Bone Structure and Function: Anatomy & Physiology Study Notes

Bone Structure and Function

Functions of the Skeletal System

The skeletal system is essential for multiple physiological processes and structural support in the human body.

• Support: Provides a rigid framework that supports the body and cradles soft organs.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Production: Houses bone marrow, which produces blood cells (hematopoiesis).

• Triglyceride Storage: Yellow bone marrow stores fat as an energy reserve.

Classification of Bones by Shape

Bones are classified according to their shapes, which relate to their functions and locations in the body.

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

• Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., skull bones, sternum, ribs).

• Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, pelvis).

• Sesamoid Bones: Small bones embedded within tendons (e.g., patella).

• Sutural (Wormian) Bones: Small bones found within the sutures of the skull.

Structure of Long Bones vs. Short, Flat, Irregular, and Sesamoid Bones

Bone structure varies according to shape and function.

• Long Bones:

  • Consist of a shaft (diaphysis) and two ends (epiphyses).

  • Medullary cavity within diaphysis contains bone marrow.

  • Covered by periosteum (dense connective tissue) and lined internally by endosteum.

  • Articular cartilage covers joint surfaces.

  • Composed of compact bone surrounding spongy bone at the ends.

• Short, Flat, Irregular, and Sesamoid Bones:

  • Do not have a shaft or epiphyses.

  • Consist of thin layers of compact bone surrounding spongy bone (diploë in flat bones).

  • Contain bone marrow but no medullary cavity.

  • Sesamoid bones are found within tendons and help reduce friction.

Red vs. Yellow Bone Marrow

Bone marrow exists in two forms, each with distinct functions.

• Red Bone Marrow:

  • Contains hematopoietic cells that produce red blood cells, white blood cells, and platelets.

  • Found in spongy bone of flat bones, epiphyses of long bones, and in children’s medullary cavities.

• Yellow Bone Marrow:

  • Contains adipocytes that store triglycerides (fat).

  • Found in the medullary cavity of long bones in adults.

Organic and Inorganic Components of the Extracellular Bone Matrix

The bone matrix is a composite material that provides both strength and flexibility.

• Organic Matrix (Osteoid):

  • Makes up about 35% of bone ECM.

  • Composed of collagen fibers, proteoglycans, glycosaminoglycans (GAGs), and glycoproteins.

  • Provides tensile strength and flexibility.

• Inorganic Matrix:

  • Makes up about 65% of bone ECM.

  • Composed mainly of hydroxyapatite crystals ($Ca_{10}(PO_4)_6(OH)_2$), bicarbonate, potassium, magnesium, and sodium.

  • Provides compressive strength and hardness.

Functions of Bone Tissue Cells

Bone tissue contains several cell types, each with specialized roles in bone formation and maintenance.

• Osteogenic Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-building cells that secrete osteoid and promote mineralization.

• Osteocytes: Mature osteoblasts trapped in the matrix; maintain bone tissue and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells derived from blood cell lineages; responsible for bone resorption by secreting enzymes and acids.

Structures of Compact and Spongy Bone

Bone tissue is organized into two main structural types, each with unique features.

• Compact Bone:

  • Dense and solid; forms the outer layer of bones.

  • Composed of osteons (Haversian systems), which are cylindrical structures with concentric lamellae.

  • Central canals contain blood vessels and nerves.

  • Provides strength for weight-bearing.

• Spongy Bone (Cancellous Bone):

  • Porous and lightweight; found at the ends of long bones and inside flat, short, and irregular bones.

  • Composed of trabeculae (bony struts) with spaces filled by bone marrow.

  • No osteons; blood supply comes from vessels in the surrounding compact bone.

  • Provides structural support and flexibility.

Table: Comparison of Bone Types by Shape

Bone Type	Shape	Example	Main Features
Long	Length > width	Femur, humerus	Diaphysis, epiphyses, medullary cavity
Short	Length ≈ width	Carpals, tarsals	Thin compact bone, spongy interior
Flat	Thin, broad	Skull, sternum, ribs	Diploë (spongy bone) between compact layers
Irregular	Complex shape	Vertebrae, pelvis	Varied structure
Sesamoid	Small, round	Patella	Embedded in tendons
Sutural	Small, flat	Skull sutures	Within cranial sutures

Example: Bone Marrow in Clinical Context

Bone marrow biopsies are used to diagnose blood disorders such as leukemia and anemia. Red marrow is especially important in children and in adults with increased hematopoietic demand.

Additional info: Some details, such as the chemical formula for hydroxyapatite and the clinical application of bone marrow, were inferred for completeness and academic context.