The Integumentary System: Structure, Function, and Clinical Relevance

The Integumentary System

Overview

The integumentary system consists of the skin and its derivatives, including sweat and oil glands, hair, and nails. It serves as the body's primary barrier against environmental hazards and plays a vital role in protection, sensation, and homeostasis.

Functions of the Integumentary System

Major Functions

• Protection: Acts as a chemical and physical barrier. The skin contains melanin, sebum, and acid mantle to defend against pathogens. The physical barrier is provided by keratinized cells and tight cell junctions. Substances absorbed include fat-soluble vitamins (A, D, E, K), oxygen, carbon dioxide, and some drugs.

• Body Temperature Regulation: Achieved through sweat production and blood vessel dilation/constriction. Sweat glands can release up to 12L/day during heat stress. Blood vessels constrict in cold, reducing heat loss.

• Cutaneous Sensation: Skin contains sensory receptors for touch, pressure, pain, and temperature, allowing interaction with the environment.

• Metabolic Functions: Synthesis of vitamin D from cholesterol, activation of steroid hormones, and excretion of nitrogenous wastes.

• Blood Reservoir: Skin holds up to 5% of the body's blood volume, which can be redistributed as needed.

• Excretion: Sweat removes nitrogenous wastes, salts, and water.

Characteristics and Structure of Skin

General Properties

• Surface area: 1.5–2.0 m2

• Weight: 9–11 pounds (~7% of body weight)

• Every square inch contains:

  • 15 ft of blood vessels

  • 4 yards of nerves

  • 650 sweat glands

  • 1,500 sensory receptors

  • 3 million cells constantly replaced

• Thickness: 0.5 to 4.0 mm

Layers of the Skin

Epidermis

The epidermis is composed of keratinized stratified squamous epithelium with four distinct cell types and four or five layers. It is replaced every 25–45 days.

• Cell Types:

  • Keratinocytes: Produce keratin, providing protection and waterproofing.

  • Melanocytes: Produce melanin, which gives skin its color and protects against UV radiation.

  • Merkel cells: Sensory receptors for touch.

  • Langerhans cells: Macrophages that activate the immune system.

• Layers of the Epidermis:

  • Stratum basale (germinativum): Deepest layer, site of cell division.

  • Stratum spinosum: Several layers of keratinocytes, contains spiny cells.

  • Stratum granulosum: Three to five layers of flattened cells, contains keratohyalin granules.

  • Stratum lucidum: Thin, clear layer found only in thick skin (palms, soles).

  • Stratum corneum: Outermost layer, dead keratinized cells.

Dermis

The dermis is composed of connective tissue proper and contains collagen and elastic fibers, blood vessels, nerves, and lymphatics. It supports the epidermis and houses skin appendages.

• Major Areas:

  • Papillary layer: Thin, loose CT with dermal papillae, contains Meissner's corpuscles (touch receptors).

  • Reticular layer: Dense irregular CT, provides strength and elasticity, contains Pacinian corpuscles (pressure receptors).

Hypodermis

The hypodermis (subcutaneous layer) is not part of the skin but anchors it to underlying structures. It consists mainly of adipose tissue and acts as a shock absorber and insulator.

• Thicker in females (breasts, thighs) than males (abdomen).

Skin Color

Pigments

• Melanin: Produced by melanocytes, transferred to keratinocytes. Production increases with sun exposure.

• Carotene: Yellow-orange pigment from plant products, most prominent in palms and soles.

• Hemoglobin: Pinkish hue due to oxygenated blood in skin capillaries.

Other factors affecting skin color include emotional states and disease:

• Redness: Increased blood flow due to emotion, fever, or inflammation.

• Pallor: Pale skin due to anemia or low blood pressure.

• Jaundice: Yellow skin due to liver disorder.

• Bronzing: Addison's disease (adrenal cortex hypofunction).

• Cyanosis: Blue skin due to low oxygenation.

Skin Appendages

Glands

• Sudoriferous (Sweat) Glands:

  • Eccrine: Most numerous, found all over the body, secrete watery sweat for thermoregulation.

  • Apocrine: Located in axillary and genital areas, secrete sweat containing fatty acids and proteins, begin functioning at puberty.

  • Ceruminous: Modified apocrine glands in the ear canal, secrete earwax.

  • Mammary: Specialized sweat glands that secrete milk.

• Sebaceous (Oil) Glands:

  • Secrete sebum, lubricates skin and hair, kills bacteria.

  • Activated by hormones, especially during puberty.

Hair

• Composed of fused keratinized cells.

• Hair shaft projects from skin; root is embedded in skin.

• Three layers: medulla (core), cortex (middle), cuticle (outer).

• Color determined by melanin; gray/white hair due to decreased melanin production.

• Hair follicle extends deep into dermis; hair bulb is the base.

• Arrector pili muscle causes hair to stand upright.

• Sebaceous glands secrete oily sebum into the follicle.

• Hair growth cycles: growth phase (longer hair), resting phase (shedding).

Nails

• Scale-like modification of the epidermis covering the dorsal surface of fingers and toes.

• Nail matrix is responsible for growth.

• Cells become keratinized and slide distally over the nail bed.

• Structures: free edge, hyponychium, nail body, lateral and proximal folds, lunula, eponychium (cuticle).

Homeostatic Imbalances

Burns

Burns are classified by severity and extent. They damage skin and underlying tissues, often leading to fluid loss and infection.

• First-degree: Epidermis damaged (e.g., sunburn).

• Second-degree: Epidermis and upper dermis damaged; blisters form.

• Third-degree: Entire thickness of skin damaged; risk of infection and fluid loss.

Rule of Nines: Used to estimate the percentage of body surface area affected by burns.

Body Region	% of Total Body Area
Anterior and posterior head and neck	9%
Anterior and posterior upper limbs	18%
Anterior and posterior trunk	36%
Anterior and posterior lower limbs	36%
Perineum	1%

Frostbite

• Damage to skin cells by exposure to extreme cold.

• Classified similarly to burns.

Cancer

• Basal Cell Carcinoma: Least malignant, most common, slow-growing, rarely metastasizes.

• Squamous Cell Carcinoma: Arises from stratum spinosum, grows rapidly, may metastasize.

• Malignant Melanoma: Most dangerous, arises from melanocytes, highly metastatic, resistant to chemotherapy.

ABCDE Rule for recognizing melanoma:

• A = Asymmetry

• B = Border irregularity

• C = Color

• D = Diameter

• E = Elevation

Key Terms and Concepts

• Keratin: Fibrous protein providing protection and waterproofing.

• Melanin: Pigment responsible for skin color and UV protection.

• Stratified squamous epithelium: Multiple layers of flat cells, found in the epidermis.

• Adipose tissue: Fat-storing tissue in the hypodermis.

• Sudoriferous glands: Sweat glands involved in thermoregulation.

• Sebaceous glands: Oil glands that lubricate skin and hair.

Formulas and Equations

• Rule of Nines Equation:

$\text{Total Burn Area} = \sum \text{(Area % for each region affected)}$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table for Rule of Nines has been recreated for study purposes.