The Skeletal System - Human and Social Biology

Introduction to the Skeletal System

The skeletal system is the body's structural framework, composed of bones, cartilage, ligaments, and tendons. An adult human has 206 bones that perform crucial functions for survival and movement.

Key Functions:

Classification of Bones

Bones are classified by their shape and function into five main categories:

Long Bones

Femur, Humerus

Short Bones

Carpals, Tarsals

Flat Bones

Skull, Sternum

Irregular Bones

Vertebrae, Pelvis

Sesamoid Bones

Patella

Characteristics of Bone Types:

Bone Type Structure Function Examples
Long Bones Longer than wide, shaft with ends Levers for movement Femur, radius, phalanges
Short Bones Cube-shaped, spongy bone Stability, shock absorption Carpals, tarsals
Flat Bones Thin, curved plates Protection, muscle attachment Skull, ribs, scapula
Irregular Bones Complex shapes Specialized functions Vertebrae, facial bones
Sesamoid Bones Small, round bones in tendons Reduce friction Patella (kneecap)

Bone Structure

Bones are complex living organs with multiple tissue types and structural components.

Diaphysis (Shaft) Epiphysis (End) Epiphysis (End) Compact Bone Spongy Bone Marrow Cavity

Figure 2: Structure of a long bone with cross-sectional view

Microscopic Bone Structure:

Joints and Movement

Joints are points where bones connect, enabling movement and flexibility.

1. Fibrous Joints

Fibrous Tissue Bone Bone

2. Cartilaginous Joints

Cartilage Bone Bone

3. Synovial Joints

Synovial Fluid Bone Bone

Synovial Joint Types

Hinge Ball-and-Socket Pivot Gliding

Figure 3: Types of synovial joints and their movements

Bone Development and Growth

Bones develop through two processes and continue growing until early adulthood.

1. Mesenchymal Cells 2. Ossification Center 3. Bone Formation 4. Compact Bone 1. Cartilage Model 2. Primary Ossification 3. Bone Replacement 4. Growth Plates

Figure 4: Bone development processes - intramembranous (top) and endochondral (bottom)

Ossification Processes:

Common Skeletal Disorders

Disorder Causes Symptoms Treatment/Prevention
Osteoporosis Calcium deficiency, aging Brittle bones, fractures Calcium/Vitamin D, weight-bearing exercise
Arthritis Joint inflammation, wear Pain, stiffness, swelling Anti-inflammatories, joint replacement
Scoliosis Abnormal spine curvature Uneven shoulders/hips Braces, physical therapy
Fractures Trauma, osteoporosis Pain, deformity, swelling Casting, surgery

Glossary of Skeletal System Terms

Articulation
A joint where two or more bones meet.
Cartilage
Flexible connective tissue found in joints and other structures.
Diaphysis
The shaft or central part of a long bone.
Epiphysis
The end part of a long bone, initially growing separately.
Hematopoiesis
The formation of blood cellular components in bone marrow.
Ligament
Fibrous connective tissue that connects bones to other bones.
Osteoblast
Bone-forming cell.
Osteoclast
Bone-resorbing cell.
Osteocyte
Mature bone cell that maintains bone tissue.
Tendon
Fibrous connective tissue that connects muscle to bone.

Self-Assessment Questions

1. List five functions of the skeletal system.
Support, protection, movement, mineral storage, blood cell production (hematopoiesis).
2. Compare compact bone and spongy bone in terms of structure and function.
Compact bone: Dense, solid appearance with osteons; provides strength and support.
Spongy bone: Porous with trabeculae; lighter weight and contains red marrow for blood cell production.
3. What is the difference between tendons and ligaments?
Tendons: Connect muscle to bone (enable movement).
Ligaments: Connect bone to bone (provide joint stability).
4. Describe how a long bone grows in length.
Growth occurs at the epiphyseal plate (growth plate) where cartilage is continuously replaced by bone through endochondral ossification until adulthood when the plate ossifies.
5. Name three types of synovial joints and give an example of each.
1) Hinge joint - elbow
2) Ball-and-socket joint - hip
3) Pivot joint - neck (atlas/axis)
6. What are the components of a synovial joint?
Articular cartilage, joint capsule, synovial membrane, synovial fluid, ligaments, and sometimes bursae and menisci.
7. Explain how calcium homeostasis is maintained by the skeletal system.
Bones act as calcium reservoirs. When blood calcium is low, parathyroid hormone stimulates osteoclasts to release calcium from bone. When calcium is high, calcitonin promotes calcium deposition in bones.
8. What are the differences between intramembranous and endochondral ossification?
Intramembranous: Forms flat bones directly from mesenchymal tissue without cartilage model.
Endochondral: Forms most bones by replacing hyaline cartilage models with bone tissue.
9. How does exercise benefit the skeletal system?
Stimulates bone growth/remodeling, increases bone density, strengthens connective tissues, improves joint flexibility, and reduces osteoporosis risk.
10. Describe two ways the skeletal system protects internal organs.
1) The skull's bony structure protects the brain from mechanical injury.
2) The rib cage surrounds and shields the heart and lungs from trauma.

Summary of Key Points