Integumentary System: Structure and Function (EXS 111 Study Notes)

Integumentary System

Overview and Components

The integumentary system is the body's external covering, providing protection and serving multiple physiological functions. It consists of the skin, hair, nails, and various glands.

• Skin: The largest organ of the body, acting as a barrier against injury, pathogens, and dehydration.

• Hair: Provides protection, sensory input, and regulates body temperature.

• Nails: Protect the distal phalanges and aid in fine motor functions.

• Glands: Includes sweat (sudoriferous) and oil (sebaceous) glands, which help regulate temperature and lubricate the skin.

• Cutaneous receptors: Sensory receptors such as nociceptors (pain), mechanoreceptors (touch/pressure), and thermoreceptors (temperature).

Functions of the Integumentary System

• Protection: Shields internal tissues from mechanical damage, pathogens, chemicals, and UV radiation.

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

• Thermoregulation: Sweat glands and blood vessels regulate heat loss.

• Metabolic Functions: Synthesis of vitamin D in response to UV radiation.

• Excretion: Removal of metabolic wastes through sweat.

Structure of the Skin

Layers of the Skin

The skin is composed of three main layers, each with distinct functions and structures:

• Epidermis: The outermost layer, made of keratinized stratified squamous epithelium. Provides a waterproof barrier and creates skin tone.

• Dermis: The middle layer, consisting of connective tissue, blood vessels, nerve endings, and appendages (hair follicles, sweat glands).

• Hypodermis (Subcutaneous Layer): Lies deep to the dermis, composed mainly of areolar and adipose tissue. Not technically part of the skin, but important for insulation and energy storage.

Skin Thickness

• Varies from 1.5 to 4.4 mm depending on location.

• Accounts for approximately 7-8% of total body weight.

Detailed Structure (See Diagram)

• Hair shaft: Visible part of hair above the skin surface.

• Hair follicle: Structure in the dermis from which hair grows.

• Sweat pore: Opening on the skin surface for sweat secretion.

• Dermal papillae: Projections of the dermis that increase surface area for exchange and form fingerprints.

• Arrector pili muscle: Smooth muscle that causes hair to stand up (goosebumps).

• Adipose tissue: Fat storage in the hypodermis.

• Nerve endings: Sensory structures for touch, pain, and temperature.

Epidermis

Cell Types in the Epidermis

• Keratinocytes: Most abundant cell type; produce keratin, a tough, fibrous protein that provides protection.

• Melanocytes: Located in the basal layer; manufacture and secrete the pigment melanin, which protects against UV radiation.

• Tactile (Merkel) cells: Basal layer; attached to sensory nerve endings, involved in touch sensation.

• Dendritic (Langerhans) cells: Stratum spinosum; part of the immune system, act as macrophages.

Layers of the Epidermis

• Stratum basale (germinativum): Deepest layer; site of cell division and melanocyte activity.

• Stratum spinosum: Several layers of keratinocytes; contains tonofilaments and dendritic cells.

• Stratum granulosum: Contains keratohyaline granules (form keratin) and lamellar granules (waterproofing glycolipids).

• Stratum lucidum: Present only in thick skin (palms, soles); few rows of flat, dead keratinocytes.

• Stratum corneum: Outermost layer; many layers of dead, keratinized cells, providing protection against abrasion and penetration.

Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer that supports and nourishes the epidermis.

• Papillary layer: Superficial layer; contains dermal papillae for exchange of gases, nutrients, and waste. Forms friction ridges (fingerprints).

• Reticular layer: Deep layer; makes up 80% of dermal thickness, contains dense irregular connective tissue, collagen, and elastic fibers for strength and flexibility.

• Blood supply: Dermal vascular plexuses play a key role in temperature regulation and nutrient delivery.

Hypodermis (Subcutaneous Layer)

Structure and Function

• Composed of areolar and adipose connective tissue.

• Anchors skin to underlying structures (muscle, bone).

• Provides insulation and energy storage.

• Distribution varies between males and females.

Skin Color

Pigments Contributing to Skin Color

• Melanin: Brown-black pigment produced by melanocytes; protects against UV damage.

• Carotene: Yellow-orange pigment from diet (carrots, tomatoes); accumulates in stratum corneum and fat tissue.

• Hemoglobin: Red pigment in blood; visible in areas with little melanin.

Appendages of the Skin

Nails

• Scalelike modification of the epidermis.

• Protects the distal phalanges.

• Consists of nail plate, lunule, cuticle (eponychium), and nail bed.

Hair

• Made of dead, keratinized cells.

• Consists of medulla (central core), cortex (surrounds medulla), and cuticle (outermost layer).

• Hair follicle structure includes connective tissue sheath, epithelial root sheath, and dermal papilla.

• Arrector pili muscle causes hair to stand up.

Growth and Types of Hair

• Vellus hair: Fine, pale hair found in children and adult females.

• Terminal hair: Coarse, long hair of scalp, eyebrows, and after puberty in axillary and pubic regions.

• Alopecia: Hair thinning or baldness.

Glands

• Sebaceous (oil) glands: Associated with hair follicles; secrete sebum by holocrine secretion, lubricating skin and hair.

• Sudoriferous (sweat) glands:

  • Eccrine glands: Widely distributed; produce true sweat for thermoregulation.

  • Apocrine glands: Found in axillary, anal, and genital areas; produce a special kind of sweat that may function in scent and mate attraction.

  • Mammary glands: Modified sweat glands that produce milk.

Clinical Applications

Tattooing and Removal

• Tattooing: Deposits ink into the dermis, where it is taken up by dermal macrophages.

• Laser removal: Uses lasers to break up ink particles, which are then cleared by lymphocytes and macrophages.

Summary Table: Layers of the Skin

Layer	Main Components	Functions
Epidermis	Keratinocytes, melanocytes, tactile cells, dendritic cells	Protection, waterproofing, pigmentation, sensation
Dermis	Connective tissue, blood vessels, nerve endings, hair follicles, glands	Support, nourishment, sensation, thermoregulation
Hypodermis	Adipose and areolar tissue	Insulation, energy storage, anchoring skin

Key Equations

• Body Surface Area (BSA) estimation (Mosteller formula):

$BSA = \sqrt{\frac{height (cm) \times weight (kg)}{3600}}$

• Vitamin D Synthesis: UV radiation + 7-dehydrocholesterol (in skin) → Vitamin D3 (cholecalciferol)

Additional info: Academic context and terminology have been expanded for clarity and completeness.