Skeletal System: Structure, Function, and Bone Physiology Study Notes

Ch.6 Skeletal System Study Guide

Main Divisions of the Skeleton

The human skeleton is divided into two main regions, each serving distinct structural and functional roles.

• Axial Skeleton: Comprises the skull, vertebral column, and thoracic cage. It supports and protects the brain, spinal cord, and vital organs.

• Appendicular Skeleton: Includes the limbs and girdles (pectoral and pelvic), facilitating movement and interaction with the environment.

Functions of the Skeletal System

The skeletal system performs several essential functions for the body:

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

• Protection: Shields vital organs (e.g., brain, heart, 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 marrow.

Classification of Bones by Shape

Bones are classified based on their shapes, which relate to their functions:

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

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

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

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

• Sesamoid Bones: Embedded in tendons (e.g., patella).

Anatomy of a Typical Long Bone

Long bones have specialized regions and structures:

• Diaphysis: The shaft, composed mainly of compact bone.

• Epiphysis: The ends of the bone, containing spongy bone and red marrow.

• Epiphyseal Plate: Growth plate of hyaline cartilage, site of lengthwise growth in children.

• Medullary Cavity: Central cavity containing yellow marrow in adults.

• Periosteum: Dense connective tissue membrane covering the bone's outer surface.

• Endosteum: Thin membrane lining the medullary cavity.

Bone Cells and Their Functions

Bone tissue contains specialized cells with distinct roles:

• Osteoblasts: Bone-forming cells that synthesize the bone matrix.

• Osteocytes: Mature bone cells maintaining bone tissue.

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

Comparison Table: Bone Cell Functions

Cell Type	Function
Osteoblast	Builds bone matrix
Osteocyte	Maintains bone tissue
Osteoclast	Resorbs bone matrix

Bone Matrix Composition and Flexibility

The bone matrix consists of organic and inorganic components:

• Organic: Collagen fibers provide flexibility and tensile strength.

• Inorganic: Hydroxyapatite (calcium phosphate crystals) gives hardness and ability to bear weight.

Structure and Function of Cancellous (Spongy) Bone

Cancellous bone is found at the ends of long bones and inside flat bones:

• Trabeculae: Lattice-like network providing structural support.

• Red Marrow: Site of hematopoiesis.

• Lightweight: Reduces bone weight while maintaining strength.

Appositional Bone Growth

Appositional growth increases bone diameter:

• Osteoblasts in periosteum: Add new bone tissue to the outer surface.

• Osteoclasts in endosteum: Remove bone from the inner surface, enlarging the medullary cavity.

Endochondral Ossification Centers

Bone development in long bones occurs at specific centers:

• Primary Ossification Center: Forms in the diaphysis during fetal development.

• Secondary Ossification Centers: Develop in the epiphyses after birth.

Intramembranous Ossification

Intramembranous ossification forms bones directly from mesenchymal tissue:

• Examples: Flat bones of the skull, mandible, and clavicle.

• Process: Mesenchymal cells differentiate into osteoblasts, forming bone without a cartilage model.

Endochondral Ossification

Most bones develop via endochondral ossification:

• Process: Hyaline cartilage model is replaced by bone.

• Examples: Long bones such as femur, tibia, and humerus.

Disorders Affecting Bone Growth

Several disorders can impact bone growth and development:

• Pituitary Dwarfism: Insufficient growth hormone leads to short stature.

• Achondroplasia: Genetic disorder causing abnormal cartilage formation and short limbs.

• Gigantism: Excess growth hormone before epiphyseal plate closure causes abnormal height.

• Acromegaly: Excess growth hormone after plate closure leads to enlarged extremities.

• Marfan's Syndrome: Connective tissue disorder affecting bone growth.

• Rickets: Vitamin D deficiency causing soft, weak bones in children.

• Osteomalacia: Softening of bones in adults due to vitamin D deficiency.

Steps in Bone Repair

Bone repair after fracture involves several stages:

1. Hematoma Formation: Blood clot forms at the fracture site.

2. Fibrocartilaginous Callus Formation: Collagen and cartilage stabilize the break.

3. Bony Callus Formation: Osteoblasts produce new bone.

4. Bone Remodeling: Bone is reshaped to its original form.

Hormonal Regulation of Calcium Homeostasis

Calcium levels in the body are tightly regulated by hormones:

• Parathyroid Hormone (PTH): Major regulator of blood Ca2+ levels.

• Calcitonin: Lowers blood Ca2+ levels by inhibiting osteoclast activity.

Regulation Equation:

$\text{Ca}^{2+}_{\text{blood}} = \text{Ca}^{2+}_{\text{intake}} - \text{Ca}^{2+}_{\text{excretion}} + \text{Ca}^{2+}_{\text{bone resorption}}$

Effects of Parathyroid Hormone (PTH)

PTH increases blood calcium by:

• Stimulating osteoclasts to resorb bone.

• Promoting formation of active vitamin D (calcitriol) in kidneys.

• Increasing renal reabsorption of Ca2+.

Calcitonin Secretion and Osteoclast Activity

Calcitonin is secreted by the thyroid gland in response to high blood Ca2+ levels:

• Inhibits osteoclast activity, reducing bone resorption.

• Lowers blood calcium levels.

Example: After a meal rich in calcium, calcitonin secretion increases to prevent hypercalcemia.

Summary Table: Hormonal Effects on Bone

Hormone	Source	Effect on Bone	Effect on Blood Ca2+
Parathyroid Hormone (PTH)	Parathyroid gland	Stimulates osteoclasts	Increases
Calcitonin	Thyroid gland	Inhibits osteoclasts	Decreases
Vitamin D (Calcitriol)	Kidney (activated)	Promotes Ca2+ absorption	Increases

Additional info: Vitamin D is essential for calcium absorption in the intestines, and its deficiency can lead to bone disorders such as rickets and osteomalacia.