The Integumentary System

Overview

The integumentary system is composed of the skin and its associated structures, including hair, nails, and glands. It serves as the body's primary barrier against environmental hazards and plays a vital role in protection, sensation, temperature regulation, and metabolic functions.

Hair

Structure and Function

• Hair consists of dead keratinized cells and is located on most skin surfaces except palms, soles, lips, nipples, and parts of external genitalia.

• Functions of hair:

  • Warns of insects on skin

  • Protects head against physical trauma

  • Reduces heat loss

  • Shields skin from sunlight

Structure of a Hair

• Hair (pili): Flexible strands of dead, keratinized cells produced by hair follicles.

• Regions:

  • Shaft: Area where keratinization is complete; extends above scalp surface.

  • Root: Area within scalp; keratinization is ongoing.

• Three parts of hair shaft:

  1. Medulla (middle): Central core of large cells and air spaces.

  2. Cortex: Several layers of flattened cells surrounding medulla.

  3. Cuticle: Outer layer consisting of overlapping layers of single cells.

Structure of a Hair Follicle

• Extends from epidermal surface to dermis.

• Hair bulb: Expanded area at deep end of follicle.

• Hair follicle receptor (root hair plexus): Sensory nerve endings that wrap around bulb; acts as a touch receptor.

• Wall of follicle composed of:

  • Peripheral connective tissue sheath: Derived from dermis.

  • Glassy membrane: Thickened basal lamina.

  • Epithelial root sheath: Derived from epidermis.

• Hair matrix: Actively dividing area of bulb that produces hair cells.

• Arrector pili: Small band of smooth muscle attached to follicle; responsible for "goose bumps".

• Hair papilla: Dermal tissue containing a knot of capillaries that supplies nutrients to growing hair.

Types and Growth of Hair

• Vellus hair: Pale, fine body hair of children and adult females.

• Terminal hair: Coarse, long hair found on scalp and eyebrows; appears at puberty in axillary and pubic regions of both sexes.

• Follicles cycle between active and regressive phases; average growth is 2.25 mm per week, and 90 scalp hairs are lost daily.

Hair Thinning and Baldness

• Alopecia: Hair thinning in both sexes after age 40.

• True (frank) baldness: Genetically determined and sex-influenced; male pattern baldness caused by follicular response to DHT.

Nails

Structure and Function

• Nails are scale-like modifications of the epidermis that act as protective covers for the distal surface of fingers and toes.

• Components:

  • Free edge

  • Nail plate

  • Root

• Nail matrix: Thickened portion responsible for nail growth.

• Nail folds: Skin folds that overlap border of nail.

• Eponychium: Nail fold that projects onto surface of nail body (cuticle).

• Hyponychium: Area under free edge of plate that accumulates dirt.

• Lunula: Thickened nail matrix, appears white.

Clinical - Homeostatic Imbalance

• Nail appearance can help diagnose some diseases:

  • Yellowing: May indicate respiratory or thyroid gland disorder.

  • Thickened yellow nails: May signal fungal infection.

  • Koilonychia (spoon nail): Outward concavity; may signal iron deficiency.

  • Beau's lines: Horizontal lines; may indicate severe illnesses such as uncontrolled diabetes, heart attack, or cancer chemotherapy.

Glands of the Skin

Types of Cutaneous Glands

• Sudoriferous (Sweat) Glands: All skin surfaces except nipples and parts of external genitalia contain sweat glands. About 3 million per person.

  • Eccrine (merocrine) sweat glands: Most numerous; abundant on palms, soles, and forehead. Ducts connect to pores; function in thermoregulation. Regulated by sympathetic nervous system. Secrete "sweat" (99% water, salts, vitamin C, antibodies, dermcidin).

  • Apocrine sweat glands: Axillary and anogenital areas; secrete viscous milky or yellowish sweat containing fatty substances and proteins. Begin functioning at puberty; ducts empty into hair follicles.

