Skeletal System and Bone Function

Getty Images/ROGER HARRIS/Science Photo Library

The skeletal system supports and protects the body while giving it shape and form. This system is composed of connective tissues including bone, cartilage, tendons, and ligaments. Nutrients are provided to this system through blood vessels that are contained within canals in bone. The skeletal system stores minerals, fats, and produces blood cells. Another major role of the skeletal system is to provide mobility. Tendons, bones, joints, ligaments and muscles work in concert to produce various movements. 

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Skeleton Components

Skeletal X-Ray Shoulder
Skeletal System, Colored X-ray of normal shoulder. DR P. MARAZZI/Science Photo Library/Getty Images

The skeleton is composed of fibrous and mineralized connective tissues that give it firmness and flexibility. It consists of bone, cartilage, tendons, joints, and ligaments.

  • Bone - a type of mineralized connective tissue that contains collagen and calcium phosphate, a mineral crystal. Calcium phosphate gives bone its firmness. Bone tissue may be compact or spongy. Bones provide support and protection for body organs.
  • Cartilage - a form of fibrous connective tissue that is composed of closely packed collagenous fibers in a rubbery gelatinous substance called chondrin. Cartilage provides flexible support for certain structures in adult humans including the nose, trachea, and ears.
  • Tendon - a fibrous band of connective tissue that is bonded to bone and connects bone to bone.
  • Ligament - a fibrous band of connective tissue that joins bones and other connective tissues together at joints.
  • Joint - a site where two or more bones or other skeletal components are joined together.

Skeleton Divisions

Bones are a major component of the skeletal system. Bones that comprise the human skeleton are divided into two groups. They are the axial skeletal bones and appendicular skeletal bones. An adult human skeleton contains 206 bones, 80 of which are from the axial skeleton and 126 from the appendicular skeleton.

Axial Skeleton
The axial skeleton includes bones that run along the medial sagittal plane of the body. Imagine a vertical plane that runs through your body from front to back and divides the body into equal right and left regions. This is the medial sagittal plane. The axial skeleton forms a central axis that includes bones of the skull, hyoid, vertebral column, and thoracic cage. The axial skeleton protects numerous vital organs and soft tissues of the body. The skull provides protection for the brain, the vertebral column protects the spinal cord, and the thoracic cage protects the heart and lungs.

Axial Skeleton Components

  • Skull - includes bones of the cranium, face, and ears (auditory ossicles).
  • Hyoid - U-shaped bone or complex of bones located in the neck between the chin and larynx.
  • Vertebral Column - includes spinal vertebrae.
  • Thoracic Cage - includes ribs and sternum (breast bone).

Appendicular Skeleton
The appendicular skeleton is comprised of body limbs and structures that attach limbs to the axial skeleton. Bones of the upper and lower limbs, pectoral girdles, and pelvic girdle are components of this skeleton. Although the primary function of the appendicular skeleton is for bodily movement, it also provides protection for organs of the digestive system, excretory system, and reproductive system.

Appendicular Skeleton Components

  • Pectoral Girdle - includes shoulder bones (clavicle and scapula).
  • Upper Limbs - includes bones of the arms and hands.
  • Pelvic Girdle - includes hip bones.
  • Lower Limbs - includes bones of the legs and feet.
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Skeletal Bones

Bone Marrow Broken Finger
This colored scanning electron micrograph (SEM) is showing the internal structure of a broken finger bone. Here, the periosteum (outer bone membrane, pink), compact bone (yellow) and bone marrow (red), in the medullary cavity, can be seen. STEVE GSCHMEISSNER/Science Photo Library/Getty Images

Bones are a type of mineralized connective tissue containing collagen and calcium phosphate. As a component of the skeletal system, a major function of bone is to assist in movement. Bones work in concert with tendons, joints, ligaments and skeletal muscles to produce various movements. Nutrients are provided to bone through blood vessels that are contained within canals in bone.

Bone Function

Bones provide several important functions in the body. Some major functions include:

  • Structure - Bones compose the skeleton, which provides structure and support for the body.
  • Protection - Bones provide protection for numerous vital organs and soft tissues of the body. For example, the vertebral column protects the spinal cord, and the thoracic (rib) cage protects the heart and lungs.
  • Mobility - Bones work in conjunction with skeletal muscle and other skeletal system components to assist in enabling body movement.
  • Blood Cell Production - Blood cells are produced by bone marrow. Bone marrow stem cells develop into red blood cells, white blood cells, and platelets.
  • Storage - Bones store important minerals and mineral salts, including calcium, phosphorus, and calcium phosphate. Calcium phosphate gives bone its firmness. Bone also stores fat in yellow bone marrow.

Bone Cells

Bone consists primarily of a bone matrix, which is composed of collagen and calcium phosphate minerals. Bones are constantly being broken down and rebuilt to replace old tissue with new tissue in a process called remodeling. There are three main types of bone cells that are involved in this process.

  • Osteoclasts
    These large cells have several nuclei and function in resorption and the assimilation of bone components. Osteoclasts attach to bone surfaces and use acids and enzymes to decompose bone.
  • Osteoblasts
    Osteoblasts are immature bone cells which form bone. They help to control bone mineralization and produce the proteins needed for bone formation. Osteoblasts produce osteoid (organic substance of bone matrix), which mineralizes to form bone. Osteoblasts may develop into osteocytes or into lining cells, which cover bone surfaces.
  • Osteocytes
    Osteocytes are mature bone cells. They have long projections that keep them in contact with each other and with lining cells on the bone surface. Osteocytes assist in bone and matrix formation. They also aid in maintaining a proper blood calcium balance.

Bone Tissue

There are two primary types of bone tissue: compact bone and cancellous bone. Compact bone tissue is the dense, hard outer layer of bone. It contains osteons or haversian systems that are tightly packed together. An osteon is a cylindrical structure consisting of a central canal, the Haversian canal, which is surrounded by concentric rings (lamellae) of compact bone. The Haversian canal provides a passageway for blood vessels and nerves. Cancellous bone is located within compact bone. It is spongy, more flexible, and less dense than compact bone. Cancellous bone typically contains red bone marrow, which is the site of blood cell production.

Bone Classification

Bones of the skeletal system can be classified into four major types. They are categorized by shape and size. The four main bone classifications are long, short, flat and irregular bones. Long bones are bones that have greater length than width. Examples include arm, leg, finger, and thigh bones. Short bones are almost the same in length and width and are close to being cube shaped. Examples of short bones are wrist and ankle bones. Flat bones are thin, flat, and typically curved. Examples include cranial bones, ribs, and the sternum. Irregular bones are atypical in shape and can not be classified as long, short, or flat. Examples include hip bones, cranial bones, and vertebrae.

Source:

  • SEER Training Modules, Introduction to the Skeletal System. U. S. National Institutes of Health, National Cancer Institute. Accessed 01/18/2013 (http://training.seer.cancer.gov/anatomy/skeletal/)