Skeletal System Overview

Introduction to the Skeletal System

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. It is composed of bones, joints, ligaments, and tendons, and is divided into two main divisions: the axial and appendicular skeleton.

• Axial skeleton: Includes the skull, vertebral column, and rib cage.

• Appendicular skeleton: Comprises the limbs and girdles (shoulder and pelvic).

• Total bones: The adult human skeleton consists of 206 bones.

• Ligaments: Connect bone to bone.

• Tendons: Connect muscle to bone.

Skeletal System Functions

Main Functions of the Skeletal System

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

• Support: Provides a rigid framework that supports 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 essential minerals, primarily calcium and phosphate, which can be released into the bloodstream as needed.

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

• Blood Cell Formation (Hematopoiesis): Red bone marrow produces blood cells (erythrocytes, leukocytes, platelets).

Bone Classification

Types of Bones

Bones are classified based on their shape and structure, which relate to their function.

• Long Bones: Characterized by a shaft (diaphysis) and two ends (epiphyses). Mostly composed of compact bone, with spongy bone in the interior. Examples: femur, tibia, fibula, humerus, fingers of hand.

• Short Bones: Cube-shaped and contain mostly spongy bone, with a thin surface layer of compact bone. Examples: bones of carpus (wrist) and tarsus (ankle).

• Flat Bones: Thin, flattened, and usually curved. Consist of a layer of spongy bone sandwiched between two layers of compact bone. Examples: sternum, ribs, skull, scapula.

• Sesamoid Bones: Small, round, and flat bones found near tendons. Examples: patella.

• Irregular Bones: Complex shapes that do not fit other classifications. Examples: vertebrae, hip bones.

Bone Structure Examples

• Flat bone: Sternum, ribs, skull, scapula.

• Irregular bone: Vertebra, hip.

• Sesamoid bone: Patella.

Bone Tissue Types

Compact vs. Spongy Bone

Bones are composed of two main types of tissue: compact bone and spongy bone, each with distinct structure and function.

• Compact Bone (Cortical Bone): Dense and solid, forms the outer layer of bones. Contains osteons (Haversian systems) and blood vessels. Provides strength for weight-bearing.

• Spongy Bone (Cancellous Bone): Porous and less dense, found inside bones, especially at the ends. Contains trabeculae and spaces filled with red bone marrow. Important for hematopoiesis and reducing bone weight.

Example: The shaft of a long bone is primarily compact bone, while the ends contain spongy bone.

Bone Anatomy

Features of a Long Bone

Long bones have a distinct anatomy that supports their function in movement and support.

• Diaphysis: The shaft, composed of compact bone surrounding a medullary cavity filled with yellow bone marrow (in adults).

• Epiphysis: The ends of the bone, containing spongy bone filled with red bone marrow (in infants and some adults).

• Epiphyseal Line (Growth Plate): Region where long bones grow in length during development; located between diaphysis and epiphysis.

• Articular Cartilage: Hyaline cartilage covering the external surface of the epiphysis, reducing friction at joint surfaces.

• Periosteum: Dense connective tissue layer covering the bone, containing osteoblasts and osteoprogenitor cells for bone growth and repair.

• Endosteum: Thin membrane lining the medullary cavity, involved in bone growth and remodeling.

Example: The humerus has a diaphysis, two epiphyses, and an epiphyseal plate during growth.

Additional info:

• Further details on bone cell types, bone growth, calcium homeostasis, and fracture repair are covered in subsequent slides and notes.