The Integumentary System: Structure, Function, and Homeostasis

Homeostatic Contributions of the Integumentary System

Contributions of the Integumentary System for All Body Systems

General Organ System Protection: The skin and hair serve as primary physical and chemical barriers that safeguard internal body organs from environmental physical damage, chemical abrasion, and infectious micro-organisms.
Thermoregulatory Support: Sweat glands and dermal blood vessels dynamically regulate internal body temperature, maintaining conditions necessary for enzyme function and structural stability across all organ systems.
Skeletal System Interactions: Skin activation of vitamin D enables the body to produce calcitriol, which is essential for the intestinal absorption of dietary calcium and phosphorus necessary for bone matrix deposition and mineralization.
Muscular System Interactions: By enabling calcium ion absorption through synthesized calcitriol, the integumentary system helps supply the free calcium ions required for muscular contraction.
Nervous System Interactions: Sensory nerve endings located in the dermis and subcutaneous tissue provide crucial tactile, thermal, pressure, and nociceptive (pain) sensory input directly to the central nervous system.
Endocrine System Interactions: Epidermal keratinocytes perform the initial enzymatic reaction converting inactive precursors into active vitamin D (calcitriol), an endocrine hormone governing calcium and phosphorus homeostasis.
Cardiovascular System Interactions: Local metabolic and chemical changes in the dermis cause vasodilation and vasoconstriction of skin blood vessels, directly adjusting systemic dermal blood flow and thermoregulation.
Lymphoid and Immune System Interactions:
- Serves as the body's first physical line of defense through mechanical barriers and antimicrobial chemical secretions that inhibit microbial growth.
- Intraepidermal macrophages (Langerhans cells) present in the epidermis process exogenous antigens and trigger specific adaptive immune responses.
- Dermal macrophages phagocytize pathogens that successfully penetrate the superficial epidermal layer.
Respiratory System Interactions: Hair inside nasal passages filters incoming atmospheric dust and particulate matter. Deep pain receptor stimulation within skin layers can directly alter respiratory rates.
Digestive System Interactions: Synthesis of calcitriol in the skin drives active cellular transport of dietary calcium and phosphorus across the epithelial lining of the small intestine.
Urinary System Interactions:
- Renal tubular cells convert partially activated vitamin D intermediates derived from the skin into fully functional calcitriol.
- Eccrine sweat glands excrete metabolic nitrogenous waste products including urea, uric acid, and ammonia, assisting renal excretion.
Reproductive System Interactions:
- Cutaneous nerve endings respond directly to tactile sensory stimuli during sexual excitation.
- Lactation mechanics: Infant suckling stimulates cutaneous nerve endings around the nipple, triggering neuroendocrine pathways for milk ejection.
- Mammary glands represent specialized, highly modified sudoriferous (sweat) glands engineered for nutrient milk production.
- Abdominal skin expands and stretches to accommodate fetal growth throughout pregnancy.

Structural Architecture of the Skin

Sectional view of skin and subcutaneous tissue

Major Anatomical Layers:
- Epidermis: The avascular, epithelial outer layer derived from ectoderm, comprising keratinized stratified squamous epithelium.
- Dermis: The deep, highly vascularized connective tissue layer housing accessory structures, sensory units, blood vessels, and nervous tissue.
Subcutaneous Layer (Hypodermis / subQ):
- Positioned directly deep to the reticular dermis, serving as an attachment anchor for the skin to underlying fascia and muscular tissue.
- Not formally classified as a layer of the skin itself.
- Composed structural tissue includes loose areolar connective tissue and dense clusters of adipose tissue.
- Contains large blood vessel trunks (cutaneous vascular plexus) and encapsulated nerve endings (lamellar corpuscles).

