The Skeletal System: Structure, Function, and Types of Bones
The skeletal system performs several crucial functions, including:
- Mechanical Support: Provides a framework for the body and its soft tissues, maintaining body morphology.
- Postural Maintenance: Enables the body to maintain positions like standing.
- Dynamic Support: Facilitates gait, locomotion, and body movements by acting as levers and attachment points for muscles.
- Containment and Protection: Safeguards vital organs from external pressure or impact, such as the ribs protecting the lungs.
- Metabolic Storage: Acts as a buffer and moderates the exchange of salts like calcium and phosphate.
- Vibration Transmission: Transmits vibrations throughout the body.
Additionally, the spongy bone marrow within certain bones is responsible for hematopoiesis, or blood cell differentiation.
Types of Bones
Bones are classified into three types based on their shape and size:
Flat Bones
These bones offer protection to vital organs and provide broad surfaces for muscle attachment. Examples include the ribs, sternum, and skull bones.
Long Bones
These bones support weight and facilitate movement. Examples include the bones of the upper and lower limbs.
Short Bones
These bones are resilient and capable of withstanding significant force, but their movement is limited. Examples include the vertebrae and some bones of the hands and feet.
Bone Structure and Function
The longest bone in the human body is the femur, extending from the pelvic girdle to the knees. The shortest bone is the stapes in the ear, measuring a mere 2.5 millimeters.
Bone minerals are inert components but are constantly exchanged and replaced with organic components in a process called bone remodeling. This process is regulated by hormones and nutrients that provide essential vitamins for proper tissue function. Bone tissue is remarkably strong, resistant to shocks and pressure, yet also elastic. It protects vital organs like the heart, lungs, and brain, while also enabling movement for various activities. The skeletal system forms the bone structure, or skeleton, and serves as a reservoir for calcium and phosphorus.
Bones are composed of living connective tissue and are classified as short, long, flat, or irregular. For example, arm and leg bones are long bones, facial and vertebral bones are irregular, and skull bones are flat.
Types of Bone Tissue
Bones have areas with varying bone tissue density, macroscopically and microscopically differentiated into compact and cancellous (spongy) bone. These areas lack sharp boundaries and transition seamlessly into each other.
Compact Bone
Compact bone forms the shaft of long bones, appearing as a solid, continuous mass. Its mineralized bone matrix is deposited in lamellae.
Cancellous Bone (Spongy Bone)
Cancellous bone lacks osteons, instead featuring irregularly shaped interstitial plates called trabeculae. These plates create a spongy structure with spaces filled with red bone marrow. Spongy bone is a component of the epiphysis of long bones and the interior of other bones.
Bone Tissue Composition
- Organic Matrix: Constitutes 10% of bone tissue and contains a lower concentration of glycosaminoglycans (GAGs) compared to cartilage. It includes unique bone proteins like osteocalcin and osteopontin, both bound to hydroxyapatite.
- Collagen: Makes up 90% of the organic matrix. It is type 1 collagen with numerous intermolecular bonds, insoluble in most solvents, and features hydroxylation of lysine.
- Inorganic Substances: Primarily calcium phosphate in the form of hydroxyapatite crystals. Also contains citrate, bicarbonate, fluoride, magnesium, and sodium ions. Bone has an affinity for radioactive substances, which can damage its components.
Divisions of the Skeleton
Axial Skeleton
- Skull: Comprises cranial, facial, and orbital bones, as well as skull openings.
- Hyoid Bone: A U-shaped bone in the neck that supports the tongue.
- Torso or Trunk: Consists of the vertebrae, sacrum, spine, and thorax (sternum and ribs).
Appendicular Skeleton
- Shoulder Girdle: Formed by the clavicle and scapula.
- Upper Extremity: Includes the humerus, ulna, radius, carpals, metacarpals, and phalanges.
- Pelvic Girdle: Includes the innominate or iliac bones.
- Lower Extremity: Includes the femur, fibula, tibia, patella, tarsals, metatarsals, and phalanges.