Skin structure, wound healing, and dressings | NPTE-PTA Integumentary system

Foundational skin information

The integumentary system consists of the skin, hair, nails, glands, and sensory receptors. It serves as the first line of defense against injury, infection, and dehydration while regulating temperature and sensation. Below are the layers of skin and their descriptions:

Epidermis (outermost layer)

Avascular (no blood vessels)
Composed of keratinized stratified squamous epithelium
Key cells:
- Keratinocytes - strength and waterproofing (keratin production)
- Melanocytes - pigmentation and ultraviolet light protection
- Langerhans cells - immune defense
- Merkel cells - touch sensation
The epidermis is organized into several layers, from the deep stratum basale (site of active cell division) to the superficial stratum corneum, a barrier layer of dead, keratinized cells

Dermis (middle layer)

Vascularized, supplies nutrients to the epidermis
Contains:
- Blood vessels
- Nerve endings
- Hair follicles
- Sweat and sebaceous glands
- Collagen and elastin fibers (structural support)

Hypodermis (subcutaneous tissue - deepest layer)

Composed of: fat (adipose tissue), connective tissue
Function: insulation, energy storage, shock absorption

Related to skin

Hair - protects against UV, insulates scalp
Nails - keratinized structures for protection and dexterity
Sebaceous (oil) glands - secrete sebum (moisturizes and protects skin)
Sweat glands - regulate temperature and remove waste
- Eccrine glands - watery sweat, found over most of the body
- Apocrine glands - thicker secretion, found in the axillae and groin

Wound healing phases

Inflammatory phase

Initiated immediately and lasts 3-4 days
Marked by hemostasis (stopping bleeding) and phagocytosis (removal of debris and pathogens)

Proliferative phase

Lasts until day 21 post-injury
Collagen formation: A whitish protein substance that adds to the tensile strength of the wound
Granulation tissue (beefy red, moist) fills the wound bed, new capillaries form (angiogenesis), myofibroblasts pull the wound edges together (wound contraction), and epithelial cells migrate across the surface (epithelialization)
New epithelial tissue appears light pink and translucent as it grows over the granulation tissue from the wound margins

Maturation phase

Begins at day 21 and lasts up to 1-2 years
Leads to scar formation
Collagen is remodeled, but the scar reaches only about 70-80% of the original tensile strength

Types of healing

Primary intention (first intention)

Tissue surfaces are closed with minimal tissue loss
Example: Surgical incision

Secondary intention

Extensive tissue loss prevents edges from being approximated
Example: Pressure ulcer
Longer repair time, increased scarring, and higher infection risk

Tertiary intention healing (delayed closure)

Wound is left open for a period before closure
Example: Abdominal wound left open for drainage before later closure

Factors that impair wound healing

Local factors:
- Infection - bacteria compete with healing cells for oxygen and nutrients
- Poor perfusion - reduces oxygen and nutrient delivery to the wound bed
- Pressure - compresses vessels and restricts local blood flow
- Too much or too little moisture - excess moisture causes maceration, while dryness slows cell migration
- Necrotic tissue - acts as a physical barrier to granulation
Systemic factors:
- Diabetes - impairs circulation and immune response
- Malnutrition or low protein intake - limits collagen synthesis
- Smoking - causes vasoconstriction, reducing oxygen delivery
- Corticosteroids or chemotherapy - suppress the inflammatory response needed to start healing
- Advanced age - slower cell turnover and reduced skin elasticity
- Obesity - reduces vascularity of adipose tissue and increases tension on the wound

Classification of wound drainage

Serous - Clear or pale yellow, thin and watery fluid from the serum portion of blood
Sanguineous - Bright red, bloody drainage
Serosanguineous - Pinkish drainage, a mix of blood and serum
Purulent - Thick, yellow or yellow-green pus, indicating infection

How to measure wounds accurately

Measuring wounds is essential for assessing healing progress and treatment effectiveness. Proper wound measurement includes length, width, depth, and any tunneling or undermining. Below are the tools and steps for how to measure a wound.

PTA scope of practice: The PTA collects wound data (measurements, drainage, tissue type) and recognizes changes such as signs of infection or deterioration, then reports these findings to the supervising PT. Report promptly: increasing erythema or warmth spreading into the periwound skin, new or increasing pain, purulent or foul-smelling drainage, wound enlargement, fever, or new necrotic tissue. Selecting a different dressing type or debridement method, or modifying the overall plan of care, is outside the PTA’s scope (performing dressing changes as directed in the plan of care is within it) - that responsibility belongs to the supervising PT.