Epidermal Cellular Composition and Strata

Four principal cell types in epidermis of thick skin

Principal Epidermal Cell Types:
1. Keratinocytes: Constitute approximately $90\%$ of all epidermal cells. Arranged in distinct continuous strata, they produce the fibrous structural protein keratin (which protects underlying tissues from heat, microbes, and chemicals) and secrete water-repellent lamellar granules.
2. Melanocytes: Constitute approximately $8\%$ of epidermal cells. They feature slender cytoplasm extensions that extend between surrounding keratinocytes to transfer melanin granules. Melanin absorbs harmful ultraviolet (UV) radiation and shields nuclear DNA.
3. Intraepidermal Macrophages (Langerhans Cells): Dendritic immune cells originating from bone marrow that participate in immune responses against invading pathogens by recognizing and processing foreign antigens.
4. Tactile Epithelial Cells (Merkel Cells): Located in the deepest layer of the epidermis, these flattened cells make contact with the flattened expansion of a sensory neuron axon terminal (tactile sensory corpuscle / Merkel disc) to detect touch sensations.
Epidermal Strata (Deep to Superficial):
- Stratum Basale (Stratum Germinativum):
  - Deepest single layer of cuboidal or columnar keratinocytes containing scattered keratin intermediate filaments (tonofilaments).
  - Contains stem cells undergoing continuous mitosis to yield new keratinocytes.
  - Houses scattered melanocytes and tactile epithelial cells connected to non-encapsulated sensory corpuscles.
- Stratum Spinosum:
  - Eight to ten rows of multi-sided, polyhedral keratinocytes that fit closely together.
  - Contains coarse bundles of keratin intermediate filaments.
  - Exhibits cytoplasmic projections of melanocytes and intraepidermal macrophages.
- Stratum Granulosum:
  - Three to five rows of flattened keratinocytes undergoing apoptosis; cellular organelles begin intracellular degeneration.
  - Contains dense dark-staining granules of the protein keratohyalin, which converts keratin intermediate filaments into mature keratin cross-links.
  - Houses membrane-enclosed lamellar granules that fuse with the plasma membrane to release lipid-rich, water-repellent extracellular secretions.
- Stratum Lucidum:
  - Present exclusively in thick skin regions (palms, palmar digit surfaces, soles, and plantar toe surfaces).
  - Consists of four to six rows of clear, flat, dead keratinocytes packed with immense amounts of keratin and thickened plasma membranes.
- Stratum Corneum:
  - Superficial layer consisting of a few to 50 or more rows of flat, dead, extremely thin keratin-filled cells.
  - Cells are continuously shed and replaced by deeper layers; acts as an effective physical barrier against friction, microbial invasion, and fluid loss.

Dermal Structure and Vascularization

General Organization: Composed of strong connective tissue containing woven collagen fibers and coarse elastic fiber networks. Divided into two main histological regions:
- Papillary Dermis (Superficial Region):
  - Constitutes approximately one-fifth ( $20\%$ ) of the total dermal thickness.
  - Composed of loose areolar connective tissue containing fine elastic fibers and thin collagen bundles.
  - Features finger-like surface projections termed dermal papillae that project into the epidermis.
  - Contains capillary loops (supplying epidermal nutrients), tactile corpuscles (Meissner corpuscles of touch), and free nerve endings (detecting pain, thermal sensations, tickle, and itch).
- Reticular Dermis (Deep Region):
  - Constitutes approximately four-fifths ( $80\%$ ) of the total dermal thickness.
  - Composed of dense irregular connective tissue packed with thick interweaving bundles of collagen and coarse elastic fibers.
  - The spaces between fiber bundles house scattered adipocytes, hair follicles, nerves, sebaceous (oil) glands, and sudoriferous (sweat) glands.

Skin Pigmentation and Associated Clinical Disorders

Primary Pigments:
- Melanin: Produced by melanocytes via the enzymatic conversion of the amino acid tyrosine. Melanin synthesis increases upon UV radiation exposure.
  - Pheomelanin: Synthesizes yellow-to-red pigments.
  - Eumelanin: Synthesizes brown-to-black pigments.
- Hemoglobin: Oxygen-binding red pigment contained within circulating red blood cells in dermal capillaries, imparting a pinkish hue to translucent skin.
- Carotene: Yellow-orange dietary pigment precursor to vitamin A; accumulates in the stratum corneum, fatty areas of the dermis, and subcutaneous layer upon excessive dietary consumption.
Clinical Pigmentary Conditions:
- Albinism: An inherited enzyme deficiency (typically tyrosinase) resulting in an inability to synthesize melanin. Leaves affected individuals with complete or partial absence of pigment in the skin, hair, and irises.
- Vitiligo: A chronic, progressive dermatological condition characterized by autoimmune destruction of functional melanocytes. Produces distinct, expanding white depigmented patches across various skin regions.