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

• Sebaceous (Oil) Glands: Widely distributed except for thick skin of palms and soles. Most develop from hair follicles and secrete into hair follicles. Secrete sebum (oily holocrine secretion); bactericidal properties; softens hair and skin.

Table 5.1: Summary of Cutaneous Glands

Clinical - Homeostatic Imbalance

• Acne: Usually an infectious inflammation of sebaceous glands, resulting in pimples.

• Whiteheads: Blocked sebaceous glands; if secretion oxidizes, becomes a blackhead.

• Cradle cap: Overactive sebaceous glands in infants; scalp lesions that flake off.

Functions of Skin

Main Functions

• Protection

• Body temperature regulation

• Cutaneous sensations

• Metabolic functions

• Blood reservoir

• Excretion of wastes

1. Protection

• Chemical barrier: Skin secretes chemicals (antimicrobial proteins, sweat, sebum, defensins) that kill bacteria. Acid mantle (low pH) retards bacterial multiplication. Melanin protects against UV radiation damage.

• Physical barrier: Flat, dead, keratinized cells of stratum corneum surrounded by glycolipids block most water and water-soluble substances. Some chemicals can penetrate skin (e.g., organic solvents, heavy metals).

• Biological barrier: Epidermis contains phagocytic cells (dendritic cells) and dermis contains macrophages, which activate immune responses. DNA absorbs UV radiation, converting it to harmless heat.

2. Body Temperature Regulation

• Under normal conditions, sweat glands produce about 500 mL/day of unnoticeable sweat (insensible perspiration).

• If body temperature rises, dermal vessels dilate and sweat gland activity increases, producing up to 12 L (3 gallons) of noticeable sweat (sensible perspiration).

• In cold environments, dermal blood vessels constrict, and skin temperature drops to slow passive heat loss.

3. Cutaneous Sensations

• Skin contains sensory receptors for touch, pressure, temperature, and pain.

4. Metabolic Functions

• Skin synthesizes vitamin D needed for calcium absorption in intestine.

• Chemicals from keratinocytes can disarm some carcinogens.

• Keratinoctyes can activate some hormones.

• Skin makes collagenase, aiding in natural turnover of collagen to prevent wrinkles.

5. Blood Reservoir

• Skin can hold up to 5% of the body's total blood volume.

• Skin vessels can be constricted to shunt blood to other organs, such as exercising muscle.

6. Excretion

• Skin can secrete limited amounts of nitrogenous wastes (ammonia, urea, uric acid).

• Sweating can cause salt and water loss.

Developmental Aspects of the Integumentary System

Fetal Development

• By end of 4th month, skin of fetus is developed.

• Vernix caseosa: Sebaceous gland secretion that protects skin of fetus in watery amniotic fluid.

Infancy to Adulthood

• Skin thickens and accumulates more subcutaneous fat; sweat and sebaceous gland activity increases, leading to acne.

• Optimal appearance in 20s and 30s.

• Scaling and dermatitis become more common with age.

Aging

• Epidermal replacement slows; skin becomes thin, dry, and itchy.

• Decreased sebaceous gland activity.

• Subcutaneous fat and elasticity decrease.

• Increased risk of cancer due to decreased numbers of melanocytes and dendritic cells.

• Hair thinning.

Ways to Delay Aging

• 1. [Not specified]

• 2. [Not specified]

• 3. [Not specified]

• 4. [Not specified]

Key Terms and Definitions

• Keratin: A fibrous protein forming the main structural constituent of hair, nails, and the outer layer of skin.

• Melanin: A pigment that gives skin its color and protects against UV radiation.

• Arrector pili: Small muscle attached to hair follicles, responsible for "goose bumps".

• Sebum: Oily secretion of sebaceous glands.

• Vernix caseosa: Waxy or cheese-like white substance found coating the skin of newborns.

Important Equations

• Sweat production (sensible perspiration):

Additional info: Some content was expanded for clarity and completeness, including definitions and explanations of clinical terms and developmental aspects.