The Skeletal System

Overview and Divisions

The skeletal system forms the structural framework of the human body, providing support, protection, and enabling movement. It is divided into two main parts:

• Axial skeleton: Comprises the bones along the longitudinal axis of the body, including the skull, vertebral column, and thoracic cage.

• Appendicular skeleton: Consists of the bones of the limbs and the girdles (shoulder and pelvic) that attach them to the axial skeleton.

The skeletal system includes bones, joints, cartilages, and ligaments.

Functions of Bones

Key Roles of the Skeletal System

Bones perform several vital functions necessary for survival and movement:

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

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

• Movement: Skeletal muscles attach to bones, using them as levers to produce movement.

• Storage: Bones store minerals (such as calcium and phosphorus) and fats (in the yellow marrow).

• Blood cell formation: Hematopoiesis, the formation of blood cells, occurs in the red marrow of certain bones.

Classification of Bones

Types of Bone Tissue

The adult human skeleton contains 206 bones, which are classified by their shape and internal structure:

• Compact bone: Dense, homogeneous bone tissue that forms the outer layer of bones, providing strength and support.

• Spongy bone: Composed of small needle-like pieces of bone (trabeculae) and open spaces, found mainly at the ends of long bones and inside flat bones.

Classification by Shape

Bones are also categorized based on their shapes, which relate to their functions:

• Long bones: Longer than they are wide; mostly compact bone. Examples: humerus (arm), femur (thigh).

• Short bones: Generally cube-shaped; mostly spongy bone. Examples: carpals (wrist bones), tarsals (ankle bones).

• Flat bones: Thin, flattened, and usually curved; two thin layers of compact bone sandwiching a layer of spongy bone. Examples: sternum (breastbone), skull bones.

• Irregular bones: Bones that do not fit into the above categories. Examples: vertebrae, hip bones.

Example: The femur is a long bone that supports body weight and enables movement, while the vertebrae are irregular bones that protect the spinal cord and allow flexibility of the spine.

Additional info: Bone tissue is highly vascularized and dynamic, constantly being remodeled in response to mechanical stress and metabolic needs.