LO3 of Unit 1 – Skeletal System (2021)

Skeletal System

Learning outcomes
By the end of this section, you will be able to:

3.1 Describe the basic functions of the skeleton

3.2 Identify the bones of the axial and appendicular skeleton

3.3 Identify the classification of bones

3.4 Describe the main features of a long bone

3.5 Describe the stages of bone growth

3.6 Describe posture in terms of:

    • curves of the spine
    • neutral spine alignment
    • movement potential of the spine
  • postural deviations.


Functions of the skeleton

Our skeleton serves several essential functions:

Framework – it provides the main framework for the body

Movement – muscles attach to the skeleton, cross joints and pull on bones to enable movement

Shape – body shape and type are determined by skeletal structure (i.e., ectomorph, endomorph, mesomorph)

Storage – calcium and other minerals are stored in the bones

Production – red and white blood cells are produced in the bone marrow

Protection – it protects several vital organs

  • The rib cage protects the heart and lungs
  • The vertebral column protects the spinal cord
  • The skull protects the brain

Functions of the skeleton

Useful terms and definitions

The prefix ‘osteo‘ means ‘bone‘. Example: osteoporosis (condition of porous bone)

The suffix ‘genesis‘ meaning ‘beginning process or new‘ Example: osteogenesis

The suffix ‘olysis‘ means ‘destruction‘ or ‘breaking down‘ Example: lipolysis (the breakdown of triglycerides or fat)

The prefix ‘peri‘ means ‘going around‘ The prefix ‘endo‘ means ‘found within‘ The suffix ‘blast‘ means to ‘build‘ Example: osteoblast (

The suffix ‘osis‘ means ‘condition of‘ Example: osteoporosis (condition of porous bone)

The prefix ‘chondro‘ refers to ‘cartilage‘ 

The word ‘cyte‘ means ‘cell‘ Example Example: osteocyte (bone cell), myocyte (muscle cell)

The prefix ‘hypo‘ means ‘low‘ Example: hypoglycaemia

The suffix ‘eamia‘ refers to ‘relating to blood‘ Example anaemia (

Collagen flexible fibrous proteins that give connective tissue tensile strength 

Inorganic does not contain carbon 

Organic contains carbon

Bones of the skeleton

The skeletal system consists of the 206 bones and ligaments and cartilage.

The human skeleton is divided into:

  1. The axial section (80 bones) – the cranium and facial bones, cervical vertebrae, thoracic vertebrae, lumbar vertebrae, sacrum and coccyx, costal bones and sternum.
  2. The appendicular section (126 bones) – the arm, pelvic and leg bones, also known as the upper and lower extremities.

Appendicular and axial skeleton plus name of bones (Image 3.2)

Bone classifications

Bones are classified by their shape and not their size.

There are five types of bones in the human body.

Sesamoid bones 

Sesamoid bones are embedded within a tendon or a muscle and function to diminish friction and alter the muscle’s pull direction—E.g. patella.

Long bones

Long bones are longer than they are wide, with a hollow centre. These bones act as levers to create movement, produce blood cells and store minerals—E.g. femur, tibia, fibula, humerus, radius, ulna, metacarpals, metatarsals.

Short bones

Sort bones are as wide as they are long. Their primary function is providing support and stability with little movement. E.g. carpals and tarsals.

Flat bones

Flat bones are made up of a layer of spongy bone between two thin layers of compact bone. As their name suggests, these bones are ‘flat’ in appearance and have broad surfaces for muscular attachment. E.g. cranium, sternum, ribs

Irregular bones 

Irregular bones consisting of spongy bone coated by a thin layer of compact bone, their shapes depend on the functions they fulfil within the body. e.g. provides significant mechanical support for the body, yet protects multiple anchor points for skeletal muscle attachment. E.g. vertebrae, facial bones, pubis, ischium

General features of a long bone

Spongy bone

Spongy or cancellous bone has large open spaces for bone marrow. Red and white blood cells are produced here.

Epiphyseal plate

The site where longitudinal bone growth occurs in children and adolescents. Once growth is complete, it is called the epiphyseal line.

