The Human Skeleton
The human skeleton is made up of 206 bones. The functions of the
skeleton are to provide support, give our bodies shape, provide protection
to other systems and organs of the body, to provide attachments for
muscles, to produce movement and to produce red blood cells.
The main bones of the human skeleton are:
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The Skull - Cranium, Mandible, and Maxilla
Shoulder girdle - clavicle and scapula
Arm - humerus, radius, and ulna
Hand - Carpals, Metacarpals, and Phalanges
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Chest - Sternum, and Ribs
Spine - Cervical area (top 7 vertebrae), Thoracic (next 12), Lumbar (bottom 5
vertebrae), Sacrum (5 fused or stuck together bones) and Coccyx (the tiny bit at the
bottom of the spine).
Pelvic girdle - Ilium, Pubis, and Ischium.
Leg - Femur, Tibia, and Fibula
Ankle - Talus and calcaneus (not shown above)
Foot - Tarsals, Metatarsals, and Phalanges.
The skeleton can be divided into two parts known as axial and the appendicular. The axial
skeleton consists of the central core of the skull, spine, and ribs whilst the appendicular is
composed of the arms and legs.
How are bones formed?
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Bones are formed by the ossification of cartilage. What this really means is all bones
start off as cartilage (normally in the womb) and they gradually turn to hard bone
(ossification) over a period of years.
Calcium is needed for strong bone growth. Read more on the structure of bone.
What is the function of the skeleton?
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It provides protection to the major organs, in particular, the chest and rib cage.
Muscles attach to bones to enable movement.
Production of red blood cells within the bone marrow (a spongy substance is found in
the cavities of long bones). Red blood cells carry oxygen around the body which is
important in the production of energy.
The Axial & Appendicular Skeleton
The Human Skeleton can be divided up into two parts, the axial skeleton
which is the central core of the body and the appendicular skeleton which
forms the extremities of the arms and legs.
Bones of the Axial Skeleton
The Axial Skeleton is the central core of the human body housing and protecting its vital
organs. The axial skeleton consists of 80 bones:
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29 bones in the head - (8 cranial and 14 facial bones) and then also 7 associated bones
(6 auditory ossicles and the Hyoid Bone)
25 bones of the thorax - (the sternum and 24 ribs)
26 bones in the vertebral column (24 vertebrae, the sacrum, and the coccyx)
The function of the Axial skeleton
The Axial Skeleton has 2 functions. The first is to support and protect the organs in the
dorsal and ventral cavities. The second: it creates a surface for the attachment of muscles.
The intervertebral disc(which lies between the adjacent vertebrae in the spine) is a classic
example of a joint within the Axial skeleton in that it is very strong and will only permit
limited movement.
Of the 206 bones in the human body 126 of these make up the appendicular skeleton.
Bones of the Appendicular skeleton:
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4 bones in the shoulder girdle (clavicle and scapula each side)
6 bones in the arm and forearm (humerus, ulna, and radius)
58 bones in the hands (carpals 16, metacarpals 10, phalanges 28 and sesamoid 4)
2 pelvis bones
8 bones in the legs (femur, tibia, patella, and fibula)
56 bones in the feet (tarsals, metatarsals, phalanges, and sesamoid)
Structure of Bone
It is important for bones to be strong to support our body weight and in
some cases provide protection such as the skull and ribs. However, they
must also be light enough to make movement possible.
A long bone consists of several sections:
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Diaphysis: This is the long central shaft
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Epiphysis: Forms the larger rounded ends of long bones
Metaphysis: Area between the diaphysis and epiphysis at both ends of the bone
Epiphyseal Plates: Plates of cartilage, also known as growth plates which allow the
long bones to grow in length during childhood. Once we stop growing, between 18 and
25 years of age the cartilage plates stop producing cartilage cells and are gradually
replaced by bone.
Covering the ends of bones, where they form a joint with another bone, is a layer of hyaline
cartilage. This is a firm but elastic type of cartilage which provides shock absorption to the
joint and has no neural or vascular supply.
