Skeletal System: Structure, Function, and Bone Physiology Study Guide

Skeletal System

Functions of the Skeletal System

The skeletal system is a vital organ system that provides structure, protection, and support for the human body. It also plays key roles in movement, mineral storage, and blood cell production.

• Support: Provides a framework for the body and maintains its shape.

• 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 red and white blood cells (hematopoiesis).

• Energy Storage: Yellow bone marrow stores lipids as an energy reserve.

Bone Classification and Examples

Bones are classified based on their shapes, which relate to their functions.

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

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

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

• Flat Bones: Thin and often curved; protect internal organs and provide surfaces for muscle attachment (e.g., sternum, skull bones).

Parts of a Long Bone

Long bones have distinct anatomical regions, each with specific functions.

• Diaphysis: The shaft or central part of a long bone; composed mainly of compact bone.

• Epiphysis: The ends of a long bone; contains spongy bone and red marrow.

• Articular Cartilage: Smooth tissue covering the ends of bones at joints; reduces friction and absorbs shock.

• Epiphyseal Plate/Line: Growth plate in children; becomes the epiphyseal line in adults after growth stops.

• Endosteum: Thin membrane lining the medullary cavity; involved in bone growth and repair.

• Medullary Cavity: Central cavity within the diaphysis; contains yellow bone marrow.

Microscopic Anatomy of Bone

Bones are composed of specialized cells and matrix, organized differently in compact and spongy bone.

Spongy Bone

• Trabeculae: Network of bony struts providing structural support and housing marrow.

• Osteocytes: Mature bone cells located in lacunae within the trabeculae.

Compact Bone

• Osteocyte: Mature bone cell responsible for maintaining bone tissue.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange.

• Central Canal (Haversian Canal): Contains blood vessels and nerves; runs longitudinally through the bone.

• Osteon (Haversian System): Structural unit of compact bone, consisting of concentric lamellae around a central canal.

Bone Cells and Their Functions

• Osteoblasts: Cells that synthesize new bone matrix; responsible for bone formation.

• Osteoclasts: Large cells that break down bone matrix; involved in bone resorption and remodeling.

• Osteocytes: Mature bone cells derived from osteoblasts; maintain bone tissue.

Parathyroid Hormone (PTH)

Parathyroid hormone is a key regulator of calcium homeostasis in the body.

• Released by the parathyroid glands in response to low blood calcium levels.

• Stimulates osteoclast activity to increase bone resorption, releasing calcium into the bloodstream.

• Enhances calcium reabsorption in the kidneys and activates vitamin D to increase intestinal absorption of calcium.

Types of Bone Fractures

• Closed (Simple) Fracture: Bone breaks but does not penetrate the skin.

• Open (Compound) Fracture: Bone breaks and pierces the skin, increasing risk of infection.

Fracture Repair: Four Events

Bone repair is a complex process involving several stages:

1. Hematoma Formation: Blood clot forms at the fracture site.

2. Fibrocartilaginous Callus Formation: Soft callus of collagen and cartilage bridges the broken bone ends.

3. Bony Callus Formation: Osteoblasts produce new bone, converting the soft callus into a hard bony callus.

4. Bone Remodeling: Bone is reshaped to restore its original structure and strength.

Summary Table: Bone Cell Functions

Cell Type	Function
Osteoblast	Bone formation; synthesizes bone matrix
Osteoclast	Bone resorption; breaks down bone tissue
Osteocyte	Maintains bone tissue; regulates mineral content

Key Equations

• Calcium Homeostasis:

$\text{PTH} \uparrow \Rightarrow \text{Osteoclast Activity} \uparrow \Rightarrow \text{Blood Calcium} \uparrow$

$\text{Bone Remodeling Rate} = \text{Osteoblast Activity} - \text{Osteoclast Activity}$

Additional info: Academic context and definitions have been expanded for clarity and completeness.