Chapter 5: The Integumentary System – Structured Study Notes

Integumentary System Overview

Components of the Integumentary System

The integumentary system is composed of the skin and its accessory structures, playing a vital role in protection and homeostasis.

• Skin (cutaneous membrane): Includes the epidermis (outer layer) and dermis (deeper layer).

• Accessory structures: Hair, nails, and glands.

Major Functions

• Protection: Acts as a barrier against pathogens, chemicals, and physical injury.

• Temperature regulation: Controls heat loss via sweat and blood flow.

• Sensation: Contains sensory receptors for touch, pain, and temperature.

• Excretion: Removes waste products through sweat.

• Vitamin D synthesis: Produces vitamin D3 when exposed to UV light.

• Immune defense: Contains cells that help fight infection.

Exam Trap: The skin synthesizes vitamin D, not vitamin C.

Layers of the Skin

Epidermis

The epidermis is the superficial layer of the skin, composed of stratified squamous epithelium and is avascular.

• Key Function: Provides a barrier against water loss and bacterial invasion.

• Cell Types:

  • Keratinocytes: Produce keratin, the main structural protein.

  • Melanocytes: Synthesize melanin for UV protection.

  • Dendritic cells: Immune defense cells.

  • Merkel (tactile) cells: Sensory receptors for touch.

• Epidermal Layers (Deep to Superficial):

  1. Stratum basale

  2. Stratum spinosum

  3. Stratum granulosum

  4. Stratum lucidum (only in thick skin)

  5. Stratum corneum

• Cell Migration: Cells move upward and die; rapid movement causes psoriasis.

Dermis

The dermis lies beneath the epidermis and is composed of connective tissue, providing structural support and housing blood vessels, nerves, glands, and hair follicles.

• Papillary Layer: Areolar tissue with capillaries and sensory receptors.

• Reticular Layer: Dense irregular connective tissue with collagen and elastic fibers.

• Key Concepts:

  • Skin turgor: Elasticity; decreased turgor indicates dehydration.

  • Stretch marks: Occur when the dermis is overstretched.

  • Subpapillary plexus: Superficial blood vessel network.

Subcutaneous Layer (Hypodermis)

The hypodermis is not technically part of the skin but lies beneath it, consisting mainly of adipose tissue.

• Functions: Insulation, energy storage, and cushioning.

Skin Color & Pigments

Melanin

Melanin is produced by melanocytes and protects against UV radiation. Increased sun exposure stimulates melanin production.

• Albinism: Lack of melanin production.

Carotene

Carotene is a yellow/orange pigment obtained from the diet.

Hemoglobin

Hemoglobin is a red pigment in blood; increased blood flow causes skin to appear red (blushing).

Clinical Color Changes

• Jaundice: Yellow skin due to liver dysfunction.

• Cyanosis: Blue skin due to low oxygen levels.

Vitamin D Synthesis

Role of Skin in Vitamin D Production

Exposure to UV light enables the skin to synthesize vitamin D3, which is essential for calcium absorption.

• Deficiency: Leads to weak bones and rickets.

Hair

Structure of Hair

Hair consists of the visible shaft, the root within the skin, and the bulb at the base where growth occurs.

• Hair Types:

  • Vellus: Soft, fine hair.

  • Terminal: Thick hair (scalp, eyelashes, eyebrows).

• Growth Center: The hair matrix is where cell division and hair growth occur.

• Other Structures:

  • Arrector pili: Muscle causing goosebumps.

  • Hair papilla: Provides blood supply to the hair bulb.

Glands

Sebaceous (Oil) Glands

These glands produce sebum, which lubricates the skin and hair. Overactivity can cause dandruff and inflammation.

Sweat Glands

• Eccrine (Merocrine): Found throughout the body; involved in cooling and thermoregulation.

• Apocrine: Located in axilla and groin; produce thicker secretions.

Special Glands

• Ceruminous glands: Produce earwax.

Nails

Structure and Growth

Nails are made of keratin, and growth occurs at the nail root.

Skin Damage & Repair

Dehydration

Decreased skin turgor results in skin that remains tented when pinched, indicating dehydration.

Scars

Scars form from dense, inflexible collagen fibers during repair.

Burns

Burns increase the risk of infection and sepsis, which is a major cause of death in burn patients.

Skin Conditions

Psoriasis

Characterized by rapid cell division in the epidermis.

Stretch Marks

Result from skin being stretched beyond its elastic limit.

Dandruff

Often caused by overactive sebaceous glands.

Skin Cancer

Types of Skin Cancer

• Basal cell carcinoma: Most common type.

• Squamous cell carcinoma: Second most common.

• Melanoma: Most dangerous; memorize this for exams.

Common Exam Traps

• Skin does not produce vitamin C.

• The epidermis is the barrier, not the dermis.

• There are no blood vessels in the epidermis.

• Neurons are not the main cells in the epidermis.

• Hair growth occurs at the matrix, not the papilla.

Final Cheat Sheet

• Epidermis: Barrier

• Dermis: Support, blood, nerves

• Hypodermis: Fat

• Melanin: UV protection

• Carotene: Yellow/orange pigment

• Hemoglobin: Red pigment

• Hair growth: Matrix

• Nail growth: Root

• Sweat: Thermoregulation

• Sebum: Oil/lubrication

• Skin cancer: Melanoma is the worst

Summary Table: Skin Layers and Functions

Layer	Main Tissue	Key Functions
Epidermis	Stratified squamous epithelium	Barrier, UV protection, water loss prevention
Dermis (Papillary)	Areolar connective tissue	Capillaries, sensory receptors
Dermis (Reticular)	Dense irregular connective tissue	Strength, elasticity, glands, hair follicles
Hypodermis	Adipose tissue	Insulation, energy storage, cushioning

Summary Table: Skin Pigments

Pigment	Source	Effect on Skin Color
Melanin	Melanocytes	Brown/black; UV protection
Carotene	Diet	Yellow/orange
Hemoglobin	Blood	Red (blushing)

Key Formula: Vitamin D Synthesis

UV light converts 7-dehydrocholesterol in the skin to vitamin D3:

$\text{7-dehydrocholesterol} \xrightarrow{UV\ light} \text{Vitamin\ D}_3$

Vitamin D3 is then converted in the liver and kidneys to its active form, which is essential for calcium absorption.