Integumentary System: Structure, Function, and Clinical Aspects

Integumentary System

Overview and Functions

The integumentary system is the body's largest organ system, accounting for approximately 16% of total body weight. It is the only system visible externally and serves multiple vital functions.

• Protection: Acts as a barrier against mechanical injury, pathogens, and chemical exposure.

• Excretion: Removes salts, water, and metabolic wastes through sweat.

• Thermoregulation: Maintains body temperature via sweat and blood flow regulation.

• Vitamin D Synthesis: Produces vitamin D, essential for calcium metabolism.

• Nutrient Storage: Stores lipids in adipose tissue.

• Sensory Detection: Detects touch, pressure, pain, and temperature.

Cutaneous Membrane

Structure of the Skin

The skin consists of three main layers: the epidermis, dermis, and subcutaneous layer (hypodermis). Each layer has distinct structures and functions.

Epidermis

• Composed of stratified squamous epithelium; avascular (lacks blood supply).

• Most abundant cells are keratinocytes.

• Renews every 25-45 days; about 40 pounds of skin flakes shed in a lifetime.

• Thickness: Thick skin (palms, soles) has 5 layers (~0.5 mm); thin skin (elsewhere) has 4 layers (~0.08 mm).

Layer	Key Features
Stratum Corneum	15-30 rows of flat, dead, keratin-filled cells; waterproof; cells joined by desmosomes; shed in sheets; exposed cells last ~2 weeks.
Stratum Lucidum	Only in thick skin; 3-5 rows of clear, flat, dead cells with eleidin (precursor to keratin).
Stratum Granulosum	3-5 rows of flat cells with keratohyalin granules; no cell division; cells die due to lack of nutrients.
Stratum Spinosum	8-10 rows; spine-like projections; pre-keratin filaments; contains Langerhans cells (immune function).
Stratum Basale (Germinativum)	Single layer of cuboidal/columnar cells; mitosis occurs here; contains melanocytes (produce melanin) and Merkel cells (touch receptors); forms epidermal ridges (fingerprints).

Dermis

• Thicker than epidermis; mainly connective tissue (collagen and elastin fibers).

• Contains nerve fibers, blood vessels, lymphatics, hair follicles, and glands.

Layer	Key Features
Papillary Layer	Areolar connective tissue; outer 20%; forms dermal papillae (fit to epidermal ridges); rich capillary supply.
Reticular Layer	Dense irregular connective tissue; lower 80%; contains arterioles, venules, nerves, hair follicles, glands; forms fissure lines and stretch marks.

• Blister: Separation of epidermal and dermal layers by fluid.

Subcutaneous Layer (Hypodermis)

• Not technically part of the skin.

• Loose connective tissue, mainly adipose; accommodates fluid (site for injections).

• Contains large arteries and veins.

Epidermal Pigmentation and Color Changes

• Carotene: Orange/yellow pigment; excess intake (e.g., carrots) can cause skin to appear orange; converted to vitamin A.

• Melanin: Brown/black pigment produced by melanocytes; all humans have similar numbers of melanocytes, but production varies; more melanin provides greater UV protection.

• Dermal Circulation: Blood flow affects skin color (blushing, pallor, cyanosis).

• Disease Coloration: Jaundice (yellow), Vitiligo (loss of pigment).

Accessory Structures (Epidermal Derivatives)

Hair

• Functions: Insulation, sensory detection, visual identification, protection.

• Growth: ~1 mm every 3 days; head hair lasts 2-5 years; normal loss is 70-100 hairs/day.

• Color: Determined by melanin; white hair results from lack of pigment or air bubbles.

Part	Description
Cuticle	Outermost layer; keratinized scale-like cells.
Shaft	Visible portion above skin.
Root	Below skin surface.
Follicle	Surrounds root; associated with sebaceous gland and root hair plexus (touch sensitive).
Bulb	Base of root; contains papilla (blood supply) and matrix (growth by cell division).
Arrector Pili	Muscle causing "goose-bumps" via involuntary contraction.

• Types: Vellus (peach fuzz), Terminal (eyelashes, scalp hair); puberty converts vellus to terminal in certain regions.

• Male pattern baldness: Terminal hair reverts to vellus type due to hormonal changes.

Nails

• Plates of tightly packed, hard, keratinized epidermal cells.

• Lunula: Whitish, crescent area; thick stratum germinativum beneath.

• Nail root: Buried in skin fold; site of nail growth.

• Nail body: Covers nail bed.

• Growth: ~1 mm per week.

Glands

• Sebaceous (Oil) Glands: Holocrine secretion; sebum released by arrector pili contraction; active in fetal life, reactivates at puberty; keeps hair/skin soft, inhibits bacteria; absent on palms/soles.

• Sweat Glands: 3-4 million total.

  • Apocrine: Armpits, groin, nipples; secrete onto hair follicles; controlled by nervous/hormonal signals; actually use merocrine secretion.

  • Merocrine (Eccrine): More numerous; highest on palms/soles; discharge directly to skin; functions in cooling, excretion, and protection; sweat contains water, salts, urea, uric acid, amino acids, ammonia, sugar, lactic acid, ascorbic acid.

  • Ceruminous: Modified sweat glands in ear canal; produce cerumen (earwax) for protection.

• Control: Sebaceous and apocrine glands are regulated by the autonomic nervous system (ANS) globally; merocrine glands have regional control.

Sensory Endings in the Skin

• Meissner’s Corpuscles: Detect touch; located in papillary dermis.

• Pacinian Corpuscles: Detect pressure; found in reticular dermis and hypodermis.

• Free Nerve Endings: Detect pain; can also sense intense touch or temperature.

Pain Type	Description
Somatic	From skin receptors.
Visceral	From internal organs.
Referred	Pain perceived at a site other than origin.
Phantom	Pain from a missing body part.

Injury and Repair

1. Bleeding occurs.

2. Scab forms (fibrin and RBCs); debris removed by phagocytes.

3. Collagen fibers form; clot dissolves.

4. Scar tissue develops.

Burns

Rule of Nines

The Rule of Nines estimates the percentage of body surface area affected by burns:

Body Region	% of Surface Area
Head and Neck (anterior & posterior)	9%
Each Arm (anterior & posterior)	9%
Trunk (anterior & posterior, incl. buttocks)	36%
Each Leg (anterior & posterior)	18%
Perineum	1%

Burn Depths

Degree	Involvement	Features
First	Surface epidermis	Sunburn; skin functions intact.
Second	Entire epidermis, part of dermis	Some function lost; heals in 1-2 weeks.
Third	Epidermis, dermis, and derivatives	Functions lost; less pain (nerves destroyed); requires grafts; risk of sepsis.
Fourth	Electrical injury	Damage depends on voltage, current, contact time; entrance/exit wounds; most damage at exit.

Skin Disorders

• Aging: Thinning, decreased blood flow, reduced cellular activity, slower repair.

• Skin Cancer:

  • Basal Cell Carcinoma: 75% of cases; arises from stratum basale; rarely metastasizes.

  • Squamous Cell Carcinoma: From epidermis; rarely metastasizes; often from sun-exposed skin.

  • Malignant Melanoma: From melanocytes; metastasizes rapidly; can be fatal within months.

• Acne: Inflammation of sebaceous glands; bacteria feed on sebum; common at puberty.

• Dermatitis: Inflammation of the papillary dermis.

• Decubitus Ulcer (Bedsore): Caused by prolonged pressure and blood deficiency over bony areas; results in ulceration.