The Integumentary System: Structure, Function, and Components

Chapter 5: The Integumentary System

Overview

The integumentary system is composed of the skin, hair, nails, and associated glands. It serves as the body's primary barrier against the external environment and plays a vital role in protection, sensation, and regulation.

• Main components: Skin, accessory structures (glands, sweat & oil, hair, nails)

• Primary function: Protection (mechanical and chemical)

• Definition: Integument means covering; refers to an enveloping layer of an organism or one of its parts.

Structure of the Skin

Layers of the Skin

The skin consists of three main layers, each with distinct functions and structures.

• Epidermis: Outermost layer; composed mainly of epithelial cells.

• Dermis: Middle layer; contains connective tissue, blood vessels, nerves, and accessory structures.

• Hypodermis (Subcutaneous layer): Deepest layer; primarily adipose tissue, provides insulation and cushioning.

Diagram: Skin Structure

Refer to labeled diagrams for visual representation of skin layers and associated structures.

Epidermis

Cell Types in the Epidermis

The epidermis contains several specialized cell types, each contributing to skin function.

• Keratinoctyes: Produce keratin, a protein that provides strength and waterproofing.

• Basal cells: Stem cells located in the deepest layer; undergo mitosis to replenish epidermal cells.

• Merkel cells: Sensory cells involved in touch perception.

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

• Dendritic (Langerhans) cells: Immune cells that help fight infection.

Layers of the Epidermis

• Stratum corneum: Outermost layer; dead, keratinized cells.

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

• Stratum granulosum: Cells begin to die and keratinize.

• Stratum spinosum: Several layers of keratinocytes.

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

Dermis

Layers of the Dermis

The dermis is divided into two layers, each with unique features.

Layer	Main Features
Papillary layer	Loose connective tissue; contains dermal papillae, capillaries, and sensory receptors
Reticular layer	Dense irregular connective tissue; provides strength and elasticity, contains hair follicles, sweat glands

Dermal Ridges

• Dermal ridges: Found on palms, soles, and ends of fingers; form fingerprints and aid in grip.

Cleavage and Flexure Lines

• Flexure lines: Occur where the dermis is tightly secured to deeper structures, causing visible skin creases.

• Cleavage lines: Arranged by collagen fibers; important in surgery for healing.

Hypodermis

Structure and Function

• Composition: Adipose tissue and areolar connective tissue.

• Functions: Insulation, energy storage, shock absorption.

Accessory Structures

Hair (Pili)

Hair is a filamentous structure that grows from follicles in the dermis and serves protective and sensory functions.

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

• Distribution: Covers most of the body except palms, soles, lips, nipples, and parts of external genitalia.

• Growth phases:

  • Anagen: Active growth phase (2-6 years).

  • Catagen: Transitional phase (few weeks).

  • Telogen: Resting phase (2-4 months).

Nails

Nails are hard, keratinized plates that protect the distal phalanges and aid in grasping objects.

• Main parts: Nail plate, nail bed, cuticle, lunula.

• Growth: Produced by the nail matrix at the base.

Glands

Gland Type	Location	Function
Sebaceous (oil) glands	Associated with hair follicles	Secrete sebum for lubrication and waterproofing
Sudoriferous (sweat) glands	Distributed over most of the body	Regulate temperature, excrete wastes
Apocrine glands	Axillary and genital areas	Produce scent; active after puberty
Eccrine glands	Most skin surfaces	Produce watery sweat for cooling

Skin Color

Determinants of Skin Color

• Melanin: Brown-black pigment produced by melanocytes.

• Carotene: Yellow-orange pigment from diet.

• Hemoglobin: Red pigment in blood; gives pinkish hue to skin.

Sensory Receptors in Skin

Types and Functions

• Merkel cells: Detect light touch.

• Meissner's corpuscles: Detect fine touch and vibration.

• Pacinian corpuscles: Detect deep pressure and vibration.

• Ruffini endings: Detect stretch.

• Free nerve endings: Detect pain and temperature.

Protection: Barriers of the Skin

Physical Barriers

• Stratum corneum cells surrounded by lipids form a waterproof barrier.

• Keratin and glycolipids block most water and water-soluble substances.

• Limited penetration by lipid-soluble substances, plant oleoresins, salts of heavy metals, and some drugs.

Chemical Barriers

• Skin secretions (acid mantle) inhibit bacterial multiplication.

• Melanin protects against UV radiation damage.

Biological Barriers

• Dendritic cells and macrophages provide immune defense.

• DNA absorbs UV radiation and dissipates heat.

Functions of the Integumentary System

Temperature Regulation

• Evaporation of sweat cools the body.

• Blood vessel dilation/constriction regulates heat loss.

Equation:

$Q = m c \Delta T$

Where $Q$ is heat energy, $m$ is mass, $c$ is specific heat, and $\Delta T$ is change in temperature.

Other Functions

• Cutaneous sensation: Sensory receptors detect touch, pressure, pain, and temperature.

• Metabolic functions: Synthesis of vitamin D, collagenase.

• Blood reservoir: Up to 5% of body's blood volume is in the skin.

• Excretion: Nitrogenous wastes and salt are eliminated in sweat.

Clinical Considerations

Burns

• First-degree burns: Affect only the epidermis; redness and pain.

• Second-degree burns: Affect epidermis and part of dermis; blistering and more severe pain.

• Third-degree burns: Affect all skin layers; may require skin grafts.

Additional info:

• Skin is the largest organ of the body, accounting for about 15% of total body weight.

• Vitamin D synthesis in the skin is essential for calcium absorption and bone health.