Tools needed for wound measurement

Disposable ruler (centimeters or millimeters)
Sterile cotton-tipped applicator (for depth)
Wound probe (for tunneling or undermining)
Gloves (to maintain infection control)
Transparent film grid (optional for tracing irregular shapes)

Step-by-step wound measurement

Measuring length and width

Patient positioning: Ensure the patient is in the same position each time for consistency
Use a ruler: Place a disposable ruler over the wound
Length: measure head-to-toe (12 to 6 o’clock), with the patient’s head at 12 o’clock
Width: measure side-to-side (3 to 9 o’clock), perpendicular to length
Use the same method at every visit so measurements stay comparable over time
Documentation format: Length × Width (e.g., 5.2 cm × 3.4 cm)

Measuring wound depth

Hold a sterile cotton-tipped applicator perpendicular to the skin surface at the deepest part of the wound
Grasp or mark the applicator where it is level with the wound edge
Remove and measure against a ruler
Example documentation: Depth = 0.8 cm

Measuring wound volume

Measuring wound volume can be done by multiplying length, width, and depth
Alternatively, measure the amount of hydrogel required to fill the wound

Definitions

Tunneling: Tract extending deeper into tissue
Undermining: Cavity or space under wound edges
Granulation tissue: New, pink-red, moist connective tissue that fills the wound bed during healing
Slough: Soft, yellow or white dead tissue in the wound bed, distinct from healthy granulation tissue
Eschar: Hardened, dry, black or brown dead tissue covering a wound
Maceration: Softened, whitish periwound skin caused by excess moisture
Induration: Abnormal firmness of the periwound tissue, often a sign of underlying inflammation or infection

Steps for measuring tunneling and undermining

Insert a sterile probe into the suspected tunnel/undermined area
Measure using a ruler
Use the clock method to describe location (e.g., “Tunneling at 3 o’clock position, depth 1.5 cm”)

Additional documentation details

Shape and edges: Irregular, round, oval, jagged
Tissue type: Granulation, slough, eschar
Exudate: amount (none, scant, moderate, heavy) and type (serous, purulent, sanguineous)
Periwound skin: Erythema, edema, maceration, induration

Types of dressing

Dressing selection follows the principle of moist wound healing: the wound bed should stay moist enough to support cell migration and granulation, but not so wet that the surrounding skin breaks down (maceration). Match the dressing’s absorbency to the wound’s exudate level - add moisture to a dry wound, absorb moisture from a heavily draining one, and choose a middle-ground option for light-to-moderate drainage.

Pressure injury stages (I-IV, unstageable, and deep tissue injury) are introduced in the later chapter on debridement and wound classification by etiology. Arterial, venous, and diabetic/neuropathic ulcers are covered in that same chapter, and burns in burns and other common skin conditions.

Dressing	Exudate level	Best for	Avoid
Gauze	Moderate-heavy	Packing deep or surgical wounds; deeper pressure injuries	Caution: dry wounds (can adhere to the wound bed and delay healing)
Transparent film	None-minimal	Superficial wounds, abrasions, IV sites; shallow pressure injuries	Contraindicated: infected or heavily exudating wounds
Hydrocolloid	Light-moderate	Shallow pressure injuries, venous ulcers; keeps the wound bed moist	Contraindicated: infected wounds or heavy exudate
Hydrogel	None-minimal	Dry wounds, burns, painful wounds, necrotic tissue (autolytic debridement)	Caution: heavily exudating wounds (risk of maceration)
Foam	Moderate-heavy	Venous ulcers, diabetic foot ulcers; cushions deeper pressure injuries	Caution: dry wounds or minimal exudate
Alginate	Heavy	Infected or tunneling wounds; seaweed-derived, absorbs large fluid volumes	Caution: dry wounds or minimal exudate
Collagen	Light-moderate	Chronic non-healing or granulating wounds	Contraindicated: collagen (often bovine-derived) allergy. Caution: dry wounds or wounds with eschar
Antimicrobial (silver or iodine)	Moderate-heavy	Infected wounds, burns, surgical wounds at risk of infection	Contraindicated: silver or iodine allergy; iodine is also contraindicated with thyroid disorder or pregnancy/lactation. Caution: long-term use (cytotoxicity risk)

When an alginate dressing is removed, it can look yellowish-tan or brown and have a mild, unpleasant odor - this is normal and comes from the interaction between the seaweed-based fibers and the wound’s exudate, not necessarily a sign of infection.

