive and quite multifunctional. Several duties they carry out
are, movement, providing points of attachment, protection and support of softer tissues,
Hematopoieis, fat and inorganic salt storage, and passageways for blood vessels and nerves.
Without our skeletons, our bodies would be formless masses. However, by providing a
framework for the head, face, thorax, and limbs, the skeleton gives the body its shape. It also
protects vital organs and tissues. For example, the thoracic cage—composed of a sternum,
twenty-four ribs, and twelve thoracic vertebrae—protects the heart, lungs, and some of the upper
abdominopelvic viscera. In the lower abdominopelvic cavity, the pelvic girdle—comprised of
two coxal bones—protects the reproductive organs.
Bones, cartilages, and ligaments form joints, or articulations. These structures make bone
growth possible and change parts of the skeleton during childbirth. By working directly with the
skeletal muscles, articulations are involved with the movement of the body as well. Processes—
prominent projections on bones—allow tendons and ligaments (both made of dense regular
fibrous connective tissue) to attach skeletal muscles to bones and bind the bones together. When
a limb (the arm) is flexing, the bones and muscles work as a levers. These are four-part
mechanisms that include a rigid bar ( the forearm), a pivot (the elbow), an object to move against
resistance (the hand), and a source of energy (the biceps brachii). When flexion occurs, the
insertion (the moveable tendon) is pulled towards the origin (the immoveable tendon), which, in
this case is attached to the acromion process and the greater tubercle of the humerus. The
forearm bends on the elbow and the hand lifts the resistance (e.g. a weight).
The storing of inorganic salts (calcium phosphates) is done by the bone matrix of osseous
connective tissue—specifically in the compact bone. This matrix is deposited in thin layers
called lamellae that also include osteocytes residing in cavities called lacunae. The matrix and
osteocytes form concentric structures—osteons—around central, or Haversian, canals—which
provide passageways for the blood vessels and nerves. These osteons tightly clustered together
form the compact bone. Calcium phosphate is stored to make bones crush-resistant and rigid.
Collagen protein fibers also aid in keeping bones strong. When calcium levels are depleted,
bones become soft and deformed. This brings us to the homeostasis of bone tissue and the
maintenance of calcium blood levels. First, it’s important to know we cannot have too much or
too little bone mass. Through resorption by osteoclasts (taking in old bone matrix) and
deposition by osteoblasts (putting out fresh bone matrix) the bone tissue of an adult skeleton
remains relatively constant. The hormones of the thyroid and parathyroid that govern these
processes also control calcium phosphorus levels. For example, when blood calcium levels are
reduced, the parathyroid cells are notified. The parathyroid is stimulated to produce PTH
Hormone to cause the osteoclasts (effectors) to perform osteoclasis and secret an acid to break
down bone matrix. This action releases stored calcium into the system. The intestines and
kidneys are also stimulated to release calcium prepared for excretion back into the body to bring
calcium blood levels to normalcy. If the opposite occurs, and our calcium blood levels rise to
unhealthy points, the thyroid cells are notified and osteoclast activity is inhibited. At this point,
the thyroid is roused to release Calcitonin. This hormone triggers osteoblasts to perform
osteogenesis and create more bone matrix to store excess calcium.
It is understood that our hearts and blood vessels pump and carry blood throughout the
body. They do not, however, manufacture the blood cells—Hematopoises. That task is reserved
for the bones. More specifically, the trebicular bones in the epiphysis have Red Bone Marrow to
perform Hematopoises. This marrow—which also makes leukocytes and blood platelets— fills
in the gaps between these criss-crossing bony plates and is comprised of a soft connective tissue.
It is found in generous amount in infants and decreases with age. The red color of the marrow
comes from the protein hemoglobin, which is highly oxygenated.
The medullary cavity in the shaft—or dyaphysis—of a long bone is filled with fat-storing
Yellow Bone Marrow. This has no role in Hematopoises, but it does increase as people age. Red
and Yellow Bone Marrow can replace each other as well. For example, if a particular area
requires more blood, the Red Bone Marrow will replace Yellow with extensions. One the blood
deficiency has been corrected, Red will revert back to Yellow.
By examining these detailed functions of the skeletal system, it is evident that this
system—like all the others—is a vital component of our bodies. It does not merely function as an
individual segment but it is integrated with the muscular system, the cardiovascular system, and
every other part of our body. It is a living factory that makes our blood and stores fats and
calcium phosphates. Clearly, bones are not “just” a part of our organism—they are the
component that holds us together.