Lecture 5 Guide

Skeletal System Overview

Introduction to the Skeletal System

The skeletal system provides the structural framework for the human body, supporting movement, protecting organs, and serving as a reservoir for minerals. It is divided into two main divisions: the axial and appendicular skeletons.

• Axial Skeleton: Consists of the skull, vertebral column, and bony thorax (rib cage and sternum). It forms the central axis of the body and provides protection for the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Includes the bones of the limbs and girdles (shoulder and pelvic girdles) that attach the limbs to the axial skeleton. It is primarily involved in movement.

• Cartilage: A flexible connective tissue found in various forms in the larynx and respiratory tract, at the ends of ribs, between vertebrae, and in joints.

Functions of the Skeletal System

Major Functions

The skeletal system performs several vital functions necessary for survival and homeostasis.

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

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

• Movement: Serves as levers for muscles to act upon, enabling movement.

• Mineral Storage: Acts as a reservoir for minerals, especially calcium and phosphorus, which can be released into the bloodstream as needed.

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

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

Classification of Bones by Shape

Types of Bones

Bones are classified according to their shapes, which relate to their functions.

• Long Bones: Longer than they are wide; primarily found in the limbs (e.g., femur, humerus). Function as levers for movement.

• Short Bones: Approximately equal in length and width; found in the wrist (carpals) and ankle (tarsals). Provide stability and support with limited motion.

• Flat Bones: Thin, flattened, and usually curved; found in the skull, ribs, and sternum. Provide protection and broad surfaces for muscle attachment.

• Irregular Bones: Complex shapes that do not fit other categories; found in the vertebrae and certain facial bones.

Structure of Long Bone (Macroscopic Anatomy)

Major Regions of a Long Bone

Long bones have a unique structure that supports their function in movement and weight-bearing.

• Diaphysis: The central shaft of the bone, composed of compact bone that provides strength.

• Medullary (Marrow) Cavity: The central cavity within the diaphysis, containing yellow marrow (fat storage) in adults.

• Epiphyses: The expanded ends of the bone, which articulate with other bones. Composed mainly of spongy (cancellous) bone, which contains red marrow in certain locations.

• Articular Cartilage: A layer of hyaline cartilage covering the ends of bones within joints, reducing friction and absorbing shock.

• Periosteum: A tough, fibrous membrane covering the outer surface of the bone, except at joint surfaces. Contains nerves, blood vessels, and is essential for bone growth and repair.

• Endosteum: A thin membrane lining the medullary cavity and spaces within spongy bone; involved in bone growth, repair, and remodeling.

Bone Marrow

• Red Marrow: Site of hematopoiesis (blood cell formation); found in the epiphyses of long bones and in flat bones (e.g., hip, sternum).

• Yellow Marrow: Consists mainly of adipose tissue; found in the medullary cavity of long bones in adults.

• Marrow Conversion: In cases of increased demand for red blood cells, yellow marrow can revert to red marrow.

Structure of Long Bone (Microscopic Anatomy)

Osteon (Haversian System)

The osteon is the fundamental functional unit of compact bone, providing strength and structure.

• Osteocytes: Mature bone cells located in small spaces called lacunae.

• Lacunae: Small cavities within the bone matrix that house osteocytes.

• Lamellae: Concentric rings of calcified matrix surrounding the central canal.

• Central (Haversian) Canal: Runs longitudinally through the osteon, containing blood vessels and nerves.

• Canaliculi: Tiny channels connecting lacunae to each other and to the central canal, allowing for nutrient and waste exchange.

Bone Matrix Composition

• Inorganic Component: Calcium salts (primarily hydroxyapatite) provide hardness and strength for weight-bearing.

• Organic Component: Collagen fibers provide flexibility and tensile strength.

• Cellular Component: Osteocytes and other bone cells (osteoblasts, osteoclasts).

Deficiencies:

• Insufficient vitamin D or calcium: Soft bones (rickets in children, osteomalacia in adults).

• Insufficient collagen: Brittle bones (osteogenesis imperfecta, scurvy).

Summary Table: Classification of Bones by Shape

Type	Example	Function
Long Bone	Humerus, Femur	Leverage, movement
Short Bone	Carpals, Tarsals	Stability, support
Flat Bone	Parietal bone, Sternum	Protection, muscle attachment
Irregular Bone	Vertebrae	Complex shape, protection

Example

Application: The femur is a long bone that supports body weight and enables movement by acting as a lever for muscles.

Additional info: The provided notes and images are highly relevant to a college-level Anatomy & Physiology course, specifically covering the skeletal system's structure and function.