Image of a long bone (image labelled with the above names) 3.4

Medullary cavity

The central cavity of the bone shaft where red and yellow bone marrow (adipose tissue) is stored


A cellular layer that lines the marrow cavity inside a long bone’s shaft. The endosteum actively participates in bone growth and repair.


Connective tissue with a fibrous outer layer and a cellular (osteogenic) inner layer surrounding bone; actively participates in bone growth and repair.

Compact bone

Compact bone has a sturdy, calcified matrix with very few spaces. This layer not only forms a protective shell around the spongy bone tissue, but it also gives our bones their rigidity, strength and resistance.

Hyaline cartilage

The word hyaline means ‘glass like’ and bluey-grey in colour. It is the tissue covering the ends of bones promoting smoother movement, with less friction between the bones as they move. Hyaline cartilage is the most widespread cartilage in the body. It contains no nerves or blood vessels


The Epiphysis is the expanded end of the long bones, which ossifies separately from the bone shaft but becomes fixed to the shaft when full growth is attained. The epiphysis is made of spongy bone covered by a thin layer of compact bone.


The diaphysis is the shaft of a long bone. It is made up of cortical bone and usually contains bone marrow and adipose tissue.

Bone formation and growth across a lifespan

Bone formation

The bone formation process called ossification starts with cartilage becoming ossified as it is converted into bone as calcium is laid down. There are two stages of development:

  1. Primary – bone formation during foetal development.
  2. Secondary – bone formation from birth to adulthood.

Bone tissue is composed of various inorganic and organic substances such as calcium, phosphate, and collagen.

The mineral calcium is of particular importance to bone health and together with the protein collagen, gives bones their strength and flexibility.

Stages of bone growth: primary and secondary (step 1-3 foetal development)

Foetal development 

Step 1: 0-2 months

During the first two months in the womb, the immature skeleton is formed and wholly composed of hyaline cartilage.

Step 2: Three months

Blood vessels grow around the edges of the cartilage (perichondrium), Cartilage cells convert to osteoblasts, creating a thin layer of bone along the outer shaft.

Step 3: Nine months

Blood vessels increase and invade, supplying nutrients to the developing bone’s central region. Bone formations spread along the shaft towards the bone end in ‘primary ossification’.

Step 4: Birth 

Bone remodelling and growth continue; the bone becomes longer, creating the central marrow cavity. The bone becomes thicker, and the epiphyses cartilage is replaced by bone called ‘secondary ossification’.

Stages of bone growth: primary and secondary (step-4 birth)

Step 5: Childhood

Cartilage remains only on the bone ends and in the growth plates. The shaft continues to develop with the bone becoming longer and thicker.

Bone mass changes across a lifespan 

Bone remodelling continually occurs throughout our lifespan. It is a dynamic process involving the balance between bone resorption (where bone tissue is removed from the skeleton) by cells called osteoclasts and bone deposition (bone tissue formation) by cells called osteoblasts.

At around 10-15 years of age, girls and boys go through puberty, where rapidly rising sex hormones influence bone growth. In girls, it is the hormone oestrogen that greatly accelerates bone mass development resulting in a stature change. The hormone testosterone for boys causes more significant bone mass development. By age twenty, 90% of bone mass is achieved in both girls and boys.

Bone mass increases a further 10% during the third decade of life. From the age of 30, bone mass starts to gradually fall in both men and women due to declining levels of sex hormones.

Men can achieve a higher bone mass than women due to several factors such as a larger skeleton, greater overall muscle mass and ten times the testosterone level.

Hormone levels continue to fall as men and women age, resulting in further bone mass reductions. However, women experience an acceleration of bone loss around 50 due to the female menopause

Bone Mass Changes Across a Lifespan

Bone remodelling process

Bone is a dynamic tissue subjected to continuous renewing during everyone’s life by bone remodelling. This physiological process is necessary because of the following:

  • It allows the replacement of infantile bone with secondary bone, which is more mechanically competent
  • To remove damaged or micro fractured bone
  • As part of normal calcium homeostasis.

