Study Guide: The Skeletal System – Part I

The Skeletal System

General Functions and Structures

The skeletal system is a vital organ system that provides structural support, protection, movement, mineral storage, and blood cell production. It consists of bones, cartilage, ligaments, and joints.

• Support: Bones form the framework that supports the body and maintains its shape.

• 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 to produce movement.

• Mineral Storage: Bones store minerals such as calcium and phosphorus.

• Blood Cell Production: Hematopoiesis occurs in the red bone marrow.

Number of Bones in the Human Body

The average adult human skeleton consists of 206 bones. Infants have more bones, which fuse during development.

• Axial skeleton: 80 bones (skull, vertebral column, rib cage)

• Appendicular skeleton: 126 bones (limbs and girdles)

Physiological Aspects of the Skeletal System

The skeletal system provides support and protection for the body, facilitates movement, and maintains mineral homeostasis.

• Support and Protection: Bones support body weight and protect vital organs.

• Movement: Bones serve as attachment points for muscles.

• Mineral Homeostasis: Bones release or absorb minerals to maintain balance.

Anatomy of Bone

Compact Bone

Compact bone is dense and forms the outer layer of bones, providing strength and protection.

• Structure: Composed of osteons (Haversian systems) with concentric lamellae.

• Function: Supports weight and withstands stress.

Spongy Bone

Spongy bone (cancellous bone) is found at the ends of long bones and inside flat bones. It is less dense and contains trabeculae.

• Structure: Network of trabeculae with spaces filled with bone marrow.

• Function: Reduces bone weight and supports hematopoiesis.

Bone Microanatomy Structures

Bones are composed of several microscopic structures that contribute to their function.

• Lacunae: Small spaces housing osteocytes.

• Lamellae: Layers of bone matrix.

• Canaliculi: Tiny channels connecting lacunae for nutrient exchange.

Cancellous Bone

Cancellous bone is synonymous with spongy bone and is characterized by a porous structure.

• Location: Ends of long bones, inside vertebrae.

• Function: Supports bone marrow and reduces bone mass.

Trabeculae

Trabeculae are the lattice-like structures in spongy bone that provide structural support and house bone marrow.

• Relation to Spongy Bone: Form the framework of spongy bone.

Epiphysis and Diaphysis

Bones have distinct regions with specific functions.

• Epiphysis: The ends of long bones, containing spongy bone and red marrow.

• Diaphysis: The shaft of long bones, composed mainly of compact bone.

Osteoclasts and Osteoblasts

Bone remodeling is regulated by two main cell types:

• Osteoclasts: Break down bone tissue (bone resorption).

• Osteoblasts: Build new bone tissue (bone formation).

• Maintenance: Balance between these cells maintains bone health.

Purpose of Red Bone Marrow

Red bone marrow is responsible for hematopoiesis, the production of blood cells.

• Location: Found in spongy bone of flat bones and epiphyses of long bones.

• Function: Produces red blood cells, white blood cells, and platelets.

Bone Growth and Repair

Bones grow and repair through a process involving cell division and mineralization.

• Ossification: Formation of bone tissue.

• Remodeling: Continuous renewal of bone.

• Repair: Healing after fractures involves hematoma formation, callus formation, and remodeling.

Epiphyseal Plate/Cartilage

The epiphyseal plate (growth plate) is a region of cartilage that allows bones to grow in length during childhood and adolescence.

• Function: Site of longitudinal bone growth.

• Closure: Plate ossifies and becomes the epiphyseal line in adults.

Vitamins and Bone Health

Certain vitamins are essential for healthy bones.

• Vitamin D: Facilitates calcium absorption.

• Vitamin C: Required for collagen synthesis.

• Vitamin A: Supports osteoblast function.

Minerals Needed by the Skeletal System

Bones require minerals for strength and function.

• Calcium: Main mineral in bone matrix.

• Phosphorus: Combines with calcium for bone rigidity.

• Magnesium: Involved in bone structure.

Fetal Bone Development

Bone development in the fetus occurs through two processes:

• Endochondral ossification: Bone forms from cartilage models (most bones).

• Intramembranous ossification: Bone forms directly from mesenchymal tissue (flat bones).

Exercise and Bone Health

Physical activity influences bone strength and density.

• Weight-bearing exercise: Stimulates bone formation and increases bone mass.

• Inactivity: Leads to bone loss and increased fracture risk.

Bone Identification

Students should be able to identify the major bones of the human body, including the skull, vertebrae, ribs, pelvis, and limb bones.

Rickets

Rickets is a disease caused by vitamin D deficiency, leading to soft and weak bones in children.

• Symptoms: Bone pain, deformities, delayed growth.

• Prevention: Adequate vitamin D and calcium intake.

Regulation of Calcium Levels

Calcium levels in the blood are regulated by hormones.

• Parathyroid hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Lowers blood calcium by inhibiting bone resorption.

• Vitamin D: Enhances calcium absorption from the gut.

Effect of Sex Hormones on Bone Growth

Sex hormones play a significant role in bone growth and maintenance.

• Estrogen and Testosterone: Stimulate bone growth during puberty.

• Hormonal Changes: Decreased estrogen (e.g., menopause) leads to bone loss and increased risk of osteoporosis.

Summary Table: Key Bone Structures and Functions

Structure	Location	Function
Compact Bone	Outer layer of bones	Strength, protection
Spongy Bone	Ends of long bones, inside flat bones	Lightweight, supports marrow
Red Bone Marrow	Spongy bone regions	Blood cell production
Epiphyseal Plate	Between epiphysis and diaphysis	Bone length growth
Osteoblasts	Bone surface	Bone formation
Osteoclasts	Bone surface	Bone resorption

Key Equations

• Calcium Homeostasis: $\text{Blood Calcium Level} = \text{Intestinal Absorption} + \text{Bone Resorption} - \text{Renal Excretion}$

Additional info: Some explanations and examples have been expanded for academic completeness and clarity.