Skeletal System and bones overview 2024

The Skeletal System Overview

The human skeletal system is an intricate framework that provides structure, support, and protection to the body, while also facilitating movement and blood cell production.

Learning Outcomes

In this module, after completing the session, you are expected to:

  1. Describe the structure and function of various cells and tissues in the human body.

  2. Identify and articulate the structure of major body systems.

  3. Relate normal physiology of body systems to their functions.

  4. Discuss the normal function of all major body systems and understand pathologies that occur.

Key Questions About Bones

  • What are bones?

  • What is the composition of bones?

  • How are bones formed?

  • What are the primary functions of the skeletal system?

Structure of the Skeletal System

The skeletal system is divided into two main parts:

  • Axial Skeleton

  • Appendicular SkeletonBone itself is a dynamic living tissue composed of minerals, proteins, blood, nerves, and fatty bone marrow. The functions of bone include facilitating movement, protecting soft tissues, supporting the body, producing blood cells, and storing inorganic salts.

Axial Skeleton

The axial skeleton is primarily involved in protection and consists of:

  • Skulls (28 bones: 8 cranial and 22 facial)

  • Hyoid bone (1)

  • Inner ear (6 bones)

  • Spinal column (26 bones: cervical (7), thoracic (12), lumbar (5), sacrum (1), coccyx (1))

  • Thoracic cage (25 bones: 24 ribs and sternum (1)) This skeleton plays a crucial role in protecting vital organs such as the brain, spinal cord, and thoracic organs.

Appendicular Skeleton

The appendicular skeleton supports limb attachment and movement, consisting of:

  • Shoulder/pectoral girdle (4 bones)

  • Upper limbs (6 bones)

  • Hands (54 bones)

  • Pelvis (6 bones)

  • Lower limbs (8 bones)

  • Feet (52 bones)

Types of Bones

Understanding the classification of bones is crucial:

  • Long Bones: Longer than wide, e.g., femur; essential for movement and weight-bearing.

  • Short Bones: Cube-shaped, e.g., wrist and ankle bones; provide support.

  • Flat Bones: Protect organs, e.g., skull; serve as muscle attachment sites.

  • Sesamoid Bones: Found within tendons, e.g., patella; add strength.

  • Irregular Bones: Complex shape, e.g., pelvic bones; protect organs and serve as muscle attachment points.

Functions of Bones

The skeletal system serves five primary functions:

  1. Support: Maintains body shape and supports structural weight.

  2. Movement: Facilitates mobility through the muscular system.

  3. Protection: Shields vital organs, such as the brain and heart.

  4. Storage: Stores minerals and lipids.

  5. Blood Cell Production: Hematopoiesis occurs in red marrow found in various bones.

Blood Cell Production (Hematopoiesis)

In adults, blood cells originate from red marrow located in the medullary cavity of long bones and spongy bones. Primary sites include the skull, sternum, ribs, vertebrae, and femoral head, producing:

  • Red blood cells (erythrocytes)

  • White blood cells (leucocytes)

  • Platelets

Storage of Inorganic Salts

Bones serve as reservoirs for inorganic salts, including calcium, potassium, sodium, and magnesium. Calcium plays a critical role in metabolic processes such as muscle contraction and nerve function, regulated by hormones like parathyroid hormone which signals osteoclast activity to release calcium into the bloodstream when levels are low.

Long Bone Structure

Long bones, such as those in limbs, have distinct structures:

  • Epiphyses: Ends of the bone, composed of spongy bone and covered by articular cartilage.

  • Diaphysis: The shaft, primarily made of compact bone.

  • Epiphyseal Plate: Growth plate critical for bone lengthening, becoming the epiphyseal line post-growth.

Layers of Long Bone

  • Periosteum: Outermost layer for protection and attachment for ligaments and tendons.

  • Medullary Cavity: Contains yellow marrow, lined with endosteum, facilitating bone growth and repair.

  • Connective Tissue: Endosteum is a vascular membrane that maintains the inner surface of long bones.

Compact vs. Spongy Bone

  • Compact Bone: Densely packed, strengthening the diaphysis.

  • Spongy Bone: Lighter and found in epiphyses, reducing weight without compromising strength.

Microscopic Structure of Bone

Bone consists of:

  • Lamellae: Thin layers surrounding blood vessels (Haversian canals).

  • Osteocytes: Bone cells in lacunae contributing to maintenance and repair.

  • Canaliculi: Channels for nutrient transport.

  • Osteons: The structural unit of compact bone, resembling tree rings.

Bone Development and Growth

  • Ossification (Osteogenesis): The process of bone formation, begins during embryonic development. Two types:

    1. Intramembranous Ossification: Forms flat bones such as the skull.

    2. Endochondral Ossification: Occurs in long bones, where hyaline cartilage is replaced with bone, beginning in the diaphysis.

Factors Affecting Bone Growth

Factors influencing bone development include:

  • Loading: Influence of physical activity on bone density.

  • Nutrition: Importance of vitamins and minerals, especially calcium and vitamin D.

  • Hormones: Roles of various growth-related hormones.

  • Genetics: Conditions affecting bone health like Osteogenesis Imperfecta.

  • Trauma: Impact of injuries on bone growth patterns.

Wolff's Law

Julius Wolff's principle states that bone adapts to the load placed on it, which promotes density increases as stress is applied through activities. Systematic loading influences cellular responses to maintain bone health.

Conclusion

Understanding the structure, classification, and functions of bones is critical for recognizing their roles in overall health and physiology, the processes of growth and adaptation, and the impact of various factors on bone integrity and function.