Bone remodelling is accomplished according to the following phases.

Bone Remodelling

1. Activation phase. Pre-osteoclasts are attracted to the remodelling sites. Pre-osteoclasts eventually form into osteoclasts. 

2. Resorption phase. Osteoclasts attach to the bone surface and begin to dissolve bone. They dig out a cavity, called a resorption pit, in spongy bone or burrow a tunnel in compact bone. Resorption requires two steps: 

  • Step 1: Acidification of the bone matrix to dissolve the inorganic component 
  • Step 2: There is a release of enzymes to degrade the organic component of bone, and calcium is released into the blood for various body functions. Once accomplished their function, osteoclasts undergo apoptosis (Cell death). Cell death is a physiological consequence needed to avoid excessive bone resorption.

3. Reverse phase. Macrophage-like cells function to remove debris produced during matrix degradation. Mesenchymal stem cells, precursors to osteoblasts, appear along the burrow or pit, where they increase in numbers and change into pre-osteoblasts.

4. Formation phase. Osteoblasts accumulate at the surface of the burrow or pit. Osteoblasts release a compound called osteoid at the site, forming a new soft non mineralised (not calcified) matrix. The new matrix is mineralized with calcium and phosphorus, and the new bone is complete.

5. Quiescence. The site of the new bone remains dormant until the next cycle. 

Calcium regulation

99% of calcium (average 1.2kg) in the body is deposited in the bones; the other 1% is a necessary component of other body tissues such as muscle. In effect, our bones act as our calcium reservoirs and can take up or release calcium.

Calcium regulation must be tightly controlled to maintain vital life-sustaining physiological functions such as the heart beating, the nervous system’s function, and muscle contractions. 1% is more important than the 99%.

Calcium regulation is maintained by a pair of hormones with opposing effects, and together they regulate the storage, absorption and excretion of calcium in the body.

  1. Parathyroid hormone (also known as PTH)
  • PTH secretion is stimulated by hypocalcaemia, and it works through three mechanisms to increase calcium levels:
  • PTH stimulates the release of calcium from bone.
  • PTH decreases urinary loss of calcium, stimulating calcium reabsorption.
  • PTH indirectly stimulates calcium absorption in the small intestine by stimulating a derivative of vitamin D in the kidney.
  1. Calcitonin secretion, on the other hand:
  • Lowers blood calcium levels have risen to above-average levels by suppressing osteoclast (cells that break down bone) activity in the bones.

The composition of bone

PTH/calcitonin regulation

Affecting Bone Mass



Heredity has a role in bone density.

Researchers have identified a gene that can account for up to 75% of bone density’s total effect.

Other non-modifiable factors include:

  • Age
  • Gender
  • Ethnicity





Adequate nutrition is paramount to good bone health.

Bone growth cannot occur without regular intake of foods containing:

  • Calcium
  • Phosphate
  • Magnesium
  • Vitamin C
  • Vitamin A
  • Vitamin K & B12
  • Vitamin D

Consume a variety of foods to include plenty of dark green leafy vegetables



Many hormones regulate bone density and are Essential for bone health. The following hormones are involved in the regulation of bone growth and maintenance:

  • Calcitonin
  • Parathyroid hormone
  • Testosterone
  • Oestrogen
  • Calcitriol
  • Growth hormone
  • Thyroxine

Hormone production, in many cases, is dependent on nutritional intake.



Mechanical forces are needed to increase and Maintain bone strength across a lifespan.

Regular weight-bearing physical activity alongside resistance training exercise and short bouts of high-impact activity during childhood increases bone mass.

It also enhances the bone’s structural characteristics that contribute to overall bone strength.

Physical activity in adulthood serves to maintain bone mass and strength and in later life, reduces the risk of losing bone mass.

The spine and posture

Spine curve development

The spine has 33 vertebrae, nine fused and 24 moveable segments. Each vertebra varies in shape and size, with the largest segments in the lumbar area and the smallest in the cervical area.

