Chapter 08

Skeletal System Overview

Objectives

• To understand the functions, composition, and structure of the skeletal system.

• To learn about the various types of bones and joints, and their roles in the human body.

Functions of the Skeletal System

• Framework for the body: The skeleton provides a solid structure that supports the body and gives it shape.

• Protects internal organs: Bones safeguard vital organs, such as the skull protecting the brain, and the ribcage shielding the heart and lungs.

• Enables movement with muscles: The skeletal system works in conjunction with muscles, allowing for a wide range of movements, from simple actions like walking to complex athletic performances.

• Stores minerals: Bones act as reservoirs for essential minerals such as calcium and phosphorus, which are crucial for various bodily functions, including nerve transmission and muscle contraction.

• Produces blood cells in bone marrow: The bone marrow, found in certain bones, is the primary site for blood cell production, including red blood cells, white blood cells, and platelets, through a process called hematopoiesis.

Key Terms

• Abduction: Movement away from the midline of the body.

• Adduction: Movement towards the midline of the body.

• Extension: Increase in the angle between two body parts.

• Flexion: Decrease in the angle between two body parts.

• Circumduction: Circular movement of a limb.

• Appendicular Skeleton: The part of the skeleton that includes the bones of the limbs and the girdles.

• Axial Skeleton: The part of the skeleton that includes the skull, vertebral column, and rib cage.

• Compact Bone: Dense, hard outer layer of bone that provides strength.

• Spongy Bone: Less dense bone found at the ends of bones, containing marrow.

• Osteoblasts: Bone-forming cells.

• Osteoclasts: Bone-resorbing cells that break down bone tissue.

• Osteocytes: Mature bone cells that maintain bone health.

Introduction to the Skeletal System

• Definition: The skeletal system is a complex network consisting of bones, joints, cartilage, and ligaments; it plays a critical role in maintaining overall bodily function.

• Composition: Bone tissue is dynamic and living, made up of a matrix of collagen fibers and minerals such as calcium and phosphorus that give it its hard structure, offering both strength and resilience.

• Bone Dynamics: It is remarkable that, on average, a 70-year-old individual has completely replaced their skeleton approximately seven times throughout their lifetime, highlighting the continuous process of bone remodeling.

• Osteoporosis: This condition is marked by a significant decrease in bone density, leading to fragile bones that are more prone to fractures. It is commonly caused by factors such as hormonal changes (decreased estrogen in postmenopausal women), a sedentary lifestyle, excessive soda consumption, and steroid use.

Bone Composition and Structure

Types of Bones

• Long Bones: These bones are longer than they are wide, facilitating movement; examples include the femur in the thigh and the humerus in the arm.

• Short Bones: Roughly cube-shaped, providing support and stability; examples include the carpals in the wrist and tarsals in the ankle.

• Flat Bones: These bones are thin and curved, offering protection and surface area for muscle attachment; examples include the skull bones, ribs, and sternum.

• Irregular Bones: These have complex shapes that fit specific needs and do not fit into the other categories; examples include vertebrae and the pelvic bones.

Bone Structure

• Long Bone Anatomy:

  • Diaphysis: The shaft of the long bone, primarily composed of compact bone that provides strength.

  • Epiphysis: The ends of the long bone, containing spongy bone and a cartilage-covered surface for joint movement.

  • Epiphyseal Disc: This region is crucial for longitudinal growth in children and consists of hyaline cartilage, allowing bones to grow during development.

  • Medullary Cavity: The hollow center of long bones, which contains bone marrow (red in infants for blood production, yellow in adults, storing fat).

  • Periosteum: A dense layer of connective tissue that wraps around the bone, playing a vital role in nourishment and attachment for tendons and ligaments.

Types of Bone Tissue

• Compact Bone: This dense tissue forms the diaphysis and provides strength to withstand stress, containing structural units called Haversian systems (osteons) that allow for nutrient delivery.

• Spongy Bone: Found at the ends of long bones, it is lighter and less dense than compact bone, housing trabeculae which support the bone structure and contain red bone marrow for blood cell production.

Bone Growth and Remodeling

• Ossification: The process through which bone is formed from cartilage.

• Intramembranous Ossification: This process is responsible for the formation of flat bones of the skull and clavicle.

• Endochondral Ossification: This process converts cartilage into bone, especially in long bones during fetal development and childhood.

Bone Growth

• Longitudinal Growth: Occurs at the epiphyseal discs, regulated by growth hormones, allowing bones to grow in length.

• Width Growth: This growth occurs through the actions of osteoblasts (building bone) and osteoclasts (breaking down old bone), ensuring bone strength and structure are maintained.

Bone Cells and Function

• Osteoblasts: Specialize in building new bone by secreting the bone matrix, crucial for growth and healing.

• Osteoclasts: Break down old or damaged bone tissue, essential for remodeling, repair, and the release of stored minerals into the bloodstream.

• Osteocytes: These mature bone cells maintain bone tissue and regulate mineral content.

Joints and Movements

Types of Joints

• Immovable Joints: Such as the sutures in the skull, provide no movement, serving to protect the brain.

• Slightly Movable Joints: Such as the intervertebral discs, allow limited movement, contributing to spinal flexibility.

• Freely Movable Joints: The most common type, including:

  • Synovial Joints: Characterized by a joint cavity, allowing for wide-ranging movement, lined with synovial membrane that secretes lubricating fluid.

    • Hinge Joints: Enable back-and-forth movement like that of the elbow and knee.

    • Ball-and-Socket Joints: Allow rotational movement as seen in the shoulder and hip.

    • Pivot Joints: Allow for rotational movement around an axis, such as the atlas-axis joint in the neck.

    • Saddle Joints: Allow for a wide range of movement, seen in the base of the thumb.

    • Gliding Joints: Allow bones to slide past each other, like wrist bones.

    • Condyloid Joints: Allow movement but no rotation, seen in the knuckles (metacarpophalangeal joints).

Joint Movements

• Flexion: Reduces the angle between two bones, such as bending the elbow.

• Extension: Increases the angle between two bones, such as straightening the arm.

• Abduction & Adduction: Refers to movement of a limb away from or towards the body midline.

• Rotation: Involves moving around an axis, like turning the head.

• Circumduction: The circular movement of a limb, combining flexion, extension, abduction, and adduction.

Common Joint Conditions

• Osteoarthritis: A degenerative joint disease characterized by the breakdown of cartilage in joints, leading to pain and stiffness.

• Gout: A form of arthritis caused by the crystallization of uric acid in joints, often affecting the big toe, resulting in severe pain and inflammation.

• Bursitis: Inflammation of the bursa—small fluid-filled sacs that reduce friction at joints due to repetitive motion or trauma.

Axial and Appendicular Skeletons

Axial Skeleton

• Composed of 80 bones which include:

  • Skull: 28 bones providing protection to the brain and forming the structure of the face.

  • Vertebral Column: 26 bones forming the spine, protecting the spinal cord and providing support.

  • Thoracic Cage: 25 bones, including ribs and sternum, protecting vital organs in the thorax.

Appendicular Skeleton

• Composed of 126 bones which include:

  • Upper Limbs: Bones including the humerus, radius, and ulna, facilitating arm and hand movement.

  • Lower Limbs: Bones including the femur, tibia, and fibula, enabling leg and foot movement.

  • Girdles: The pelvic and pectoral girdles support limb attachment and provide structural integrity.

Conclusion

The skeletal system is integral for providing support, protecting vital organs, enabling movement, storing essential minerals, and facilitating blood cell production. Maintaining bone growth and joint health is crucial for overall well-being and functionality of the body.