Chapter 5: The Integumentary System – Functional Anatomy, Physiology, and Clinical Aspects

The Integumentary System

Overview and Components

The integumentary system is the most accessible organ system, commonly referred to as the skin or integument. It accounts for approximately 16% of total body weight and covers a surface area of 1.5–2.0 m2. The skin serves as the body's first line of defense against environmental threats and consists of two major components:

• Cutaneous membrane

• Accessory structures

Cutaneous Membrane Structure

The cutaneous membrane varies in thickness from 0.5 mm to 4.0 mm and is composed of two main layers:

• Epidermis: Stratified squamous epithelium; contains 4 layers in thin skin and 5 layers in thick skin.

• Dermis:

  • Papillary layer: Areolar connective tissue

  • Reticular layer: Dense irregular connective tissue

Accessory Structures

• Hairs

• Nails

• Exocrine glands:

  • Sebaceous glands

  • Sweat glands

• Sensory receptors and nerve fibers

• Arrector pili muscles

• Cutaneous plexus: Network of blood vessels

Subcutaneous Layer (Hypodermis)

The subcutaneous layer is not part of the integument but separates it from deep fascia. It is dominated by adipose tissue and serves as an important energy storage site.

Functions of the Integumentary System

• Protection: Shields underlying tissues and organs from impact, abrasion, fluid loss, and chemical attack.

• Excretion: Removes salts, water, and organic wastes via glands.

• Thermoregulation: Maintains body temperature through insulation or evaporative cooling.

• Melanin production: Protects underlying tissue from ultraviolet radiation.

• Keratin production: Provides abrasion resistance and water repellency.

• Vitamin D synthesis: Produces a steroid hormone (calcitriol) important for calcium metabolism.

• Sensory detection: Touch receptors detect pressure and pain, sending signals to the nervous system.

The Epidermis: Structure and Layers

General Features

• Composed of multiple layers of cells called strata.

• Primary cell type: Keratinocyte.

• Deeper layers form epidermal ridges adjacent to dermal papillae, increasing surface area for attachment.

Types of Skin

• Thin skin: Covers most of the body; contains four strata.

• Thick skin: Found on palms and soles; contains five strata.

Fingerprints

• Formed by patterns of epidermal ridges on fingertips during embryological development.

• Unique to each individual and do not change over a lifetime.

Epidermal Layers (Deep to Superficial)

1. Stratum basale:

   • Attached to basement membrane by hemidesmosomes.

   • Contains basal cells (stem cells) and Merkel cells (touch receptors).

2. Stratum spinosum:

   • 8–10 layers of keratinocytes bound by desmosomes.

   • Contains dendritic (Langerhans) cells for immune defense.

3. Stratum granulosum:

   • 3–5 layers of keratinocytes.

   • Cells produce keratin and keratohyalin, grow thinner, and become less permeable.

4. Stratum lucidum (only in thick skin):

   • Flattened, densely packed dead cells filled with keratin and keratohyalin.

5. Stratum corneum:

   • 15–30 layers of keratinized cells; outermost protective region.

   • Cells tightly connected by desmosomes; water resistant but not waterproof.

   • Water loss occurs via insensible perspiration and sensible perspiration (sweat).

Mnemonic: Come Let's Get Sunburned (Corneum, Lucidum, Granulosum, Spinosum, Basale)

Skin Color

Primary Pigments

• Carotene: Orange-yellow pigment; precursor for Vitamin A.

• Melanin: Brown, yellow-brown, or black pigment produced by melanocytes in the stratum basale.

  • Differences in skin pigmentation are due to the amount of melanin produced, not the number of melanocytes.

  • Melanin is packaged into melanosomes and transferred to keratinocytes.

Effects of Blood Supply

• Hemoglobin: Red pigment in blood cells; blood flow through the subpapillary plexus affects skin color.

• Increased blood flow results in redder skin; decreased flow leads to pale or bluish skin (cyanosis).

Skin Cancer Types

• Basal cell carcinoma: Most common; originates in stratum basale due to UV mutations; rarely metastasizes.

• Malignant melanoma: Dangerous; cancerous melanocytes metastasize rapidly; early detection is critical for survival.

The Dermis and Subcutaneous Layer

Dermis Layers

1. Papillary layer: Areolar tissue; contains capillaries, lymphatic vessels, and sensory neurons.

2. Reticular layer: Dense irregular connective tissue with collagen and elastic fibers; contains blood vessels, lymphatics, nerves, and accessory organs.

Subcutaneous Layer (Hypodermis)

• Separates skin from deeper structures; dominated by adipose tissue.

• Adipose accumulation patterns differ by sex and region.

Sensory Receptors

• Free nerve endings: Sensitive to touch and pressure.

• Tactile corpuscles: Detect texture and steady pressure.

• Meissner corpuscles: Detect light touch, pressure, and vibration.

• Pacinian corpuscles: Detect deep pressure and vibration.

• Ruffini corpuscles: Sensitive to pressure and skin stretching.

Tension (Cleavage) Lines

• Formed by collagen and elastic fiber arrangement.

• Clinically significant for surgery; cuts parallel to lines heal better with less scarring.

Clinical Module: Burns

Burns and Their Effects

• Result from heat, friction, radiation, electrical shock, or chemicals.

• Can compromise essential skin functions, leading to dehydration, electrolyte imbalance, kidney impairment, and circulatory shock.

• Severity depends on depth and total area affected.

Classification of Burns

Type	Description	Effects
First-degree (Partial-thickness)	Only epidermis affected	Redness (erythema), inflammation; example: sunburn
Second-degree (Partial-thickness)	Entire epidermis and some dermis damaged	Blistering, pain, swelling; accessory structures not affected; healing in 1–2 weeks
Third-degree (Full-thickness)	Destroys epidermis, dermis, and extends into subcutaneous layer	Less painful due to nerve damage; requires skin grafting

Burns and Homeostasis

• Fluid and electrolyte balance

• Thermoregulation

• Protection from infection

Clinical Evaluation and Treatment

• Depth of burns: Assessed by pin prick; loss of sensation indicates third-degree burn.

• Rule of nines: Estimates percentage of body surface affected.

• Emergency treatment:

  • Replace fluids and electrolytes

  • Provide nutrients

  • Prevent infection (cleaning, antibiotics)

  • Assist tissue repair with skin grafts

Types of Skin Grafts

Type	Source	Notes
Autograft	Patient's own undamaged skin	Best choice; no immune rejection
Allograft	Frozen skin from a cadaver	Temporary; risk of rejection
Xenograft	Animal skin	Temporary; risk of rejection

Example

Example: A patient with third-degree burns covering 20% of their body surface will require fluid replacement, infection prevention, and likely skin grafting for recovery.