Accessory Structures: Hair

Hair and surrounding structures

Distribution and Function: Present across most skin surfaces except for the palms, palmar surfaces of digits, soles, plantar surfaces of toes, nipples, labia minora, and prepuce of the penis. Hair protects against UV exposure, prevents heat loss, and filters foreign air debris.
Anatomical Structure of Hair:
- Hair Shaft: The superficial portion of the hair projecting above the surface of the skin.
- Hair Root: The portion below the epidermal surface penetrating deep into the dermis (and occasionally the subcutaneous layer).
- Concentric Layers of Hair (Medulla, Cortex, Cuticle):
  - Medulla: Inner core composed of two or three rows of poorly pigmented, irregularly shaped cells containing air spaces.
  - Cortex: Middle structural layer forming the major structural mass of the hair shaft, consisting of elongated keratinized cells.
  - Cuticle: Outer single layer of extremely thin, flat, heavily keratinized cells arranged overlapping like roof shingles.
- Hair Follicle Structure:
  - Epithelial Root Sheath: Formed by internal root sheath (produced by hair matrix) and external root sheath (downward continuation of epidermal strata).
  - Dermal Root Sheath: Dense protective connective tissue dermis layer surrounding the epithelial root sheath.
- Hair Bulb Base: Expanded base of the follicle containing:
  - Papilla of the Hair: Vascularized nipple-shaped indentation containing areolar connective tissue and capillaries supplying nutrients.
  - Hair Matrix: Germinative layer of stem cells derived from stratum basale; responsible for growing new hair cells via active mitosis.
- Associated Structures:
  - Arrector Pili Muscle: Smooth muscle bundle extending from dermal papillae to hair follicle side. Contraction under sympathetic autonomic stimulation pulls hair vertical ("goosebumps").
  - Hair Root Plexus: Dendrites of sensory neurons surrounding the base of each follicle, sensitive to subtle tactile displacement of hair shafts.
Hair Growth Cycle:
1. Growth Stage: Hair matrix cells divide rapidly, pushing older cells upward where they keratinize and die ( $2\text{--}6$ years).
2. Regression Stage: Hair matrix cell division ceases, follicle shrinks, and hair root moves away from dermal papilla ( $2\text{--}3$ weeks).
3. Resting Stage: Old hair root remains inactive until a new growth cycle initiates and sheds the old hair shaft ( $3$ months).
Types of Hair:
- Lanugo: Extremely fine, non-pigmented embryonic hair covering a developing fetus.
- Terminal Hair: Long, coarse, thick, heavily pigmented adult functional hair (scalp, eyebrows, eyelashes, pubic and axillary regions).
- Vellus Hair: Short, extremely fine, pale "peach fuzz" hair covering most adult body surfaces.
Pigmentation Mechanisms:
- Dark hair consists primarily of true eumelanin.
- Blond and red hair contain iron and sulfur-rich variants of pheomelanin.
- Gray hair results from a progressive developmental decline in active melanocyte tyrosinase activity.
- White hair is caused by total absence of melanin production coupled with accumulation of air bubbles within the hair shaft medullary core.

Accessory Structures: Exocrine Skin Glands

Comparative Analysis of Integumentary Glands:
- Sebaceous (Oil) Glands:
  - Distribution: Lips, glans penis, labia minora, tarsal glands; small amounts in trunk and limbs; completely absent in palms and soles.
  - Location of Secretory Portion: Dermis layer.
  - Excretory Duct Termination: Connected predominantly directly into hair follicles.
  - Secretion: Sebum (complex lipid mixture containing triglycerides, cholesterol, proteins, and inorganic salts).
  - Function: Prevents hair shafts from drying out, limits cutaneous evaporation, maintains skin softness, and acts as an antibacterial agent.
  - Onset of Function: Relatively inactive during early childhood; activated during puberty by androgenic hormones.
- Eccrine Sweat Glands:
  - Distribution: Throughout skin across most regions of body; highest densities in forehead, palms, and soles.
  - Location of Secretory Portion: Deep reticular dermis (occasionally superficial subcutaneous layer).
  - Excretory Duct Termination: Direct surface sweat pores on the superficial epidermis.
  - Secretion: Watery perspiration ( $H_2O$ , $Na^+$ , $Cl^-$ , urea, uric acid, ammonia, amino acids, glucose, lactic acid).
  - Function: Thermoregulation via evaporative cooling, metabolic waste removal; stimulated during thermal elevation and emotional stress.
  - Onset of Function: Function commences soon after birth.
- Apocrine Sweat Glands:
  - Distribution: Axillae, groin, pubic region, areolae, facial hair regions in adult males, clitoris, and labia minora.
  - Location of Secretory Portion: Deep reticular dermis and superficial subcutaneous layer.
  - Excretory Duct Termination: Excretory ducts open into hair follicles.
  - Secretion: Viscous perspiration containing all eccrine sweat constituents plus elevated lipids and complex proteins.
  - Function: Stimulated during emotional stress, pain, and sexual excitement ("cold sweat").
  - Onset of Function: Activated during puberty under hormonal control.
- Ceruminous Glands:
  - Distribution: Restricted to the external acoustic meatus (ear canal).
  - Location of Secretory Portion: Subcutaneous layer.
  - Excretory Duct Termination: Direct surface of external ear canal or into hair follicle ducts of sebaceous glands.
  - Secretion: Cerumen (a sticky, thick waxy secretion).
  - Function: Forms a sticky mechanical barrier impeding foreign objects, insects, and microbes; moisture-proofs ear canal tissue.
  - Onset of Function: Commences soon after birth.

