Humans are vertebrates, animals having a vertabral
column or backbone. They rely on a sturdy internal
frame that is centered on a prominent spine. The human
skeletal system consists of bones, cartilage, ligaments
and tendons and accounts for about 20 percent of the
body weight.
The living bones in our bodies use oxygen and give
off waste products in metabolism. They contain active
tissues that consume nutrients,
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require a blood supply and change shape or remodel in response
to variations in mechanical stress.
Bones provide a rigid frame work, known as the skeleton,
that support and protect the soft organs of the body.
The skeleton supports the body against the pull of gravity.
The large bones of the lower limbs support the trunk when
standing.
The skeleton also protects the soft body parts. The fused
bones of the cranium surround the brain to make it less vulnerable
to injury. Vertebrae surround and protect the spinal cord
and bones of the rib cage help protect the heart and lungs
of the thorax.
Bones work together with muscles as simple mechanical lever
systems to produce body movement.
Bones contain more calcium
than any other organ. The intercellular matrix of bone contains
large amounts of calcium salts, the most important being calcium
phosphate.
When blood calcium levels decrease below normal, calcium
is released from the bones so that there will be an adequate
supply for metabolic needs. When blood calcium levels are
increased, the excess calcium is stored in the bone matrix.
The dynamic process of releasing and storing calcium goes
on almost continuously.
Hematopoiesis, the formation of blood cells, mostly takes
place in the red marrow of the bones.
In infants, red marrow is found in the bone cavities. With
age, it is largely replaced by yellow marrow for fat storage.
In adults, red marrow is limited to the spongy bone in the
skull, ribs, sternum, clavicles, vertebrae and pelvis. Red
marrow functions in the formation of red blood cells, white
blood cells and blood platelets.
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