Skeletal_System

Overview of the Skeletal System

  • The skeletal system consists of bones and various tissues:

    • Bone tissue

    • Cartilage

    • Dense connective tissues (tendons, ligaments)

    • Blood vessels and nerves

  • Multifunctional roles:

    • Support and protect softer tissues

    • Enable movement

    • Facilitate blood cell formation

    • Store minerals

  • Unique adaptations:

    • Human skeleton adapted for bipedalism (walking on two legs)

    • Grasping ability in hands

  • Divisions of the skeletal system:

    • Axial skeleton

    • Appendicular skeleton

Bone Shape and Structure

  • Bones differ in:

    • Size

    • Shape

Similar Features

  • Bones share several structural, developmental, and functional characteristics:

    • Structure includes outer compact bone and inner spongy bone

    • Development processes such as ossification and remodeling

    • Functional roles include supporting weight and facilitating movement.

Bone Classification by Shape

  1. Long Bones

    • Characteristics: Long, narrow shape with expanded ends

    • Example: Femur

  2. Short Bones

    • Characteristics: Cube-like, equal length and width

    • Example: Carpals

    • Special type: Sesamoid bones (e.g., patella) embedded in tendons

  3. Flat Bones

    • Characteristics: Plate-like with broad surfaces

    • Example: Scapula

  4. Irregular Bones

    • Characteristics: Varied shape, complex structure

    • Example: Vertebrae

Structure of a Long Bone

  • Key components of a long bone:

    • Epiphysis: Expanded end of the bone

    • Diaphysis: Long shaft of the bone

    • Metaphysis: Region between diaphysis and epiphysis

    • Articular Cartilage: Covers the epiphysis

    • Periosteum: Dense connective tissue surrounding the bone

    • Medullary Cavity: Hollow chamber within the diaphysis containing marrow

    • Endosteum: Lines the internal spaces of the bone

    • Bone Marrow: Found in medullary cavity (red or yellow)

Microscopic Structure of Bone

  • Osteocytes: Mature bone cells located in lacunae

  • Canaliculi: Tiny passageways connecting osteocytes for nutrient and waste exchange

  • The extracellular matrix consists of:

    • Collagen fibers providing resilience

    • Inorganic salts giving hardness to the bone

Types of Bone: Compact vs. Spongy Bone

  • Compact Bone:

    • Structure: Composed of cylindrical units called osteons

    • Characteristics: Solid, strong, and weight-bearing

  • Spongy Bone:

    • Structure: Made up of trabeculae (branching plates)

    • Characteristics: Lighter, somewhat flexible, with spaces reducing weight

Bone Functions

  • Major functions of bones include:

    • Providing shape and support to the body

    • Protecting vital organs (e.g., skull protects the brain)

    • Aiding in body movement (joint functionality)

    • Blood cell production (hematopoiesis in red bone marrow)

    • Storing minerals (inorganic salts)

Hematopoiesis and Bone Marrow

  • Blood cell formation occurs in red bone marrow:

    • Produces red blood cells, white blood cells, and platelets

  • Age Variation: Red bone marrow is gradually replaced by yellow bone marrow (fat storage)

  • Adult red marrow is located in specific bones: skull, ribs, vertebrae, and pelvis

Inorganic Salt Storage

  • Around 70% of bone matrix is made of inorganic mineral salts, primarily hydroxyapatite (calcium phosphate)

  • Other minerals include magnesium, sodium, potassium, carbonate ions

  • Osteoporosis: Loss of bone density due to mineralization loss

Hormonal Control of Blood Calcium Levels

  • Parathyroid glands regulate blood calcium levels by releasing parathyroid hormone (PTH) or calcitonin based on the levels.

  • Hormonal response involves:

    • Osteoblasts depositing calcium when levels are high (calcitonin)

    • Osteoclasts breaking down bone to release calcium when levels are low (PTH)

Bone Development and Repair

  • Bone development begins during early fetal development and proceeds through adulthood.

  • Two types of ossification:

    • Intramembranous Ossification: Flat bones form from connective tissue (e.g., skull bones)

    • Endochondral Ossification: Bones replace hyaline cartilage (e.g., long bones)

Intramembranous vs. Endochondral Ossification

  • Intramembranous:

    1. Begins with connective tissue

    2. Mesenchymal cells differentiate into osteoblasts

  • Endochondral:

    1. Initial cartilage model forms

    2. Chondrocytes enlarge, and the matrix breaks down

    3. Osteoblasts invade to form bone

Steps of Fracture Repair

  • Stages of healing:

    1. Hematoma formation (blood clot)

    2. Soft callus (fibrocartilage formation)

    3. Hard callus (bone replacement)

    4. Bone remodeling (restoration to original shape)

Clinical Applications: Bone Health

  • Osteoporosis and fragility fractures:

    • Common in older adults, especially postmenopausal women

    • Prevention through exercise, calcium, and vitamin D intake

Overall Skeletal Organization

  • Number of Bones: Approximately 206 in adult skeleton

  • Bone types in the skeleton include:

    • Axial Skeleton

      • Skull: 22 bones

      • Vertebral column: 26 bones

      • Thoracic cage: 25 bones

    • Appendicular Skeleton

      • Upper limbs: 60 bones

      • Lower limbs: 60 bones

      • Pectoral girdle: 4 bones

      • Pelvic girdle: 2 bones

Differences Between Male and Female Pelvis

  • Female pelvis: wider for childbirth, with larger pelvic inlet and pubic arch angle

  • Male pelvis: narrower structure, typically heavier and less flared.

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