Chapter 5: The Integumentary System

5.1 Functions of the Integumentary System

The integumentary system, primarily composed of the skin and its derivatives, serves as the body's first line of defense and performs several vital physiological roles.

• Protection: Acts as a barrier against mechanical injury, pathogens, and harmful substances.

• Sensation: Contains sensory receptors for touch, pain, temperature, and pressure.

• Temperature Regulation: Regulates body temperature through sweat production and blood flow.

• Vitamin D Production: Synthesizes vitamin D when exposed to ultraviolet (UV) light.S

• Excretion: Eliminates small amounts of waste products through sweat.

5.2 Skin

The skin, also known as the cutaneous membrane, is the largest organ of the body and consists of two primary layers: the epidermis and the dermis.

• Epidermis: The outermost layer, composed of stratified squamous epithelium. It is avascular and provides a protective barrier.

• Dermis: The deeper, thicker layer made of connective tissue, containing blood vessels, nerves, and accessory structures.

Epidermis

• Cell Types:

  • Keratinoctyes: Produce keratin, a protein that strengthens the skin.

  • Melanocytes: Synthesize melanin, the pigment responsible for skin color and UV protection.

  • Langerhans Cells: Immune cells involved in defense against pathogens.

  • Merkel Cells: Sensory receptors for touch.

• Strata (Layers): The epidermis is organized into 4-5 layers (strata):

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

  • Stratum spinosum: Provides strength and flexibility.

  • Stratum granulosum: Cells begin to die and accumulate keratin.

  • Stratum lucidum: Present only in thick skin (palms, soles).

  • Stratum corneum: Outermost, dead, keratinized cells.

Thick and Thin Skin

• Thick Skin: Found on palms and soles; contains all five strata, including stratum lucidum.

• Thin Skin: Covers most of the body; lacks stratum lucidum and has a thinner stratum corneum.

Skin Color

Skin color is determined by the interaction of pigments and blood supply:

• Carotene: Yellow-orange pigment from diet, accumulates in the stratum corneum.

• Melanin: Brown-black pigment produced by melanocytes; protects against UV radiation.

• Dermal Blood Supply: Oxygenated blood gives a pinkish hue; reduced oxygenation causes cyanosis (bluish color).

Dermis

• Papillary Layer: Superficial, areolar connective tissue with dermal papillae that form fingerprints.

• Reticular Layer: Deeper, dense irregular connective tissue; provides strength and elasticity.

• Cleavage Lines: Patterns of collagen fibers; important in surgical incisions for healing.

5.3 Subcutaneous Tissue

The subcutaneous tissue (hypodermis) lies deep to the skin and connects it to underlying structures.

• Composition: Primarily adipose (fat) and areolar connective tissue.

• Functions:

  • Insulation and energy storage

  • Cushioning and protection

  • Anchors skin to underlying tissues

5.4 Accessory Skin Structures

Accessory structures include hair, glands, and nails, each with specialized functions.

Hair

• Types: Vellus hair (fine, peach fuzz) and terminal hair (thicker, on scalp, eyebrows, etc.).

• Functions: Protection, sensation, and reduction of heat loss.

Glands

• Sebaceous Glands: Secrete sebum (oil) to lubricate skin and hair.

• Sweat Glands: Two types:

  • Eccrine: Widely distributed, regulate temperature.

  • Apocrine: Found in axillae and groin, active after puberty.

• Mammary Glands: Modified sweat glands that produce milk.

• Ceruminous Glands: Produce earwax (cerumen) in the ear canal.

Nails

• Location: Found on the tips of fingers and toes.

• Function: Protect distal phalanges, enhance sensation, and aid in grasping.

5.5 Physiology of the Integumentary System

The skin performs several physiological functions essential for homeostasis and protection.

Protection

• Mechanical protection for underlying tissues

• Prevents entry of pathogens and harmful substances

• Melanin protects against UV damage

• Hair and nails provide additional protective measures

• Reduces water loss from the body

Sensation

• Contains receptors for touch, pressure, pain, and temperature

Temperature Regulation

• Sweat glands and blood vessel dilation/constriction help maintain body temperature

Vitamin D Production

• UV light converts 7-dehydrocholesterol in the skin to vitamin D3 (cholecalciferol)

• Vitamin D is essential for calcium absorption and bone health

Excretion

• Elimination of small amounts of urea, salts, and water through sweat

Clinical Impact 5.1: Skin Cancer

Skin cancer is a significant clinical concern, with three main types:

1. Basal Cell Carcinoma: Most common, arises from stratum basale, rarely metastasizes.

2. Squamous Cell Carcinoma: Originates from stratum spinosum, can metastasize if untreated.

3. Melanoma: Arises from melanocytes, highly malignant and likely to metastasize.

ABCDE Rule for Melanoma Detection:

• Asymmetry: One half of the mole does not match the other.

• Border: Edges are irregular, ragged, or blurred.

• Color: Varies from one area to another; shades of tan, brown, black, or sometimes white, red, or blue.

• Diameter: Larger than 6 mm (about the size of a pencil eraser).

• Evolving: Changes in size, shape, or color over time.