Integumentary System: Structure and Function (Chapter 5 Study Notes)

Integumentary System

Overview

The integumentary system, primarily composed of the skin, is the body's largest organ system and serves as a protective barrier between the internal environment and the outside world. It includes the skin, hair, nails, and various glands, each contributing to the system's diverse functions.

Primary Functions of the Skin

Protection

The skin provides multiple layers of protection against physical, chemical, and biological threats.

• Chemical Protection: Sweat and oily secretions (sebum) contain antimicrobial substances that kill bacteria. Melanin pigment absorbs and disperses ultraviolet (UV) radiation, preventing DNA damage.

• Physical Protection: The skin acts as a physical barrier, preventing the entry of pathogens and minimizing mechanical injury.

• Biological Protection: Dendritic cells (Langerhans cells) in the epidermis activate immune responses. Macrophages in the dermis destroy invading microorganisms and debris.

Temperature Regulation

The skin helps maintain homeostasis of body temperature through sweat production and blood flow regulation.

• Insensible Perspiration: Continuous, unnoticeable sweat loss that aids in basic thermoregulation and prevents dehydration.

• Sensible Perspiration: Noticeable sweat produced during heat stress, facilitating heat loss via evaporation.

• Vasoconstriction: In cold environments, blood vessels constrict, reducing blood flow to the skin and minimizing heat loss. Blood is redirected to deeper tissues, helping preserve core temperature.

Sensation & Metabolic Functions

• Sensation: The skin contains sensory receptors (nociceptors, thermoreceptors, mechanoreceptors) that detect pain, temperature, and touch.

• Metabolic Functions: The skin is involved in vitamin D synthesis when exposed to UV light. Vitamin D is essential for calcium absorption and bone health.

Blood Reservoir & Excretion

• Blood Reservoir: Up to 5% of the body's total blood volume can be found in the skin, which can be redirected as needed by the central nervous system.

• Excretion: The skin allows for the elimination of small amounts of nitrogenous wastes (urea, ammonia, uric acid) through sweat.

Structure of the Integument (Skin)

Layers of the Skin

• Epidermis: The avascular, outermost layer composed primarily of epithelial cells.

• Dermis: The vascularized, deeper layer made up of connective tissue, providing strength and flexibility.

Additional Layer: Hypodermis

The hypodermis (subcutaneous layer) lies deep to the dermis and is not technically part of the skin. It consists mostly of adipose tissue and serves several important functions:

• Storage: Provides an accessible energy reserve.

• Protection/Shock Absorption: Cushions underlying tissues against trauma.

• Insulation: Reduces heat loss from the body.

• Anchor: Attaches skin to underlying muscles while maintaining flexibility.

Microscopic Structure of the Skin

Epidermis

The epidermis is composed of several cell types and distinct layers:

• Keratinocytes: Produce keratin, a protein that provides strength and waterproofing. These cells proliferate from the basal layer and migrate upward, replacing lost cells every 25-45 days. Persistent friction leads to callus formation.

• Melanocytes: Located in the basal layer, these cells produce melanin pigment within organelles called melanosomes. Melanin is transferred to keratinocytes and accumulates on the "sunny side" to protect against UV damage.

• Dendritic (Langerhans) Cells: Immune cells that help activate the body's defense mechanisms.

• Tactile (Merkel) Cells: Sensory receptors at the epidermal-dermal junction, allowing perception of touch.

Layers of the Epidermis (from deep to superficial)

• Stratum Basale: Deepest layer, contains stem cells that divide to produce new keratinocytes. Also contains melanocytes and Merkel cells.

• Stratum Spinosum: Several layers of keratinocytes with pre-keratin filaments; dendritic cells are most abundant here.

• Stratum Granulosum: Cells accumulate keratohyaline granules (for keratin formation) and lamellar granules (water-resistant glycolipids).

• Stratum Lucidum: Thin, clear layer of dead cells found only in thick skin (e.g., palms, soles).

• Stratum Corneum: Outermost layer of dead, flattened keratinocytes; provides waterproofing and protection against friction.

Dermis

The dermis is a strong, flexible connective tissue layer containing fibroblasts, macrophages, and a rich matrix of fibers.

• Papillary Dermis: Superficial layer of areolar connective tissue with finger-like projections (dermal papillae) that form friction ridges (fingerprints). Contains pain receptors and tactile corpuscles.

• Reticular Dermis: Deeper layer of dense irregular connective tissue. Contains cleavage lines (from collagen fiber separation) and flexure lines (skin folds at joints).

Skin Color

Pigments Determining Skin Color

• Melanin: Dark pigment synthesized by melanocytes; amount and distribution determine skin color and tanning response.

• Carotene: Yellow-orange pigment stored in the stratum corneum and adipose tissue; used for vitamin A synthesis, important for epidermal health.

• Hemoglobin: Red pigment in blood; gives skin a pinkish hue, especially in fair-skinned individuals.

Homeostatic Imbalances of Skin Color

• Jaundice: Yellowing due to liver dysfunction.

• Albinism/Melanism: Genetic mutations affecting melanin production.

• Cyanosis: Bluish tint from low oxygenated hemoglobin.

• Argyria: Gray-blue discoloration from silver exposure.

Skin Appendages

Hair

Hair is composed of dead, keratinized cells and serves protective and sensory functions.

• Root: Embedded in the skin.

• Shaft: Projects above the skin surface.

• Layers: Medulla (central core), cortex (flattened cells), cuticle (outermost, heavily keratinized).

• Follicle: Invagination of epidermis into dermis; contains the hair matrix (site of cell division).

• Associated Structures: Root hair plexus (sensory nerve endings), hair papilla (capillaries), arrector pili (smooth muscle causing hair to stand upright).

Hair Texture: Determined by follicle shape (round = straight, oval = wavy, flat = curly). Hair Color: Determined by melanin type and amount; gray/white hair results from decreased melanin and air bubbles in the shaft.

Nails

Nails are hard keratin structures composed of a root, nail plate, and free edge. The nail matrix is responsible for growth. Nails protect the distal phalanges and aid in dexterity.

Skin Glands

• Eccrine Sweat Glands: Most numerous; secrete watery sweat directly to skin surface for temperature regulation.

• Apocrine Sweat Glands: Located in axillary and anogenital areas; secrete sweat into hair follicles, containing fatty substances and proteins. Modified versions include ceruminous (earwax) and mammary glands.

• Sebaceous (Oil) Glands: Secrete sebum into hair follicles; lubricates skin/hair, prevents water loss, and inhibits bacterial growth.

Homeostatic Imbalances of the Skin

Skin Cancer

Skin cancer is the most common type of cancer, with several forms:

Type	Origin	Features
Basal Cell Carcinoma	Stratum basale cells	Most common, least malignant, slow-growing, rarely metastasizes
Squamous Cell Carcinoma	Stratum spinosum cells	Second most common, can metastasize, appears as scaly red lesions
Melanoma	Melanocytes	Most dangerous, rapid metastasis, often arises from moles

ABCD(E) Rule: Used for early detection of melanoma: Asymmetry, Border irregularity, Color variation, Diameter (>6mm), Evolving.

Burns

Degree	Features	Healing
First-degree	Pain, redness, inflammation	No scarring, heals quickly
Second-degree	Pain, redness, blisters	May scar, longer healing
Third-degree	Full thickness, skin loss	Requires medical intervention (IV fluids, skin grafts)

Additional info: The integumentary system also plays a role in immunity, communication, and synthesis of biochemical compounds essential for health.