The Integumentary System: Structure, Function, and Clinical Relevance

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The Integumentary System

Overview and Functions

The integumentary system consists of the skin and its derivatives, including sweat and oil glands, hair, and nails. It serves as the primary barrier between the body and the external environment, providing protection, sensation, and other vital functions.

Protection: Acts as a physical, chemical, and biological barrier against pathogens, trauma, and UV radiation.
Waterproofing: Prevents excessive water loss and entry.
Washable and Durable: Resists wear and tear, and can be cleaned.
Self-Healing: Repairs minor injuries through wound healing phases.
Air-Conditioned: Regulates body temperature via sweat and blood flow.
Sensation: Detects touch, pain, temperature, and pressure.
Metabolic Functions: Synthesizes vitamin D and participates in other metabolic processes.

Structure of the Skin

Major Regions

The skin is composed of two main layers: the epidermis and the dermis. Beneath these lies the hypodermis (subcutaneous tissue), which is not technically part of the skin but shares some protective functions.

Epidermis: Outermost layer; made of keratinized stratified squamous epithelium.
Dermis: Deeper layer; composed of connective tissue, blood vessels, nerves, and accessory structures.
Hypodermis: Subcutaneous tissue; consists of areolar and adipose tissue, providing insulation and shock absorption.

Cells of the Epidermis

Keratinocytes: Main cell type; produce keratin for strength and waterproofing. Lifespan is 25-45 days.
Melanocytes: Located in the deepest layer; produce melanin, which protects against UV damage and determines skin color.
Dendritic (Langerhans) Cells: Star-shaped immune cells; act as macrophages and activate the immune system.
Tactile (Merkel) Cells: Located at the epidermis/dermis boundary; function as touch receptors.

Layers of the Epidermis

The epidermis is organized into distinct layers, each with specific functions and cell types.

Stratum Corneum: Outermost layer; 20-30 layers of dead, keratinized cells.
Stratum Lucidum: Clear layer found only in thick skin (palms, soles); contains eleidin, a lipid-rich protein.
Stratum Granulosum: Contains keratohyalin granules; cells begin to die and lose organelles.
Stratum Spinosum: Several layers of keratinocytes; provides strength and flexibility.
Stratum Basale: Deepest layer; site of active mitosis and contains melanocytes.

Additional info: Eleidin in the stratum lucidum provides some waterproofing and is converted to keratin as cells move outward.

Thick Skin vs. Thin Skin

Thick Skin: Found on palms and soles; contains all five epidermal layers, including stratum lucidum.
Thin Skin: Covers most of the body; lacks stratum lucidum and has fewer layers in the stratum corneum.

Dermis

Layers of the Dermis

The dermis is a connective tissue layer that provides structural support and houses blood vessels, nerves, and accessory structures.

Papillary Layer: Superficial; made of areolar connective tissue, contains dermal papillae that form friction ridges (fingerprints).
Reticular Layer: Deep; dense irregular connective tissue with thick bundles of collagen fibers, source of lines of cleavage (important for surgical incisions).

Special Features

Friction Ridges: Enhance grip and form fingerprints.
Flexure Lines: Dermal folds at joints (palms, fingers) where skin is tightly secured to underlying structures.

Hypodermis (Subcutaneous Tissue)

The hypodermis anchors the skin to underlying tissues, provides insulation, and stores fat.

Composition: Areolar connective tissue, adipose tissue, blood vessels.
Functions: Shock absorption, insulation, energy storage.

Skin Pigments

Types of Pigments

Melanin: Only pigment produced in the skin; derived from tyrosine. Color ranges from reddish-yellow to brownish-black.
Carotene: Yellow to orange pigment from plant products; accumulates in stratum corneum and hypodermis.
Hemoglobin: From blood; gives skin a pinkish hue, especially in light-skinned individuals.

Cyanosis: Bluish discoloration due to low oxygen levels in blood.

Accessory Structures of the Skin

Hair

Hair serves protective and sensory functions and is composed of hard keratin.

Functions: Senses insects, guards head, shields eyes, filters air.
Structure: Shaft (visible part), root (embedded in skin), bulb (expanded base with papilla and root hair plexus).
Layers: Medulla (central), cortex (pigment), cuticle (outermost).
Texture: Follicle shape determines straight or curly hair; size determines thickness.
Arrector pili muscle: Contracts to raise hair and dimple skin (goosebumps).
Sebaceous gland: Secretes sebum for lubrication and waterproofing.

Hair Thinning & Baldness: Influenced by hormones, nutrition, and genetics. Hair growth cycles include active and resting phases.

Nails

Nails are protective, keratinized modifications of the epidermis.

Parts: Free edge, body, root, nail folds (lateral and proximal), lunula (crescent-shaped area).
Nail Matrix: Site of nail growth.

Glands

Sudoriferous (Sweat) Glands: Eccrine (most common; thermoregulation) and apocrine (axillary/anogenital; scent production).
Sebaceous Glands: Holocrine glands that secrete sebum; bactericidal and lubricating.
Other Glands: Ceruminous (ear wax) and mammary (milk).

Functions of the Skin

Major Functions

Protection: Physical, chemical (acid mantle, dermcidin), and biological (immune cells) barriers.
Temperature Regulation: Sweating and blood flow adjustments.
Sensation: Touch, pain, temperature, pressure.
Metabolic: Vitamin D synthesis, conversion of substances.
Blood Reservoir: Dermis holds ~5% of total blood volume.
Excretion: Removal of metabolic wastes via sweat.

Clinical Relevance: Burns

Burns and Their Classification

First-degree: Only epidermis affected.
Second-degree: Epidermis and superficial dermis affected.
Third-degree: Entire thickness of skin (epidermis + all of dermis) affected.

Rule of Nines

The Rule of Nines is used to estimate the percentage of body surface area affected by burns.

Body Region	Percentage
Head and Neck (anterior & posterior)	9%
Upper Limbs (anterior & posterior)	18%
Trunk (anterior & posterior)	36%
Perineum	1%
Lower Limbs (anterior & posterior)	36%
Total	100%

Wound Healing

Phases of Wound Healing

Inflammation: Initial response to injury; blood vessels dilate and immune cells migrate to the site.
New Vessel Formation and Tissue Remodeling: Formation of new blood vessels and replacement of damaged tissue.
Healed Wound: Restoration of tissue integrity and function.

Key Terms and Concepts

Keratin: A tough, fibrous protein produced by keratinocytes.
Melanin: Pigment responsible for skin color and UV protection.
Dermis: Connective tissue layer beneath the epidermis.
Hypodermis: Subcutaneous layer providing insulation and energy storage.
Sudoriferous Glands: Sweat glands involved in thermoregulation and excretion.
Sebaceous Glands: Oil glands that lubricate and protect the skin.

Formulas and Equations

Rule of Nines Equation: