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312201678 lecture note human anatomy and fisiologi skeletal system

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CHP 3 – SKELETAL SYSTEM
Function of the Skeletal System
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
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.
Structure of Bone Tissue
There are two types of bone tissue: compact and spongy. The names imply that the two types of differ
in density, or how tightly the tissue is packed together. There are three types of cells that contribute to
bone homeostasis. Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone, and
osteocytes are mature bone cells. An equilibrium between osteoblasts and osteoclasts maintains
bone tissue.
Compact Bone
Compact bone consists of closely packed osteons or haversian systems. The osteon consists of a
central canal called the osteonic (haversian) canal, which is surrounded by concentric rings (lamellae)
of matrix. Between the rings of matrix, the bone cells (osteocytes) are located in spaces called

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