Accessory Structures: Nails

Sagittal section of nail and photomicrograph of fingertip

Structural Features: Plates of tightly packed, hard, dead keratinized epidermal cells located over the dorsal surfaces of the distal phalanges.
Anatomical Components:
- Nail Body (Plate): Visible portion of the nail secured to underlying finger/toe tissue; appears pink due to underlying dermal blood capillaries.
- Free Edge: Distal extension of the nail body projecting past the finger or toe tip; white because of absence of vascularized nail bed beneath.
- Nail Root: Proximal buried fold of the nail plate concealed beneath a fold of skin.
- Lunula: Crescent-shaped white area at the proximal end of the nail body; appears opaque due to thickened underlying stratum basale matrix concealing capillaries.
- Nail Bed: Specialized skin layer beneath the nail plate extending from lunula to hyponychium.
- Eponychium (Cuticle): Narrow band of stratum corneum extending from and adhering to the margin of the nail wall.
- Hyponychium: Thickened stratum corneum layer securing the free edge of the nail body to the fingertip skin.
- Nail Matrix: Proximal portion of epithelium deep to the nail root containing dividing stem cells; produces new nail cells via continuous mitosis.

Comparison of Thin and Thick Skin

Thin (Hairy) Skin Characteristics:
- Distribution: Covers all body areas except palms, palmar surfaces of digits, and soles.
- Epidermal Thickness: Measures approximately $0.10\text{--}0.15\,\text{mm}$ ( $0.004\text{--}0.006\,\text{in.}$ ).
- Epidermal Strata: Stratum lucidum is completely lacking; features thinner strata spinosum and corneum.
- Epidermal Ridges: Absent due to poorly developed, fewer, less organized dermal papillae.
- Structures: Houses hair follicles, arrector pili muscles, and sebaceous glands; contains fewer sudoriferous glands and lower sensory receptor density.
Thick (Hairless) Skin Characteristics:
- Distribution: Restricted to palms, palmar digit surfaces, soles, and plantar toe surfaces.
- Epidermal Thickness: Measures approximately $0.6\text{--}4.5\,\text{mm}$ ( $0.024\text{--}0.18\,\text{in.}$ ) primarily due to a massively thickened stratum corneum.
- Epidermal Strata: Stratum lucidum present; significantly thicker strata spinosum and corneum.
- Epidermal Ridges: Prominent surface ridges present, supported by well-developed dermal papillae organized in parallel rows.
- Structures: Lacks hair follicles, arrector pili, and sebaceous glands; contains abundant sudoriferous glands and a high density of sensory touch receptors.

