Skeletal System: Structure, Function, and Classification

Skeletal System Overview

General Functions of the Skeleton

The skeletal system provides the structural framework for the human body and serves several essential physiological functions.

• Support and Movement: Bones act as attachment sites for skeletal muscles, soft tissues, and some organs. They function as a system of levers, allowing muscle contraction to produce movement.

• Hemopoiesis: Blood cell production occurs in the red bone marrow, a process known as hemopoiesis.

• Storage of Mineral and Energy Reserves: Bones store the body's reserves of calcium and phosphate, which can be released into the blood as needed. Calcium is essential for muscle contraction, blood clotting, and nerve impulse transmission. Phosphate is also stored, and lipids are stored in the yellow bone marrow of adult bones.

Bone Structure and Types

Bone as an Organ

Bones are the primary organs of the skeletal system, providing a rigid framework for the body.

Types of Bone Tissue

• Compact Bone: Dense bone tissue that appears white, smooth, and solid. It constitutes about 80% of bone mass and is composed of relatively dense connective tissue.

• Spongy Bone (Cancellous or Trabecular Bone): Located internal to compact bone, spongy bone appears porous and contains red bone marrow. It accounts for about 20% of bone mass.

Cartilage and Connective Tissues

Cartilage

Cartilage is a semirigid connective tissue that is more flexible than bone and plays a crucial role in the skeletal system.

• Hyaline Cartilage: Attaches ribs to the sternum, covers the ends of some bones, and is found within growth plates.

• Fibrocartilage: A weight-bearing cartilage that withstands compression. It is located in intervertebral discs, the pubic symphysis, and cartilage pads of the knees (menisci).

• Ligaments: Connect bone to bone, providing joint stability.

• Tendons: Connect muscle to bone, transmitting the force of muscle contraction to the skeleton.

Distribution of Cartilage in the Skeleton

Hyaline and fibrocartilage are distributed throughout the adult and juvenile skeletons, with hyaline cartilage found at articular surfaces and growth plates, and fibrocartilage at sites requiring resistance to compression.

Classification of Bones

Bone Shapes

Bones are classified based on their shapes, which relate to their functions and locations in the body.

• Long Bones: Greater in length than width, with an elongated, cylindrical shaft (diaphysis). Most common bone shape, found in the upper and lower limbs (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width. Examples include carpal bones, tarsals, sesamoid bones, and the patella.

• Flat Bones: Flat, thin surfaces that may be slightly curved. They provide surfaces for muscle attachment and protect underlying soft tissues. Locations include the roof of the skull, scapulae, sternum, and ribs.

• Irregular Bones: Complex shapes that do not fit into the other categories. Examples include vertebrae, ossa coxae (hip bones), ethmoid, and sphenoid bones.

Visual Classification of Bone by Shape

Long bones (e.g., femur), short bones (e.g., tarsal bones), flat bones (e.g., frontal bone of the skull), and irregular bones (e.g., vertebrae) can be visually distinguished based on their morphology.

Gross Anatomy of a Long Bone

Regions of a Long Bone

• Diaphysis: The elongated, usually cylindrical shaft of the bone.

• Medullary Cavity: The central cavity within the diaphysis that contains bone marrow.

• Epiphysis: The expanded ends of the bone, divided into proximal and distal epiphyses.

• Metaphysis: The region of mature bone between the diaphysis and epiphysis, containing the epiphyseal plate in growing bones.

• Epiphyseal Plate: A layer of hyaline cartilage that allows for bone growth in length during childhood and adolescence.

• Articular Cartilage: A thin layer of hyaline cartilage covering the joint surfaces of the epiphyses, reducing friction and absorbing shock in movable joints.

Example: The femur, a long bone, contains all these regions and serves as a model for understanding bone structure and function.