Integumentary System

Overview

The integumentary system is the body's outer covering and is considered the largest organ system. It includes the skin and its associated structures, serving as a protective barrier and playing vital roles in homeostasis, sensation, and metabolic functions.

Key Terms Related to the Integumentary System

Definitions and Components

• Skin: The main organ of the integumentary system, consisting of multiple layers.

• Hair: Filamentous structures that grow from follicles in the skin.

• Nails: Hard coverings on the distal ends of fingers and toes.

• Sweat (Sudoriferous) Glands: Glands that produce sweat for thermoregulation and excretion.

• Sebaceous (Oil) Glands: Glands that secrete sebum to lubricate and protect the skin.

Primary Functions of the Skin

Major Roles

The skin performs several essential functions to maintain health and homeostasis:

• Protection: Acts as a barrier against physical, chemical, and biological threats.

• Regulation of Temperature: Maintains body temperature through sweat production and blood vessel dilation/constriction.

• Sensory Perception: Contains receptors for touch, pain, temperature, and pressure.

• Synthesis of Vitamin D: Initiates vitamin D production when exposed to UV light.

• Blood Reservoir: Stores blood that can be redirected to other organs as needed.

• Excretion of Wastes: Removes small amounts of metabolic waste through sweat.

Skin Functions: Protection

Barriers Provided by the Skin

The skin protects the body through multiple types of barriers:

• Chemical Barriers:

  • Sweat contains antimicrobial proteins and defensins that kill bacteria.

  • Low pH of skin inhibits bacterial growth.

  • Melanin provides a chemical barrier against UV damage.

• Biological Barriers:

  • Phagocytic cells in the dermis engulf foreign antigens and activate the immune response.

  • Melanin absorbs harmful UV radiation, converting it to harmless heat.

• Physical Barriers:

  • Keratinized cells of the stratum corneum, sealed by glycolipids, block most water and water-soluble substances.

  • Most chemicals have limited penetration of the skin.

Skin Functions: Thermoregulation

Maintaining Body Temperature

The skin helps regulate body temperature through several mechanisms:

• Constriction and dilation of blood vessels in the dermis control heat loss.

• Sweat gland activity increases with rising temperature, producing up to 12 L (3 gallons) of sweat per day in extreme conditions.

• Subcutaneous tissue acts as insulation to reduce heat loss.

At normal body temperature, sweat glands produce unnoticeable sweat. When temperature rises, sweat production increases for cooling.

Skin Functions: Sensory Reception

Role of Sensory Receptors

The skin contains specialized sensory receptors that detect environmental stimuli:

• Touch receptors (Meissner's corpuscles)

• Pain receptors (nociceptors)

• Temperature receptors (thermoreceptors)

• Pressure receptors (Pacinian corpuscles)

These receptors relay information to the brain, allowing perception of the environment.

Skin Functions: Metabolic Functions

Vitamin D Synthesis and Other Roles

• Vitamin D Synthesis: UV exposure stimulates conversion of cholesterol in the skin to vitamin D, essential for calcium and phosphorus absorption and bone health.

• Collagenase Production: Aids in natural turnover of collagen, helping prevent wrinkles.

Skin Functions: Blood Reservoir

Vascular Supply

The skin is highly vascular and can hold up to 5% of the body's blood supply. During times of need, blood can be redirected from the skin to vital organs.

Skin Functions: Excretion

Removal of Wastes

• Skin excretes small amounts of nitrogenous wastes such as ammonia, urea, and uric acid through sweat.

• Excessive sweating can also lead to loss of salt and water.

Structure of the Skin

Main Layers

• Epidermis: Superficial region composed of epithelial tissue; avascular.

• Dermis: Underlies the epidermis; made of connective tissue and is vascular.

• Hypodermis (Subcutaneous Tissue): Not part of the skin but shares some functions; consists of adipose tissue for shock absorption and insulation.

Structure of the Skin: Epidermis

Layers and Cell Types

• Composed of keratinized stratified squamous epithelium.

• Receives nutrients from underlying tissues via diffusion.

• Thick skin (palms, soles) has 5 layers; thin skin has 4 layers.

• Major cell types:

  • Keratinocytes: Produce keratin for protection.

  • Melanocytes: Synthesize melanin pigment for UV protection.

  • Langerhans cells: Immune activators.

  • Merkel cells: Sensory receptors for touch.

Layers of the Epidermis (from deep to superficial)

• Stratum basale (Basal Layer): Deepest layer; active mitosis.

• Stratum spinosum (Prickly Layer): Keratinocytes appear spiky.