Physiological Functions of the Integumentary System

Thermoregulation:
- High Ambient Temperature / Exercise: Sweating increases evaporative cooling. Dermal blood vessels undergo vasodilation, increasing cutaneous blood flow to radiate heat outward.
- Low Ambient Temperature: Sweating decreases. Dermal blood vessels undergo vasoconstriction, shunting warm blood inward away from the body surface.
Blood Reservoir: The extensive capillary networks of the dermis hold approximately $8\text{--}10\%$ of total resting adult blood volume, which can be diverted to vital organs during systemic stress or exercise.
Protection:
- Keratin: Guards underlying tissues against mechanical wear, heat, abrasion, chemical erosion, and cellular microbe invasion.
- Lipids: Lamellar lipid secretions prevent water loss and block hydrophilic toxic fluid absorption.
- Sebum: Keeps skin hydrated and secretes bactericidal chemicals.
- Acidic Sweat: Perspiration surface pH ( $4.0\text{--}6.0$ ) retards growth of pathogenic surface bacteria.
- Melanin: Shields nuclear DNA against destructive ultraviolet light mutagens.
- Macrophages: Dendritic cells and dermal macrophages clear active pathogens via phagocytosis.
Cutaneous Sensations: Detects sensory input via specialized sensory receptors:
- Tactile Sensations: Light touch, deep pressure, vibration, tickle, and itch.
- Thermal Sensations: Warm and cool variations detected by dedicated thermal nerve endings.
- Pain: Nociceptors activated by mechanical tissue damage or extreme temperatures.
Excretion and Absorption:
- Excretion: Removes excess water, salts, carbon dioxide, urea, uric acid, and ammonia.
- Absorption: Enables passage of lipid-soluble substances (vitamins A, D, E, K, oxygen, carbon dioxide), organic solvents (acetone, carbon tetrachloride), heavy metal salts (lead, mercury), and transdermal drug delivery patches (e.g., nitroglycerin, nicotine, scopolamine).
Synthesis of Vitamin D: Ultraviolet solar radiation converts skin pre-vitamin precursor compounds into cholecalciferol. Liver and kidney enzymes subsequently convert this intermediate into the hormone calcitriol, driving active gastrointestinal calcium ion absorption.

Wound Healing Processes

Epidermal wound healing

Epidermal Wound Healing:
- Occurs when superficial injuries involve only the epidermal layer (e.g., abrasions, minor burns).
- Basal epithelial stem cells surrounding the injury detach from the basement membrane, enlarge, and migrate across the open wound.
- Cellular migration occurs in all directions until advancing cell sheets meet one another; cell contact triggers contact inhibition, halting forward migration.
- Epidermal stem cells undergo mitosis to rebuild full stratum basale density, subsequently dividing to restock superficial epidermal strata.

Deep wound healing

Deep Wound Healing: Occurs when injuries extend through the epidermis deep into the dermis and subcutaneous layer. Involves four distinct sequential phases:
1. Inflammatory Phase:
  - Vascular blood clot forms in the wound site, uniting wound edges.
  - Local blood vessel dilation increases vascular permeability, delivering neutrophils and monocytes (which differentiate into macrophages).
  - Phagocytic white blood cells clear cellular debris, microbes, and dead tissue.
2. Migratory Phase:
  - The blood clot dries into a protective surface scab.
  - Epithelial cells migrate beneath the scab to bridge the wound deficit.
  - Dermal fibroblasts migrate along fibrin threads and synthesize collagen fibers and glycoprotein ground substance.
  - Damaged blood vessels begin growing back into the damaged zone (angiogenesis). Tissue filling the deficit is termed granulation tissue.
3. Proliferative Phase:
  - Extensive growth of epithelial cells beneath the scab.
  - Fibroblasts deposit widespread collagen fibers in random patterns.
  - Continued growth and regeneration of dermal blood vessel networks.
4. Maturation Phase:
  - The superficial scab detaches as the epithelium is fully restored to normal thickness.
  - Collagen fibers become organized into dense structural bundles.
  - Fibroblast numbers decline and dermal blood vessels normalize.
  - Scar tissue formation (fibrosis) differs from normal skin due to denser collagen arrangements, reduced elasticity, missing hair follicles, suppressed gland structures, and modified vascularization.

Embryological Development of the Integumentary System

Ectodermal Derivatives:
- The epidermis develops directly from single-layered surface ectoderm tissue.
- Nails, hair follicles, and exocrine skin glands (sebaceous, sudoriferous, ceruminous) originate as downward epidermal growths of ectodermal origin.
Developmental Timeline:
- Fourth Week: Single-layer ectoderm overlies embryonic mesenchyme tissue.
- Seventh Week: Ectoderm divides into an outer protective flat cell layer called periderm and a underlying germinative basal layer.
- Eleven Weeks: The basal layer produces an intermediate cellular layer. Epidermal pegs project downward into mesenchyme while dermal papillae push upward. Neural crest melanoblasts migrate into the tissue to differentiate into melanocytes.
- Twelve Weeks: Basal layer projections generate early hair buds and downgrowths for sudoriferous gland buds.
- Fourteen Weeks: Hair buds invaginate into hair bulbs; lateral outgrowth forms developing sebaceous glands.
- Sixteen Weeks: Hair matrix forms active hair shafts that grow upward; internal dermal papilla vascularization matures.
- Eighteen Weeks: Hair root sheaths, smooth arrector muscle fibers, and convoluted secretory portions of sweat glands differentiate.
- At Birth: Complete stratification yields functional epidermal layers (strata basale, spinosum, granulosum, lucidum, corneum) and underlying fibrous dermis.
Mesodermal Derivatives:
- The dermis arises from embryonic mesoderm located deep to the surface ectoderm.
- Mesoderm gives rise to mesenchyme, which differentiates into active collagen-producing and elastic-fiber-producing fibroblasts.