If any of the warning signs listed in the PTA scope-of-practice note above appear during a dressing change, stop and notify the supervising PT before proceeding.

Wound cleansing: normal saline is an isotonic wound cleanser that does not interfere with normal wound repair, while hydrogen peroxide is cytotoxic and can delay wound healing.

Skin layers and structures

Epidermis: avascular, keratinized stratified squamous epithelium; key cells = keratinocytes (keratin), melanocytes (pigment/UV protection), Langerhans (immune), Merkel (touch); ranges from stratum basale (division) to stratum corneum (barrier)
Dermis: vascularized; contains vessels, nerves, hair follicles, glands, collagen/elastin
Hypodermis: fat/connective tissue; insulation, energy storage, shock absorption
Appendages: hair (UV/insulation), nails (protection), sebaceous glands (sebum), sweat glands (eccrine = watery/widespread; apocrine = thick/axillae-groin)

Wound healing phases

Inflammatory (0-4 days): hemostasis, phagocytosis
Proliferative (up to day 21): collagen formation, granulation tissue, angiogenesis, wound contraction (myofibroblasts), epithelialization
Maturation (day 21 to 1-2 years): collagen remodeling, scar reaches only ~70-80% original tensile strength

Types of healing & impairing factors

Primary intention: edges approximated, minimal loss (surgical incision)
Secondary intention: extensive tissue loss, edges not approximated (pressure ulcer); longer healing, more scarring/infection risk
Tertiary intention: delayed closure (wound left open then closed later)
Local impairments: infection, poor perfusion, pressure, improper moisture balance, necrotic tissue
Systemic impairments: diabetes, malnutrition, smoking, corticosteroids/chemo, advanced age, obesity

Wound drainage types

Serous: clear/pale yellow, watery
Sanguineous: bright red, bloody
Serosanguineous: pink, blood+serum mix
Purulent: thick yellow/green, indicates infection

Wound measurement

Length: 12 to 6 o’clock (head-to-toe); Width: 3 to 9 o’clock (side-to-side); document as Length × Width
Depth: sterile cotton-tipped applicator perpendicular at deepest point
Volume: Length × Width × Depth, or hydrogel fill method
Tunneling (tract into tissue) and undermining (cavity under edges) measured with probe, described using clock method
Document tissue type (granulation, slough, eschar), exudate amount/type, periwound condition (erythema, maceration, induration)
Key terms: granulation (pink-red healing tissue), slough (yellow/white dead tissue), eschar (black/brown hardened dead tissue), maceration (softened periwound skin), induration (abnormal firmness)

PTA scope of practice (wound care)

PTA collects data (measurements, drainage, tissue type), performs dressing changes per plan of care, reports changes to PT
Report immediately: spreading erythema/warmth, new/increased pain, purulent/foul drainage, enlargement, fever, new necrotic tissue
PTA cannot select dressing type, debridement method, or modify plan of care — PT’s responsibility

Dressing selection principles

Goal: moist wound healing — moist enough for cell migration/granulation, not so wet it macerates skin
Match absorbency to exudate level: add moisture to dry wounds, absorb from heavy drainage, moderate option for light-moderate
Key dressings: gauze (moderate-heavy, avoid on dry wounds), transparent film (minimal exudate, avoid if infected), hydrocolloid (light-moderate, avoid if infected), hydrogel (dry/necrotic wounds, caution with heavy exudate), foam (moderate-heavy, venous/diabetic ulcers), alginate (heavy exudate/infected/tunneling, normal yellowish-brown odor on removal), collagen (light-moderate, avoid with bovine allergy), antimicrobial silver/iodine (infected wounds; contraindicated with silver/iodine allergy, iodine also with thyroid disorder or pregnancy/lactation; caution with long-term use (cytotoxicity))
Pressure injury staging and ulcer etiology (arterial/venous/diabetic) covered in later chapters