The spine is divided into five regions:

Cervical (7)

Thoracic (12)

Lumbar (5)

Sacrum (5)

Coccyx (4)

Vertebrae form four natural curves. There are two anteriorly convex (backward bending) curves, and two anteriorly concave (forward bending) curves. Separating each vertebra are vertebral discs which consist of fibrocartilage. The discs function to allow some movement to occur between vertebral segments whilst acting as shock absorbers during movement.

There is only a single anterior concave curve during foetal development, which is referred to as the primary curve. When the child is about three months old and can hold their head erect, the cervical curve develops. The lumbar curve develops later when the child stands and walks. Due to the cervical and lumbar curves developing in the postnatal period, they are secondary curves.

The spine is a flexible column capable of moving in all directions. When viewed from the side, it becomes evident that the spine is curved, with different areas curving either in a forward or backward direction.

Side view of spine labelled

There are four natural curves of the spine: two concaves, two convex.

The neck or cervical spine, curves gently inward (lordosis) or convex anteriorly

The mid-back, or thoracic spine, is curved outward (kyphosis) or concave anteriorly.

The low back, or lumbar spine, also curves inward (lordosis) or convex anteriorly.

The spine functions to:

  1. Provide axial rigidity to maintain an erect posture
  2. Allow mobility of the head, neck and trunk in space.
  3. Support and transmit loads from the upper body to the pelvis
  4. Offer points of attachment for muscles
  5. Absorb shock from activity
  6. Protect the spinal cord


Side view of spine labelled

Movements on the spine

[IMAGE 16]

Flexion (bending forwards)

[IMAGE 17]

Extension (bending backwards)

[IMAGE 18]

Lateral flexion right (side bending to the right)

[IMAGE 19]

Lateral flexion left (side bending to the left)

[IMAGE 20]

Rotation right (twisting to the right)

[IMAGE 21]

Rotation left (twisting to the left)

What is good posture?

When instructing exercise, look to maintain the spine’s posture in a neutral position or alignment, especially during heavy weightlifting exercises such as squatting and deadlifting.

Assessing an individual’s spinal alignment is quickly done when viewed from the side. Ask the individual to stand next to a plumb line. The line should:

  1. Pass through the ear and the acromion process of the shoulder
  2. Bisect the chest symmetrically.
  3. Pass slightly anterior to the sacroiliac joint and posterior to the hip joint
  4. Pass slightly anterior to the midline of the knee
  5. Pass slightly anterior to the lateral malleolus of the ankle

Side view with plumb line

Correct standing posture (neutral spine alignment)

It is essential to hold a good posture when exercising. During exercise, a good posture is when the spine’s standard curves are maintained, allowing the muscles to work more efficiently whilst reducing any unnecessary strain on joints and ligaments, and in the case of the spine, keep it healthy.

[IMAGE 23]

Squat or the deadlift

Spine abnormalities

Spinal posture can become exaggerated or excessive where any (or all) of the spinal curves deviate from the spine’s natural posture.

Reasons for the spine deviating from standard include hereditary factors, lifestyle factors such as sitting, driving, gaming (and other sedentary activities), psychological factors (e.g. confidence), sport and activity (e.g. muscle balance and imbalance), and the effects of pregnancy.

Postural deviations or spine abnormalities typically fall into one or more of the four categories below.

Lordosis – also known as hollow back.

The lumbar curve is increased during pregnancy due to the growing baby’s weight and weakening of the abdominals.


Kyphosis – An increased thoracic curve characterises kyphosis in a forward direction.

Typically, the person will have rounded shoulders and a protruding

head. Lifestyle factors, including being sedentary and being overweight, are thought, in part, to be the cause.


Flatback – there is flattening and decreasing of the lumbar curve. 

These changes are thought to be associated with a sedentary lifestyle.



A sideways curvature characterises scoliosis, either ‘S’ or ‘C’ shaped. It is usually genetic, affecting more young girls than boys. Typically, there will be a restricted range of motion in the spine. It can be visually apparent as one shoulder usually is higher than the other when the subject is viewed from behind.