The Skeletal System: Structure, Function, and Anatomy

The Skeletal System

Functions of the Skeletal System

The skeletal system performs several essential functions that support and protect the human body.

• Support: Bears the weight of the body and provides a structural framework.

• Protection: Encases vital organs; for example, the ribcage protects the heart and lungs.

• Movement: Joints allow bones to move, enabling locomotion and manipulation.

• Storage: Stores minerals (such as calcium and phosphorus) to be released into the bloodstream; yellow bone marrow stores fat.

• Manufacturing: Produces red and white blood cells in red bone marrow, a process called hematopoiesis.

Classification and Structure of Bones

Bones are composed of a solid matrix of living cells and fibers, surrounded by calcium deposits. They are classified by shape, which relates to their function.

• Flat bones: Protection and muscle attachment (e.g., scapula).

• Irregular bones: Specialized support (e.g., vertebrae).

• Long bones: Leverage and movement (e.g., femur).

• Short bones: Stability with limited motion (e.g., wrist).

• Sesamoid bones: Embedded in tendons (e.g., patella).

Anatomical Features of a Long Bone

Long bones have distinct regions and specialized structures for growth and movement.

• Diaphysis: The shaft or long part of the bone.

• Epiphyses: The ends of the bone.

• Epiphyseal plate: A thin layer of internal cartilage between diaphysis and epiphyses, also known as the growth plate.

• Articular cartilage: Covers the ends of the epiphyses, providing smooth joint movement and cushioning from shock.

Internal Anatomy of a Long Bone

The diaphysis contains a hollow medullary cavity filled with bone marrow.

• Red bone marrow: Fills the cavity in young individuals; site of blood cell production (hematopoiesis).

• Yellow bone marrow: Replaces red marrow with age; stores fat.

Spongy Bone vs. Compact Bone

Bones have two main types of tissue: spongy bone and compact bone, each with distinct locations and functions.

• Periosteum: Tough connective tissue covering the outer layer of bone; site of muscle attachment and bone repair.

• Compact bone: Thick layer beneath the periosteum, providing strength for load bearing.

• Spongy bone: Located beneath the compact bone at the ends of long bones; provides lightweight support and shock absorption.

Structural Differences and Stress Distribution

Compact and spongy bone differ in structure and how they handle mechanical stress.

• Compact bone: Dense, arranged in cylinders called osteons; provides strength for load bearing.

• Spongy bone: Lattice of trabeculae ("little beams") aligned along stress lines for resistance to compression; spaces filled with marrow or blood vessels.

• Lamellae: Concentric circles within osteons, surrounding a central (Haversian) canal containing blood vessels and nerves.

• Perforating (Volkmann's) canals: Connect central canals perpendicularly.

Specialized Bone Cells

Bone tissue contains several specialized cell types, each with unique functions.

• Osteocytes: Mature bone cells that maintain bone tissue; connected by canaliculi for nutrient and waste exchange.

• Osteoblasts: Produce new bone matrix.

• Osteoclasts: Break down bone matrix, aiding in bone remodeling.

Bone Formation and Growth

Bone formation begins in the embryo and continues through childhood and adolescence.

• Cartilage model: Embryonic skeleton is initially made of cartilage.

• Ossification: Osteoblasts secrete mineral deposits, replacing cartilage with bone; process is called ossification.

• Chondrocytes: Cartilage cells at the epiphyseal plate divide and deteriorate as the matrix calcifies, then become osteoblasts to form spongy bone.

• Osteoclasts: Secrete acid to enlarge the medullary cavity, ensuring marrow availability.

Composition of Bone

Bones are composed of both organic and inorganic components, each contributing to bone properties.

Component	Percentage	Main Function	Example
Organic (Osteoid)	35%	Flexibility & tensile strength	Collagen
Inorganic (Mineral Salts)	65%	Strength & hardness	Hydroxyapatite: $Ca_{10}(PO_4)_6(OH)_2$

• Osteoid: Made of ground substance and collagen; lack of collagen causes "Brittle Bone Disease".

• Hydroxyapatite: Crystalline mineral salt; lack causes "Rickets".

Bone Remodeling and Homeostasis

Bone tissue is constantly remodeled to maintain calcium levels and adapt to stress.

• Calcitonin: Hormone from the thyroid gland; lowers blood calcium by promoting bone formation.

• Parathyroid Hormone (PTH): Hormone from the parathyroid glands; raises blood calcium by stimulating osteoclasts to break down bone.

Bone Homeostasis Cycle:

• If blood calcium is too high: Thyroid gland releases calcitonin → calcium is absorbed into bone → blood calcium decreases.

• If blood calcium is too low: Parathyroid gland releases PTH → osteoclasts break down bone to release calcium → blood calcium increases.

Bone Repair

Bone repair after fracture occurs in four main stages:

1. Hematoma formation: Blood enters the wound; cells begin to die; phagocytes ingest dead cells and debris.

2. Callus formation: Blood vessels grow; cartilage forms to hold bone together.

3. Callus ossification: Spongy bone replaces cartilage.

4. Compact bone formation: Osteoclasts form a larger medullary cavity; spongy bone is converted to compact bone.

Major Sections of the Skeleton

The human skeleton is divided into two major sections:

• Axial skeleton: Central axis of the body; includes skull, ribs, sternum, and vertebrae (80 bones).

• Appendicular skeleton: Pectoral and pelvic girdles; bones of the arms, legs, pelvis, and shoulders (126 bones).

Bones of the Axial Skeleton

The axial skeleton includes:

• Cranium

• Mandible

• Cervical vertebrae

• Thoracic vertebrae

• Lumbar vertebrae

• Sacrum

• Coccyx

• Ribs

• Sternum

• Manubrium

Structure of the Vertebral Column

The vertebral column extends from the skull to the pelvis, supporting and protecting the spinal cord.

• Consists of 33 vertebrae at birth; 5 sacral and 4 coccygeal vertebrae fuse in adolescence.

• Separated by intervertebral discs for cushioning and shock absorption.

• Sections:

  • Cervical vertebrae: 7

  • Thoracic vertebrae: 12

  • Lumbar vertebrae: 5

  • Sacral vertebrae: 5 (fused)

  • Coccyx: 4 (fused)

• Spine is convexly curved at birth; cervical and lumbar regions develop concave curves later in life.

• Primary (convex) and secondary (concave) curvatures allow for balance and weight distribution.

Bones of the Appendicular Skeleton

The appendicular skeleton includes:

• Pectoral girdle: Clavicle and scapula

• Pelvic girdle: Pelvis

• Upper limbs: Humerus, radius, ulna, carpals, metacarpals, phalanges

• Lower limbs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges

Joints: Structure and Types

Joints connect bones and allow for movement. They are classified by their structure:

• Fibrous joints: Immovable or slightly movable; held together by fibrous connective tissue.

• Cartilaginous joints: Immovable or slightly movable; held together by cartilage.

• Synovial joints: Highly movable; contain synovial fluid for frictionless movement.

Structure of Synovial Joints

• Joint capsule: Filled with synovial fluid, surrounds the ends of bones.

• Synovial membrane and articular cartilage: Line the joint cavity, providing smooth movement and cushioning.

• Ligaments: Connect bone to bone.

• Tendons: Connect muscle to bone.

Summary

The skeletal system provides support, protection, movement, storage, and blood cell production. Bones are classified by shape and have specialized structures and cells. Bone tissue is constantly remodeled and repaired, and the skeleton is divided into axial and appendicular sections, each with specific bones and functions. Joints connect bones and vary in structure and mobility.