The Integumentary System: Structure and Function

Ch 5: Integumentary System

Introduction to the Integumentary System

The integumentary system is the body's largest organ system, primarily composed of the skin and its derivatives. It serves as the first line of defense against environmental hazards and plays a vital role in homeostasis.

• Largest organ: The skin covers the entire body, accounting for about 7% of total body weight (approximately 4 kg in adults).

• Surface area: Covers about 2 m2 in adults.

• Complexity: Each square inch of skin contains approximately:

  • 19 million cells

  • 60 hairs

  • 90 oil glands

  • 19 feet of blood vessels

  • 625 sweat glands

  • 19,000 sensory cells

  • Over half a million bacteria

Structure of the Skin

Main Layers of the Skin

The skin is composed of three primary layers, each with distinct structures and functions:

Layer	Description	Main Functions
Epidermis	Outermost, thin, avascular layer	Protection, waterproof barrier, sensory reception
Dermis	Middle, thicker, vascular layer	Support, elasticity, houses nerves and blood vessels
Hypodermis (Subcutaneous layer)	Deepest, mostly adipose tissue	Insulation, energy storage, shock absorption, anchors skin to underlying structures

Epidermis

• Stratified squamous epithelium composed mainly of keratinocytes.

• Avascular: Receives nutrients via diffusion from the dermis.

• Major cell types:

  • Keratinocytes: Produce keratin, a tough, protective protein.

  • Melanocytes: Produce melanin pigment, contributing to skin color and UV protection.

  • Langerhans' cells: Immune cells (dendritic cells) that help defend against pathogens.

  • Merkel cells: Sensory receptors for touch.

Dermis

• Connective tissue layer beneath the epidermis.

• Contains: Blood vessels, nerves, hair follicles, sweat and oil glands.

• Two sublayers:

  • Papillary layer: Thin, superficial, areolar connective tissue with dermal papillae (finger-like projections).

  • Reticular layer: Thicker, dense irregular connective tissue providing strength and elasticity.

Hypodermis (Subcutaneous Layer)

• Not technically part of the skin but closely associated.

• Composed mainly of adipose tissue (fat) and some areolar connective tissue.

• Functions: Anchors skin to underlying muscles, stores energy, insulates, and absorbs shock.

Diagram: Layers of the Skin

• Epidermis: Outermost layer, visible to the eye.

• Dermis: Middle layer, contains most skin appendages and blood supply.

• Hypodermis: Deepest layer, primarily fat and connective tissue.

Key Functions of the Integumentary System

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

• Body temperature regulation: Through sweat production and blood vessel dilation/constriction.

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

• Metabolic functions: Synthesis of vitamin D in response to sunlight.

• Excretion: Eliminates small amounts of waste products (e.g., urea, salts) via sweat.

Example: Protective Role of the Skin

• The skin's keratinized surface prevents water loss and entry of pathogens.

• Melanin produced by melanocytes protects underlying tissues from UV radiation.

Additional info:

• The skin is also involved in immune defense, with Langerhans' cells acting as antigen-presenting cells.

• Adipose tissue in the hypodermis serves as an energy reserve and helps maintain body temperature.