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The Skeletal System

The Skeletal system

  1. Main Functions of the Skeletal System
  2. Types of bone found in the skeletal system
  3. Types of Bone Cells
  4. Joints
  5. Synovial Joints
  6. Types of synovial joint

Main Functions of the Skeletal System

  • Support – the skeletal system provides a rigid supportive framework, giving us our characteristic shape and allowing us to cope with the force of gravity.
  • Protection – the bones of the skeletal system protect the delicate organs that they surround.  Examples include: the skull which surrounds and protects the brain; the vertebrae surrounds and protects the spinal chord; the rib cage protects the heart, lungs and spleen; and the pelvis protects the reproductive organs.
  • Movement – muscles are attached to bones by tendons (strong connective tissue).  The bones act as anchor that muscles can pull on in order to create movement.  Where two bones meet, a joint is formed, allowing movement between the two bones.
  • Storage – some minerals such as phosphorous and calcium are stored within bones.  If the blood levels of these minerals drops excessively then some of the stores within the bone can be released. Some fats are also stored within bones.
  • Blood cell production – red and white blood cells, along with platelets, are produced within the bone marrow (central part of bone).  Red blood cells are responsible for carrying oxygen from the lungs to working muscles, and vital organs such as the brain and heart.  White blood cells are part of the body’s immune system that fights infection.  Whilst, platelets play an important role in the clotting of open wounds.

Types of bone found in the skeletal system:

  • Long bones – longer than they are wide. They have four main components: 1) the daphysis – this is the shaft of the bone and is made of dense compact bone with a hard outer casing; 2) the epiphysis – this the end of the bone and consists of cancellous spongy bone with an outer layer of compact bone. The end of the bone is covered by articular cartilage; 3) the articular cartilage covers the end of the bone, protecting the ends of the bone from the damage of friction or impact injuries during movement; 4) the periosteum – this is the tough outer casing of bone made of a double layered connective sheath. An example of a long bones is the femur (thigh bone).  The main function of a long bone is as a lever for movement.
  • Short bones – about the same length as width. They have no diaphyses and are made of spongy bone with an outer layer of compact bone.  They have great strength but are less mobile than long bones.  Examples of short bones are the Tarsals (foot bones).
  • Flat bones – thin flattened shape. They usually have no diaphyses or epiphyses. They are made up of a framework of cancellous bone that is sandwiched between two layers of compact bone. There main functions include attachments for muscle and the protection of vital organs.  An example of flat bones ionclude the ribs.
  • Irregular – shapes do not fit into the above categories.  These are made of spongy bone with a thin covering of compact bone.  Their main function is to protect internal organs and to give support.  Examples include the vertebrae.

Types of Bone Cells:

  • Osteoblasts – They are responsible for the production of collagen (connective tissue found in bones, cartilage etc) and proteoglycans (also a connective tissue).  Osteoblasts are responsible for the formation of the mineralised bone matrix and hence the formation of bone.  This process is called ossification.
  • Osteocytes – this is when an osteoblast becomes surrounded by bone matrix and becomes a mature bone cell.  Osteocytes are less active than osteoblasts. However, they are still able to produce the components required to maintain a healthy bone matrix.
  • Osteoclasts – are the bone cells that are responsible for the breakdown of old or damaged bone. Osteoclasts may be assisted by the osteoblasts.

At birth our bones are made of cartilage.  It is through the process of ossification, in which calcium is deposited within the cartilage, that the cartilage hardens and eventually becomes bone.  When we reach the age of 25-30, osteoblast and osteoclast activity is equally active.  It is at this stage that we are said to have our peak bone density.  Amongst healthy individuals, who partake in regular physical activity, bones may still continue to get stronger into our 30’s.  Around the age of 35 osteoclast activity increases and osteoblast activity decreases.  This leads to reductions in the level of bone density.  As we loose bone density are bones weaken and are more likely to fracture.  In some individuals bone density can decrease to such a level that they are constantly a risk of suffering bone breakages or fractures from the slightest physical activity.  This is called brittle bone disease or osteoporosis.>


A joint is formed where two or more bones meet.  All joints are held together by ligaments.  The main function of ligaments is to connect to bones together, improve the stability of the joint by allowing some movement but reducing unwanted movement.

Types of joint:

  • Immovable or fibrous joint – where two bones are joined by connective tissue, there is no joint cavity (space between the bones filled with synovial fluid), and there is little or no movement. For example the bones of the pelvic girdle.
  • Slightly moveable or cartilaginous joint – the bones are connected by a ligament and cartilage, there is no joint cavity, and there is only limited movement.  For example bones of the vertebrae.
  • Freely moveable or synovial joints – there is a gap between the joints (joint cavity) containing synovial fluid. There is far greater movement between bones connected by synovial joints.  The extent of the movement varies due to the shape of the bones, and the ligaments and tendons that surround the joint.  An example is the knee joint.

Synovial Joints

All synovial joints have the following characteristics :

  • Joint Capsule – they are surrounded by a joint capsule made of fibrous connective tissue.
  • Cartilage – this is a smooth but strong tissue covering the ends of bones.  It protects bone ends from friction and impact damage by allowing them to glide across each other as well as absorbing impact forces.  During impact, the cartilage flattens thus spreading the impact over a larger surface area.
  • Synovial membrane – this is a thin layer that lines the inner surface of the joint capsule.  It secretes synovial fluid into the joint capsule.
  • Synovial Fluid – this acts as a lubricant within the joint as well as providing nutrients to the cartilage.
  • Ligaments – fibrous tissue responsible for holding bones together and maintaining stability
  • Tendons – are made of connective tissue and attach muscle to bone. Sometimes they run across a joint.  Where they run across a joint they will contribute to the range of movement at the joint.

Types of synovial joint:

  • Plain or gliding joints – two flat surfaces in which some movement, including slight rotation, is possible.  Examples include the vertebrae and bones in hand.
  • Ball and socket joints – this is where one bone has a ball end, which fits into a socket of adjacent bone.  Ball and socket joints allow a wide amount of movement in almost every direction (multiaxonal).  Examples include the shoulder and hip.
  • Hinge joints – allow movement in one direction only (monoaxial).  There will be a convex cylinder in one bone and a concavity in the opposing bone. Examples include the elbow and knee.
  • Pivot joints – movement is restricted to rotation around a single axis (monoaxial). An example is the radius and ulna.
  • Condyloid joints – these are modified ball and socket joints.  There movement is biaxial because the movement is restricted to two axis with rotation restricted. Examples include the wrist where flexion and extension can occur.
  • Saddle joints – these are biaxial joints.  One bone surface is concave the other convex.  An example is the joint at the base of the thumb.