Chapter 6: Integumentary System – Anatomy & Physiology Study Notes

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Introduction to the Integumentary System

Overview of Organ Systems

The integumentary system is a major organ system in the human body, primarily composed of the skin and its accessory structures. Organ systems are made up of organs that work together to perform specific functions, and each organ consists of two or more tissue types working toward a common goal.

Skin is the largest organ by weight and is also called the cutaneous membrane.
Accessory structures include hair follicles, sebaceous glands, sweat glands, and nails.

These components collectively protect the body, regulate temperature, and provide sensory information.

Skin and Its Tissues

Composition and Structure

The skin is composed of several tissue types and consists of two main layers:

Epidermis: The outer layer, made of epithelial tissue.
Dermis: The inner layer, made of connective tissue.

Accessory structures (hair, glands, sensory receptors) originate from the epidermis but extend into the dermis or deeper.

Layers of the Skin

Epidermis

The epidermis is the superficial layer of the skin, composed of stratified squamous epithelium. It rests on a basement membrane and contains no blood vessels.

Cells in the deepest layer (stratum basale) are nourished by blood vessels in the underlying dermis.
As cells migrate outward, they undergo keratinization—they flatten, dehydrate, and accumulate keratin, a tough, waterproof protein.
The outermost layer (stratum corneum) consists of dead, keratinized cells that are eventually shed.

Thickness varies: thickest on palms and soles (0.8–1.4 mm), thinner elsewhere (0.07–0.12 mm).

Dermis

The dermis is the deeper layer, typically 1–2 mm thick, and is composed of dense irregular connective tissue containing collagen and elastic fibers.

Contains dermal papillae—projections that form fingerprints and anchor the epidermis.
Houses accessory structures: hair follicles, sweat glands, sebaceous glands, and sensory receptors.
Provides strength, elasticity, and nourishment to the epidermis.

Subcutaneous Layer (Hypodermis)

The subcutaneous layer (hypodermis) lies beneath the dermis and is composed mainly of areolar and adipose tissue.

Acts as an insulator, conserving body heat.
Contains major blood vessels that supply the skin.

Additional info: The hypodermis is not considered a true part of the skin but is essential for its function.

Accessory Structures of the Skin

Hair Follicles

Hair follicles are tube-like depressions of the epidermis that extend into the dermis or subcutaneous layer.

Hair develops from stem cells at the base of the follicle.
The arrector pili muscle attaches to the follicle and contracts in response to cold or fear, causing "goosebumps."
Hair color is determined by the type and amount of melanin produced.

Nails

Nails are protective coverings on the ends of fingers and toes.

Nail plate: Visible, keratinized region.
Nail bed: Skin surface under the nail plate.
Nail matrix: Active growth region at the proximal end.
Lunula: Pale, half-moon-shaped region over the matrix.

Skin Glands

The skin contains several types of exocrine glands:

Sebaceous glands: Produce sebum (oil) to keep skin and hair soft and waterproof. Overactivity can lead to acne.
Sweat (sudoriferous) glands: Eccrine glands regulate temperature; apocrine glands are associated with hair follicles and produce body odor.
Ceruminous glands: Located in the ear canal, produce earwax.
Mammary glands: Modified sweat glands that produce milk.

Table: Types of Skin Glands

Type	Description	Function	Location
Sebaceous glands	Groups of specialized epithelial cells	Keep hair soft, pliable, waterproof	Near or connected to hair follicles; absent on palms and soles
Eccrine sweat glands	Numerous, open to skin surface	Regulate body temperature	Widespread, especially forehead, neck, back
Apocrine sweat glands	Less numerous, duct ends in hair follicles	Produce body odor	Armpit and groin
Ceruminous glands	Modified sweat glands	Produce earwax	External ear canal
Mammary glands	Modified sweat glands	Produce milk	Breasts

Functions of the Skin

Protection and Homeostasis

The skin serves as a physical, chemical, and biological barrier:

Prevents water loss and protects against injury, UV radiation, chemicals, and pathogens.
Maintains homeostasis by regulating body temperature and excreting small amounts of waste.
Contains sensory receptors for touch, pressure, pain, and temperature.
Initiates Vitamin D synthesis when exposed to sunlight, essential for bone health.
Acts as a blood reservoir (dermis contains ~10% of body’s blood vessels).

Regulation of Body Temperature

Body temperature is regulated by the hypothalamus and skin mechanisms:

Heat is produced by active cells (muscle, liver, etc.).
When body is too warm: dermal blood vessels dilate, sweat glands activate, heat is lost by radiation, conduction, convection, and evaporation.
When body is too cold: dermal blood vessels constrict, sweat glands are inactive, and muscles contract involuntarily (shivering).

Equations:

Heat loss by evaporation: (where is heat lost, is mass of evaporated sweat, is latent heat of vaporization)

Clinical Applications: Temperature Disorders

Hyperthermia: Abnormally high body temperature (>38.5°C), can occur in hot, humid conditions.
Hypothermia: Abnormally low body temperature (<35°C), dangerous if prolonged.
Fever: Elevated set point by the immune system to fight infection.

Healing of Wounds and Burns

Wound Healing

Any break in skin integrity triggers a healing response:

Superficial cuts: Epidermal cells divide rapidly to fill the gap.
Deeper wounds: Blood vessels break, forming a clot and scab; fibroblasts secrete collagen to bind wound; phagocytes remove debris; new tissue forms.
Excess collagen may form a scar.

Burns

Burns are classified by depth and extent of tissue damage:

First-degree (superficial partial thickness): Only epidermis affected; redness, mild pain.
Second-degree (deep partial thickness): Epidermis and some dermis; blisters, severe pain.
Third-degree (full thickness): Destroys epidermis, dermis, and accessory structures; leathery appearance, risk of infection and scarring.

Extent of burns is estimated using the "Rule of Nines"—divides body surface into regions of 9% or multiples thereof.

Life Span Changes

Aging and the Skin

With age, the skin undergoes several changes:

Cell cycle slows; skin becomes scaly, age spots appear.
Dermis and epidermis thin; subcutaneous fat decreases, leading to feeling cold.
Elasticity and oil production decrease; skin sags and dries.
Hair whitens and thins; sensory receptors decline.
Temperature regulation and Vitamin D production become less effective.

Summary Table: Layers of the Skin

Layer	Composition	Main Functions
Epidermis	Stratified squamous epithelium	Protection, waterproof barrier, sensory reception
Dermis	Dense irregular connective tissue, collagen, elastic fibers	Strength, elasticity, nourishment, houses accessory structures
Hypodermis (Subcutaneous)	Areolar and adipose tissue	Insulation, energy storage, anchors skin, blood supply