Functions of Bone

Overview of Bone Functions

Bones are dynamic organs that serve multiple essential functions in the human body. They provide structural support, protect vital organs, enable movement, and play key roles in mineral storage, blood cell production, and energy storage.

• Support: Bones support soft tissues and maintain the shape of the body.

• Protection: Bones protect internal organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Bones act as levers for muscles, forming joints and providing points of muscle attachment.

• Mineral Storage: Bones store minerals, mainly calcium and phosphorus, which are released into the bloodstream as needed.

• Production of Blood Cells: Red bone marrow produces red blood cells, white blood cells, and platelets (hematopoiesis).

• Energy Storage: Yellow bone marrow stores fat (adipose tissue), serving as an energy reserve.

Types of Bones

Classification of Bones by Shape

Bones are classified into four main types based on their shapes and functions. This classification helps in understanding their roles in the skeletal system.

• Long Bones: Longer than they are wide; found in limbs (e.g., femur, humerus).

• Short Bones: Roughly cube-shaped; found in wrist (carpals) and ankle (tarsals). Includes sesamoid bones (e.g., patella) that develop in tendons.

• Flat Bones: Thin, flat, and often curved; found in cranial bones, ribs, sternum, and scapula.

• Irregular Bones: Complex shapes; found in vertebrae and hip bones.

Structure of a Long Bone

Anatomy of Long Bones

Long bones have a specialized structure that supports their function in movement and weight-bearing. The main regions include the diaphysis, epiphyses, and various membranes.

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

• Epiphyses: The ends of the bone, which articulate with other bones.

• Articular Cartilage: Hyaline cartilage covering the epiphyses, reducing friction and absorbing shock.

• Periosteum: A double-layered membrane covering the bone except at joint surfaces. Contains:

  • Outer Fibrous Layer: Dense irregular connective tissue.

  • Inner Osteogenic Layer: Contains bone-forming cells, blood vessels, and nerves.

• Functions of Periosteum:

  • Bone growth in thickness

  • Repair of fractures

  • Protection and nourishment

  • Attachment for ligaments and tendons

• Medullary Cavity: Central cavity in the diaphysis containing yellow marrow.

• Endosteum: Thin membrane lining the medullary cavity and spongy bone, containing bone cells.

Additional info: "Peri" means around/surrounding; "Endo" means inside.

Histology of Bone Tissue

Bone Cells and Their Functions

Bones are living tissues composed of specialized cells that maintain, build, and break down bone matrix. The three main types of bone cells are:

• Osteoblasts: Bone-forming cells that secrete collagen and other components of bone matrix. Found on inner and outer bone surfaces (endosteum and periosteum).

• Osteocytes: Mature bone cells that maintain bone tissue. Located in cavities called lacunae and connected by tiny canals called canaliculi for nutrient and waste exchange.

• Osteoclasts: Bone-resorbing cells that break down bone matrix. Derived from white blood cells and found on bone surfaces.

Bone Matrix Composition

Components of Bone Matrix

The bone matrix is a composite material that provides both strength and flexibility. It consists of water, protein fibers, and mineral salts.

• 25% Water

• 25% Protein Fibers: Mainly collagen, which gives bones flexibility and tensile strength.

• 50% Mineral Salts: Mainly calcium phosphate and calcium carbonate (hydroxyapatite), which provide hardness.

Key Equation:

Compact Bone Structure

Organization of Compact Bone

Compact bone forms the outer layer of all bones and the bulk of the diaphysis in long bones. Its structure is highly organized into units called osteons or Haversian systems.

• Osteons: Cylindrical structures composed of concentric layers (lamellae) of bone matrix.

• Lamellae: Multiple layers of matrix arranged around a central canal.

• Lacunae: Small spaces containing osteocytes, located between lamellae.

• Canaliculi: Tiny canals connecting lacunae, allowing for exchange of nutrients and waste.

• Central Canal: Contains blood vessels and nerves.

Visual Description: Osteons outline circles, creating negative spaces (lacunae) connected by canaliculi filled with extracellular fluid.

Summary Table: Bone Structure and Function

Additional info: These notes provide a comprehensive overview of bone structure, function, and histology, suitable for college-level Anatomy & Physiology students.