Bone Anatomy
If you were to cut a cross-section through a bone, you would first come across a thin layer
of dense connective tissue known as Periosteum. This can be divided into two layers, an
outer 'fibrous layer' containing mainly fibroblasts and an inner 'cambium layer' containing
progenitor cells which develop into osteoblasts (the cells responsible for bone formation).
The periosteum provides a good blood supply to the bone and a point for muscular
attachment.
Under the periosteum is a thin layer of compact bone (often called cortical bone), which
provides the bones strength. It consists of tightly stacked layers of bone which appear to
form a solid section, although do contain osteons, which like canals provide passageways
through the hard bone matrix.
On the inside of this, you would find a different kind of bone, known as spongy or
cancellous bone. This is a more porous, lightweight type of bone with an irregular
arrangement of tissue which allows maximum strength. In a long bone, this is normally
found at either end of the bone, in flat or irregular bones it is a thin layer found just inside
the compact bone. Interestingly, compact bone constitutes up to 80% of the bones weight,
with spongy bone making up the additional 20%, despite its much larger surface area.
The centre of the bone shaft is hollow and known as the Medullary Cavity. This contains
both red and yellow bone marrow. Yellow bone marrow is mainly a fatty tissue, while the
red bone marrow is where the majority of blood cells are produced. This is found in higher
proportions in the flat and irregular bones.
Bone Marrow
Bone marrow is a soft gelatin-like tissue found in the central cavities of
long bones and some short bones. It contains Stem cells and it produces
most of the new red blood cells in the human body. There are two types of
bone marrow. Redbone marrow (which is also known as myeloid tissue)
and yellow bone marrow (fatty tissue).
What is bone marrow?
Bone marrow is the gelatinous, spongy substance is found in the cavities of long bones. It
contains two types of stem cells. A stem cell is an immature cell which can change into a
new specialised cell as required. The two types of stem cell found in bone marrow are:
1. Hematopoietic cells which can be used to make all the blood cell types in the body with
over 200 billion new red blood cells produced every day in healthy tissue.
2. Stromal cells which can produce fat, cartilage, and bone.
Red blood cells, platelets, and most white blood cells arise in the red bone marrow whilst
some white blood cells develop in the yellow marrow. Yellow marrow is yellow because it
has a much higher number of fat cells. Red blood cells are important in sports performance
as they carry oxygen around the body. Red marrow is not found in all the bones of the
human body. It is found mainly in the flat bones, for example, the hip bone, vertebrae, and
shoulder blades. Two types of osseous tissue from bones, compact bone and cancellous
(spongy) bone. Red bone marrow is also found in the cancellous material found at the ends
of the long bones (the femur and humerus).
How does bone marrow produce red blood cells?
A decrease in oxygen in the blood is detected by the kidneys which then secrete the
hormone called erythropoietin (commonly known as EPO). This hormone is found naturally
in the body and stimulates erythropoiesis, the process by which red blood
cells (erythrocytes) are produced in the red bone marrow. The body regulates the process
of erythropoiesis so that red blood cells are produced at a rate that is equal to the
destruction of red blood cells. This enables the body to sustain adequate tissue oxygen
levels.
Blood doping
A good example of how this works is when an athlete has to acclimatise if they travel to a
higher altitude when undergoing altitude training. The body naturally makes more red
blood cells to increase the oxygen levels in the blood. However, if additional amounts of
the hormone EPO are taken artificially then red blood cells can increase artificially. This
process is known as blood doping and is banned in most sports. It was popular among elite
level cyclists and middle distance/endurance runners as a way of enhancing performance.
Bone Marrow Transplants
Bone marrow transplants are given to people with diseases of the blood, bone marrow and
certain types of cancer. The procedure can have many complications so is only used on
people with life-threatening diseases. The process involves harvesting the healthy stem
cells to replenish the bone marrow of the patient. These new stem cells then take over the
production of healthy new blood cells.