Comprehensive Study Guide: The Skeletal System in Anatomy & Physiology

The Skeletal System

Functions of the Skeletal System

The skeletal system is a vital organ system that provides structure, protection, and support for the human body. It is involved in several key physiological processes.

• Support: Provides a framework for the body and supports soft tissues.

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

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Stores minerals such as calcium and phosphorus.

• Blood Cell Production: Houses bone marrow, which produces blood cells (hematopoiesis).

• Energy Storage: Stores lipids in yellow bone marrow.

Types of Bone Tissue: Compact vs. Spongy Bone

Bones are composed of two main types of tissue: compact bone and spongy bone, each with distinct structures and functions.

• Compact Bone: Dense, solid tissue forming the outer layer of bones; provides strength and protection.

• Spongy Bone (Cancellous Bone): Porous, lightweight tissue found at the ends of long bones and inside other bones; contains red bone marrow.

• Key Differences: Compact bone is organized into osteons (Haversian systems), while spongy bone consists of trabeculae.

Classification of Bones by Shape

Bones are classified into four main categories based on their shapes:

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., skull, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).

Anatomy of a Long Bone

Long bones have distinct anatomical regions that contribute to their function and growth.

• Diaphysis: Shaft of the bone, composed mainly of compact bone.

• Epiphyses: Ends of the bone, containing spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis, includes the epiphyseal plate.

• Medullary Cavity: Central cavity containing yellow marrow.

• Periosteum: Outer fibrous covering of the bone.

• Endosteum: Lining of the medullary cavity.

Microscopic Structure of Compact Bone

Compact bone is organized into structural units called osteons or Haversian systems.

• Osteon: Cylindrical structure containing concentric lamellae (layers of bone matrix).

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae for nutrient exchange.

Bone Markings: Names and Shapes

Bones have various markings that serve as attachment points for muscles, ligaments, and tendons, or as passageways for nerves and blood vessels.

• Foramen: Round opening (e.g., foramen magnum).

• Fossa: Shallow depression.

• Process: Projection or bump (e.g., mastoid process).

• Crest: Prominent ridge.

• Condyle: Rounded articular surface.

• Additional info: Other markings include tubercle, trochanter, and groove.

Cells Involved in Bone Formation, Growth, and Remodeling

Bone tissue is dynamic and constantly remodeled by specialized cells.

• Osteoblasts: Bone-forming cells; synthesize bone matrix.

• Osteocytes: Mature bone cells; maintain bone tissue.

• Osteoclasts: Bone-resorbing cells; break down bone matrix.

Epiphyseal Plates and Bone Growth

The epiphyseal plate (growth plate) is a region of cartilage that allows bones to lengthen during childhood and adolescence.

• Significance: Responsible for longitudinal growth of long bones.

• Closure: Plate ossifies and becomes the epiphyseal line in adulthood, ending growth.

Appositional Growth

Appositional growth refers to the increase in bone thickness and strength.

• Process: Osteoblasts add new bone tissue to the outer surface, while osteoclasts remove bone from the inner surface.

• Result: Bones become thicker and stronger without changing length.

Homeostatic Regulation of Blood Calcium

Blood calcium levels are tightly regulated by a feedback system involving receptors, control centers, and effectors.

• Receptor: Parathyroid glands detect low blood calcium.

• Control Center: Parathyroid glands release parathyroid hormone (PTH).

• Effector: Bones, kidneys, and intestines respond to PTH to increase blood calcium.

Equation:

$\text{Blood Calcium Homeostasis:} \quad \text{Receptor} \rightarrow \text{Control Center} \rightarrow \text{Effector}$

Role of Calcitonin and Parathyroid Hormone

Calcitonin and parathyroid hormone (PTH) are key hormones in calcium regulation.

• Calcitonin: Lowers blood calcium by stimulating bone formation and inhibiting osteoclasts.

• Parathyroid Hormone (PTH): Raises blood calcium by stimulating osteoclasts, increasing calcium reabsorption in kidneys, and activating vitamin D.

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main parts:

• Axial Skeleton: Includes the skull, vertebral column, and thoracic cage; supports and protects the central nervous system and vital organs.

• Appendicular Skeleton: Includes the limbs and girdles; enables movement and interaction with the environment.