Age-Associated Changes in the Integumentary System

Structural and Functional Degradation:
- Pronounced structural wrinkling due to loss and disorganization of dermal collagen and elastic fiber networks.
- Skin dehydration, dryness, and cracking caused by declining sebaceous gland oil output.
- Decreased eccrine sweat production, impairing thermoregulatory capacity in hot environments.
- Reduction in total functional melanocytes, producing gray hair and localized atypical hyperpigmented spots ("age spots").
- Subcutaneous adipose fat loss accompanied by generalized dermal and epidermal thinning.
- Impaired dermal blood circulation, slowing cutaneous repair and increasing susceptibility to mechanical pressure ulcers.
- Nails become dry, brittle, and structurally weak.

Pathophysiology: Skin Cancer, Burns, and Pressure Ulcers

Types of skin lesions and skin cancer

Skin Cancer Etiology and Forms:
- Chronic, excessive exposure to solar ultraviolet (UV) radiation represents the primary cause of skin cancer cell transformations.
- Basal Cell Carcinoma: Represents approximately $78\%$ of skin cancers. Arises from stratum basale cells; rarely metastasizes.
- Squamous Cell Carcinoma: Represents approximately $20\%$ of skin cancers. Arises from spinosum keratinocytes; variable propensity to metastasize.
- Malignant Melanoma: Represents approximately $2\%$ of skin cancers. Arises from transformed melanocytes; rapidly metastasizes through lymphatic and cardiovascular networks.
ABCDE Early Warning Diagnostic Criteria for Malignant Melanoma:
- A = Asymmetry: The lesion lack symmetry; two halves do not match in shape.
- B = Border Irregularity: Margins are notched, indented, blurred, or irregular.
- C = Color Unevenness: Multi-colored pigmentation displaying shades of black, brown, tan, pink, or blue-black.
- D = Diameter: Lesion diameter exceeds $6\,\text{mm}$ (approximate size of a standard pencil eraser).
- E = Evolving: Lesion changes continuously over time in size, shape, elevation, or color.
Burn Severity Classification: Tissue damage triggered by extreme heat, electricity, radiation, or corrosive chemicals that denature structural skin proteins.
- First-Degree Burn: Involves damage limited to the epidermal layer; causes localized erythema (redness) and pain without blister formation (e.g., mild sunburn).
- Second-Degree Burn: Destroys the entire epidermis and upper portions of the dermis; features fluid-filled blisters (bullae), severe pain, edema, and localized redness.
- Third-Degree (Full-Thickness) Burn: Destroys the epidermis, dermis, and subcutaneous layer; sensory nerve endings are destroyed, rendering the burned core insensitive to touch pain.

Anterior view illustrating rule of nines

Rule of Nines for Adult Burn Area Estimation:
- Anterior and Posterior Head and Neck: $9\%$ ( $18\%$ in a child; anterior $4.5\%$ , posterior $4.5\%$ ).
- Anterior and Posterior Shoulders, Arms, Forearms, and Hands: $18\%$ ( $9\%$ per upper limb; anterior $4.5\%$ , posterior $4.5\%$ per limb).
- Anterior Trunk (Chest/Abdomen): $18\%$ .
- Posterior Trunk (Back/Loin): $18\%$ .
- Perineum (Genital region): $1\%$ .
- Anterior and Posterior Thighs, Legs, and Feet: $36\%$ ( $18\%$ per lower limb; anterior $9\%$ , posterior $9\%$ per limb; $14\%$ total per lower limb in a child).
- Total Combined Surface Area: $100\%$ .

Pressure ulcer on heel

Pressure Ulcers (Decubitus Ulcers / Bed Sores): Caused by prolonged continuous mechanical pressure over bony prominences that compresses cutaneous blood vessels. The resulting ischemia halts nutrient delivery, causing tissue necrosis, ulceration, and open deep skin lesions.