Chapter 5: The Integumentary System – Anatomy & Physiology Study Guide

• Chemical barrier: Antimicrobial proteins, acid mantle, melanin.

• Physical barrier: Keratinized cells, glycolipids.

• Biological barrier: Dendritic cells, macrophages, DNA absorbs UV.

Body Temperature RegulationThe Integumentary System

Overview

The integumentary system is the body's largest organ system, consisting of the skin and its derivatives: hair, nails, sweat glands, and sebaceous glands. It serves as a protective barrier, regulates temperature, and provides sensory information.

• Components: Skin, hair, nails, sweat glands, sebaceous (oil) glands

• Main Functions: Protection, regulation, sensation, metabolic activity, blood reservoir, excretion

Structure of Skin

Skin Layers

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

• Epidermis: The superficial, avascular layer made of epithelial tissue.

• Dermis: The deeper, vascular layer composed of connective tissue.

• Hypodermis (Subcutaneous layer): Not technically part of the skin, but anchors it to underlying structures; mainly adipose tissue for shock absorption and insulation.

Epidermis

Cell Types in the Epidermis

The epidermis is primarily made of keratinized stratified squamous epithelium and contains four main cell types:

• Keratinocytes: Produce keratin, the protein that gives skin its strength and waterproofing.

• Melanocytes: Produce melanin pigment, which protects against UV radiation.

• Dendritic (Langerhans) cells: Immune cells that patrol the epidermis.

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

Layers of the Epidermis

The epidermis consists of four or five distinct layers (strata), depending on skin thickness:

1. Stratum basale: Deepest layer; single row of stem cells; active mitosis; contains melanocytes.

2. Stratum spinosum: Several layers thick; contains keratinocytes, melanosomes, dendritic cells.

3. Stratum granulosum: Four to six layers; keratinization begins; cells flatten and die.

4. Stratum lucidum: Only in thick skin (palms, soles); two to three rows of clear, dead keratinocytes.

5. Stratum corneum: 20–30 rows of dead, keratinized cells; provides protection and prevents water loss.

Dermis

Structure and Layers

The dermis is a strong, flexible connective tissue layer containing nerves, blood vessels, lymphatics, hair follicles, and glands. It is divided into two layers:

• Papillary layer: Superficial; areolar connective tissue; contains dermal papillae with capillaries and touch receptors.

• Reticular layer: Deep; dense irregular connective tissue; provides strength and elasticity; contains cleavage and flexure lines.

Dermal Modifications

• Friction ridges: Enhance grip and create fingerprints.

• Cleavage (tension) lines: Parallel collagen fibers; important for surgical incisions.

• Flexure lines: Dermal folds near joints; visible on hands and feet.

Clinical Note: Stretch Marks and Blisters

• Striae: Silvery white scars from dermal tears (stretch marks).

• Blisters: Fluid-filled pockets separating epidermal and dermal layers.

Skin Color

Pigments

Three pigments contribute to skin color:

• Melanin: Produced by melanocytes; shields DNA from UV; color varies from yellow to black.

• Carotene: Yellow to orange pigment; accumulates in stratum corneum and hypodermis.

• Hemoglobin: Red pigment in blood; gives pinkish hue to fair skin.

Clinical Note: Skin Color Changes

• Cyanosis: Blue skin; low oxygenation.

• Pallor: Pale skin; anemia or low blood pressure.

• Erythema: Redness; fever, inflammation.

• Jaundice: Yellow skin; liver disorder.

• Bruises: Clotted blood beneath skin.

Hair

Structure and Function

Hair consists of dead, keratinized cells and is produced by hair follicles. Functions include protection, sensation, and regulation.

• Regions: Shaft (above skin), root (within skin).

• Parts of shaft: Medulla (core), cortex (middle), cuticle (outer layer).

• Pigmentation: Melanocytes produce melanin; color varies by type and amount.

Hair Follicle Structure

• Hair bulb: Expanded deep end; contains matrix for cell division.

• Arrector pili: Smooth muscle causing "goose bumps".

• Hair papilla: Dermal tissue supplying nutrients.

Types and Growth

• Vellus hair: Fine, pale body hair.

• Terminal hair: Coarse, long hair (scalp, eyebrows, pubic regions).