Major Bones of the Skull

The skull consists of several bones that protect the brain and form the structure of the face.

• Cranial Bones: Frontal, parietal, temporal, occipital, sphenoid, ethmoid.

• Facial Bones: Maxilla, mandible, zygomatic, nasal, lacrimal, palatine, vomer, inferior nasal concha.

Hyoid Bone: Function and Location

The hyoid bone is a unique, U-shaped bone located in the neck.

• Function: Supports the tongue and serves as an attachment for muscles involved in swallowing.

• Location: Situated between the mandible and larynx.

Vertebral Column: Location, Parts, and Function

The vertebral column (spine) is a flexible, segmented structure that protects the spinal cord and supports the body.

• Regions: Cervical (7), thoracic (12), lumbar (5), sacral (5 fused), coccygeal (4 fused).

• Function: Protects spinal cord, supports head and trunk, allows movement.

Thoracic Cage: Location, Parts, and Function

The thoracic cage protects vital organs and supports respiration.

• Parts: Ribs (12 pairs), sternum, thoracic vertebrae.

• Function: Protects heart and lungs, assists in breathing.

Sternum: Location and Parts

The sternum is a flat bone located in the center of the chest.

• Parts: Manubrium, body, xiphoid process.

Shoulder Girdle: Bones and Function

The shoulder girdle connects the upper limb to the axial skeleton.

• Bones: Clavicle and scapula.

• Function: Provides mobility and stability for the upper limb.

Bones of the Upper Limb

The upper limb consists of several bones that enable complex movements.

• Arm: Humerus.

• Forearm: Radius and ulna.

• Hand: Carpals, metacarpals, phalanges.

Pelvic Girdle: Bones and Function

The pelvic girdle supports the weight of the upper body and protects pelvic organs.

• Bones: Hip bones (ilium, ischium, pubis).

• Function: Transfers weight to lower limbs, supports and protects pelvic organs.

Major Parts of a Coxal Bone

The coxal bone (hip bone) is composed of three fused parts.

• Ilium

• Ischium

• Pubis

Bones of the Lower Limb

The lower limb supports body weight and enables locomotion.

• Thigh: Femur.

• Leg: Tibia and fibula.

• Foot: Tarsals, metatarsals, phalanges.

Classification of Joints by Movement

Joints are classified based on their degree of movement.

• Synarthroses: Immovable joints (e.g., sutures of the skull).

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

• Diarthroses: Freely movable joints (e.g., synovial joints).

Structure and Function of Joint Types

Joints are also classified by their structure:

• Fibrous Joints: Bones joined by dense connective tissue; mostly immovable.

• Cartilaginous Joints: Bones joined by cartilage; slightly movable.

• Synovial Joints: Bones separated by a fluid-filled cavity; freely movable.

Examples of Joint Types

• Fibrous: Sutures in the skull.

• Cartilaginous: Pubic symphysis, intervertebral discs.

• Synovial: Shoulder, knee, hip joints.

Six Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow.

Type	Structure	Function/Movement	Example
Ball-and-Socket	Spherical head fits into cup-like socket	Multiaxial movement	Shoulder, hip
Hinge	Cylindrical end fits into trough-shaped surface	Flexion/extension	Elbow, knee
Pivot	Rounded end fits into ring of bone/ligament	Rotation	Atlas/axis, proximal radioulnar joint
Condyloid	Oval articular surface fits into depression	Flexion, extension, abduction, adduction	Wrist
Saddle	Both surfaces are concave and convex	Same as condyloid, greater range	Thumb (carpometacarpal joint)
Plane	Flat surfaces	Gliding	Intercarpal joints

Aging and the Skeletal System

The skeletal system undergoes anatomical and physiological changes with age.

• Bone Density: Decreases due to loss of minerals.

• Joint Flexibility: Reduces due to cartilage wear.

• Fracture Risk: Increases with osteoporosis.

Homeostatic Relationships with Other Body Systems

The skeletal system interacts with other body systems to maintain homeostasis.

• Muscular System: Provides attachment for muscles, enabling movement.

• Circulatory System: Produces blood cells in bone marrow.

• Endocrine System: Regulates bone growth and calcium balance.

• Additional info: The skeletal system also stores minerals for nervous and muscular function.