Skeletal System Overview

Main Components

The skeletal system is a vital organ system that provides structural support, protection, and movement for the human body. It consists of several key components:

• Bones: Rigid organs that form the framework of the body.

• Joints: Structures where two or more bones meet, allowing for movement and flexibility.

• Cartilage: Semi-rigid connective tissue that covers bone surfaces at joints and provides cushioning.

• Ligaments: Strong bands of connective tissue that connect bones to other bones, stabilizing joints.

Bones: Structure and Functions

Functions of Bones

Bones perform several essential functions in the body:

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

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

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

• Mineral Storage: Store minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

• Blood Cell Formation: House bone marrow, which produces blood cells (hematopoiesis).

• Triglyceride Storage: Store fat in yellow bone marrow as an energy reserve.

Bone Classification

By Tissue Type

Bones are composed of two main types of tissue:

• Compact Bone: Dense and solid, forms the outer layer of bones and provides strength.

• Spongy Bone (also called cancellous bone): Porous and lightweight, found mainly at the ends of long bones and inside flat bones. Contains trabeculae and spaces filled with bone marrow.

By Shape

Bones are classified into four main shapes, each with distinct characteristics and functions:

• Long Bones: Longer than they are wide, with a shaft and two ends (e.g., femur, humerus). Primarily found in limbs and function as levers for movement.

• Short Bones: Cube-shaped and mostly spongy bone (e.g., carpals in the wrist, tarsals in the ankle).

• Flat Bones: Thin, flattened, and usually curved (e.g., skull, ribs, sternum). Provide protection and broad surfaces for muscle attachment.

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

Anatomy of Long Bones

Structural Regions

Long bones have distinct anatomical regions:

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

• Epiphysis: The ends of the bone (proximal and distal), consisting mostly of spongy bone covered by a thin layer of compact bone.

• Metaphysis: The region between the diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in children and the epiphyseal line in adults.

Key Structures

• Articular Cartilage: Hyaline cartilage covering the ends of bones at joints, reducing friction and absorbing shock.

• Periosteum: Dense connective tissue membrane covering the outer surface of bones, containing nerves and blood vessels. It is essential for bone growth, repair, and attachment of ligaments and tendons.

• Endosteum: Thin membrane lining the internal bone surfaces, including the medullary cavity.

• Medullary Cavity: Central cavity within the diaphysis, containing yellow bone marrow (fat storage) in adults and red bone marrow (blood cell production) in children.

• Epiphyseal Plate/Line: Growth plate made of hyaline cartilage in children, which ossifies to become the epiphyseal line in adults.

Example: Anatomy of the Humerus

• Proximal Epiphysis: End closest to the shoulder.

• Diaphysis: Shaft of the bone.

• Distal Epiphysis: End closest to the elbow.

• Articular Cartilage: Covers both epiphyses.

• Medullary Cavity: Runs through the diaphysis.

Summary Table: Bone Classification by Shape

Additional info:

• The periosteum contains Sharpey's fibers, which anchor it to the bone surface.

• Red bone marrow is the site of hematopoiesis (blood cell formation), especially in children and in certain adult bones (e.g., pelvis, sternum).

• Yellow bone marrow stores fat and increases with age.