• Growth: Influenced by nutrition and hormones; follicles cycle between active and regressive phases.

Clinical Note: Hair Disorders

• Hirsutism: Excessive hairiness due to androgen excess.

• Alopecia: Hair thinning after age 40.

• Male pattern baldness: Genetic, influenced by DHT.

• Telogen effluvium: Abrupt hair thinning due to stress or illness.

Nails

Structure and Function

Nails are scale-like modifications of the epidermis containing hard keratin. They protect the distal phalanges and aid in manipulation.

• Parts: Free edge, nail plate, root, nail bed, nail matrix (growth area).

• Nail folds: Skin folds overlapping nail border.

• Eponychium: Cuticle.

• Hyponychium: Area under free edge.

• Lunule: White, thickened area of matrix.

Clinical Note: Nail Disorders

• Yellow nails: May indicate respiratory or thyroid disorder.

• Thickened yellow nails: Fungal infection.

• Koilonychia: Spoon-shaped nails; iron deficiency.

• Beau’s lines: Horizontal lines; severe illness.

Cutaneous Glands

Sweat Glands

• Eccrine (merocrine) glands: Most numerous; thermoregulation; secrete watery sweat.

• Apocrine glands: Found in axillary/anogenital areas; secrete viscous sweat; function as scent glands.

• Modified apocrine glands: Ceruminous (earwax) and mammary (milk) glands.

Sebaceous (Oil) Glands

• Location: Everywhere except palms and soles; usually associated with hair follicles.

• Function: Secrete sebum (oily, bactericidal); softens skin and hair.

• Activation: Stimulated by hormones, especially androgens.

Summary Table: Cutaneous Glands

Gland Type	Function	Secretion	Location
Eccrine Sweat	Temperature control, antibacterial	Watery, hypotonic filtrate	Palms, soles, forehead
Apocrine Sweat	Sexual scent	Viscous, fatty/protein-rich	Axillary, anogenital
Sebaceous	Lubricate, antibacterial	Oily sebum	Everywhere except palms/soles

Clinical Note: Gland Disorders

• Acne: Infectious inflammation of sebaceous glands.

• Whiteheads/Blackheads: Blocked sebaceous glands.

• Cradle cap (seborrhea): Overactive sebaceous glands in infants.

Functions of Skin

Protection

The skin acts as a barrier against physical, chemical, and biological threats.

• Insensible perspiration: Unnoticeable sweat (500 ml/day).

• Sensible perspiration: Noticeable sweat (up to 12 L/day) for cooling.

• Cold response: Dermal vessels constrict to conserve heat.

Cutaneous Sensations

• Exteroreceptors: Respond to external stimuli (touch, temperature).

• Free nerve endings: Sense pain.

Metabolic Functions

• Vitamin D synthesis: Needed for calcium absorption.

• Keratinocytes: Disarm carcinogens, activate hormones, produce collagenase.

Blood Reservoir

• Skin holds up to 5% of blood volume.

• Vessels can constrict to redirect blood.

Excretion

• Excretes nitrogenous wastes: Ammonia, urea, uric acid.

• Sweating: Causes salt and water loss.

Skin Cancer and Burns

Skin Cancer

• Basal cell carcinoma: Most common, least malignant; arises from stratum basale.

• Squamous cell carcinoma: Second most common; arises from stratum spinosum; can metastasize.

• Melanoma: Most dangerous; cancer of melanocytes; highly metastatic.

Burns

• First-degree: Epidermal damage only; redness, pain.

• Second-degree: Epidermal and upper dermal damage; blisters.

• Third-degree: Entire skin thickness; gray-white, red, or black; no pain (nerve destruction); requires grafting.

• Rule of Nines: Used to estimate fluid loss and burn severity.

Developmental Aspects of the Integumentary System

Fetal Development

• Lanugo coat: Delicate hairs in 5th/6th month.

• Vernix caseosa: Sebaceous secretion protecting fetal skin.

Infancy to Adulthood

• Skin thickens, fat accumulates, gland activity increases.

• Optimal appearance in 20s/30s; aging leads to thinning, dryness, wrinkles, increased cancer risk.

Aging Skin

• Decreased replacement, elasticity, fat, and immune cells.

• Hair thinning.

• Prevention: UV protection, nutrition, hydration, hygiene.