• Stratum granulosum (Granular Layer): Keratinization begins; cells above this layer die.

• Stratum lucidum (Clear Layer): Only in thick skin; two to three rows of clear, dead keratinocytes.

• Stratum corneum (Horny Layer): Outermost; 20-30 layers of dead, keratinized cells; provides water resistance and protection.

Structure of the Skin: Dermis

Layers and Features

• Thicker than epidermis; made of flexible connective tissue.

• Contains fibroblasts, macrophages, mast cells, blood and lymphatic vessels, nerves, sensory receptors, and glands.

• Two main layers:

  • Papillary Layer: Upper layer; contains phagocytes, nerve endings, and dermal ridges (fingerprints).

  • Reticular Layer: 80% of dermis; provides strength, elasticity, and hydration; contains collagen fibers and blood vessels.

Stretching of the dermis (e.g., pregnancy) can cause striae (stretch marks).

Skin Colour

Pigments and Clinical Significance

• Melanin: Brown-black pigment produced in skin; protects against UV radiation.

• Carotene: Yellow-orange pigment; most apparent in palms and soles.

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

Alterations in skin color can indicate health issues:

• Cyanosis: Blue tint; low oxygenation.

• Erythema: Redness; fever, hypertension, inflammation, allergy.

• Pallor: Pale; anemia, low blood pressure, fear, anger.

• Jaundice: Yellow; liver disorder.

Skin Appendages

Main Types and Locations

• Hair follicles

• Sweat glands (sudoriferous)

• Sebaceous glands (oil)

• Nails

Skin Appendages: Glands

Types of Cutaneous Glands

• Sweat (Sudoriferous) Glands: Eccrine and apocrine types; distributed widely except external genitalia.

• Sebaceous (Oil) Glands: Associated with hair follicles; secrete sebum.

• Modified Sweat Glands: Ceruminous glands (ear wax), mammary glands (milk).

Structure and Function of Sudoriferous (Sweat) Glands

Types and Functions

• Eccrine (Merocrine) Sweat Glands:

  • Most numerous; found on palms, soles, forehead.

  • Secrete sweat (99% water, salts, vitamin C, antibodies, metabolic wastes).

  • Function in thermoregulation; controlled by sympathetic nervous system.

• Apocrine Sweat Glands:

  • Located in axillary and anogenital areas.

  • Secrete milky or yellowish sweat containing fatty substances and proteins.

  • Bacterial breakdown leads to body odor; begin functioning at puberty.

  • May act as sexual scent glands.

• Modified Sweat Glands:

  • Ceruminous glands: Produce earwax (cerumen).

  • Mammary glands: Produce milk.

Structure and Function of Sebaceous (Oil) Glands

Holocrine Glands

• Found everywhere except thick skin (palms, soles).

• Associated with hair follicles; secrete sebum directly onto skin surface.

• Sebum keeps skin soft, inhibits bacterial growth, and prevents water loss.

• Activated at puberty by hormones (especially androgens).

Structure and Function of Hair & Hair Follicles

Hair Anatomy and Growth

• Hair is made of dead, keratinized cells produced by hair follicles.

• Consists of hard keratin (more durable than soft keratin in skin).

• Hair growth occurs in cycles; hormones affect growth rate.

• Hair color is determined by melanocytes in the follicle; gray/white hair results from decreased melanin production.

• Arrector pili muscle attached to follicle causes "goosebumps".

Types of hair:

• Lanugo: Fine hair covering fetus.

• Vellus: Fine hair on children and adult females.

• Terminal: Coarse hair on scalp, eyebrows, and after puberty in certain areas.

Hair thinning or loss can be caused by genetics, drugs, diseases, or poor nutrition.

Structure and Function of Nails

Nail Anatomy and Clinical Significance

• Nails are modifications of the epidermis containing hard keratin.

• Protect distal, dorsal surfaces of fingers and toes.

• Nail matrix is responsible for nail growth.

• Cuticle (eponychium) is the nail fold projecting onto the nail body.

• Nail color and shape can indicate health status (e.g., spoon nails for iron deficiency, clubbing for respiratory disease).

Functions of Skin Receptors

Sensory Detection

• Detect touch, pressure, pain, and temperature.

• Relay information to the central nervous system for appropriate responses.

Summary Table: Skin Barriers

Key Equations

• Vitamin D Synthesis (simplified):

Example: Excessive sweating during exercise helps cool the body by evaporative heat loss, demonstrating the skin's role in thermoregulation.

Additional info: Some details were inferred and expanded for completeness and clarity, such as the specific names of cell types and the clinical significance of nail changes.