The skeletal system in an adult body is made up of 206 individual bones. These bones are arranged into two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton runs along the body’s midline axis and is made up of 80 bones consisting of the skull, vertebral column, the ribs, and sternum.
The appendicular skeleton is made up of 126 bones and supports the appendages or limbs of the body. It consists of the shoulder girdle, upper limbs, the pelvic girdle, and lower limbs.
At birth, the skeleton has more than 300 bones; however, as we age, these bones grow together and fuse into larger bones, leaving us with 206 bones by the time we are fully developed around 23 years of age. There are five different types of bone in the skeleton defined by their shape: long bones, short bones, flat bones, irregular bones, and sesamoid bones. (These will be covered in another blog).
The skeleton makes up about 30-40% of an adult’s body mass. The skeleton’s mass is made up of non-living bone matrix and many tiny bone cells. Roughly half of the bone matrix’s mass is water, while the other half is collagen protein and solid crystals of calcium carbonate and calcium phosphate.
Living bone cells are found on the edges of bones and in small cavities inside of the bone matrix. Although these cells make up little of the total bone mass, they have several particularly important roles in the function of the skeletal system. The bone cells allow bones to:
Each bone is a complex living organ made up of multiple cells, protein fiber, and minerals. The skeleton gives support and protection for the soft tissues providing attachment points for the tendons of muscles to allow movements at the joints. Bones act as the body’s warehouse for calcium, iron, and energy in the form of fat or lipids. New blood cells are produced by the red bone marrow inside of our bones.
Red bone marrow produces red and white blood cells in a process known as haematopoiesis. Red bone marrow is found in the space inside the hollow of the bones known as the medullary cavity. Children tend to have more red bone marrow compared to their body size than adults, this is due to the body’s constant growth and development. The amount of red bone marrow declines by the end of puberty being replaced by yellow bone marrow.
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