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Wound Healing
and Skin Repair

Explore the four overlapping phases that restore your body's largest organ after injury: from the initial hemostasis response to final remodeling.

Anatomy & Physiology

Why Wound Healing Matters

Your skin is the body's first line of defense against pathogens, dehydration, and physical damage. When that barrier is broken, a complex cascade of cellular and molecular events must occur rapidly and in precise order to restore tissue integrity.

Essential Vocabulary

Hemostasis
Rapid arrest of bleeding through vasoconstriction and clot formation
Inflammation
Immune response that clears debris and prevents infection
Proliferation
Rebuilding new tissue through cell division and ECM deposition
Remodeling
Maturation and strengthening of scar tissue over weeks to years

Skin Architecture Review

Epidermis Keratinocytes, melanocytes, Langerhans cells Dermis Collagen, elastin, blood vessels, nerves Fibroblasts, mast cells, macrophages Hypodermis (Subcutaneous) Adipose tissue, larger blood vessels Increasing depth

The Four Phases of Wound Healing

1 Hemostasis 0 - 30 min 2 Inflammation 0 - 5 days 3 Proliferation 3 - 21 days 4 Remodeling 21 days - 2 yr Phases overlap: they are not strictly sequential

Phase 1: Hemostasis

1
Vasoconstriction: Damaged blood vessels immediately constrict to reduce blood loss. Smooth muscle in vessel walls contracts reflexively.
2
Platelet plug: Platelets adhere to exposed collagen, become activated, and aggregate to form a temporary plug.
3
Coagulation cascade: A fibrin mesh reinforces the platelet plug, creating a stable clot. Thrombin converts fibrinogen into fibrin strands.

Phase 2: Inflammation

Wound Site N N M M Neutrophils (0-48 hrs) Macrophages (48-96 hrs) Growth Factors Debris Clearance

Phase 3: Proliferation

Granulation
Fibroblasts deposit collagen (type III). New capillaries form via angiogenesis, creating pink, granular tissue.
Re-epithelialization
Epidermal keratinocytes migrate from wound edges across the granulation tissue to resurface the wound.
Contraction
Myofibroblasts pull wound edges together, reducing wound area by up to 40-80% in open wounds.

Phase 4: Remodeling

Type III Collagen Thin, disorganized MMP Type I Collagen Thick, aligned, cross-linked Scar reaches \( \sim 80\% \) of original skin strength

Primary vs Secondary Intention

Primary Intention Secondary Intention
Wound edges Close together (sutured) Far apart (left open)
Granulation Minimal needed Extensive: fills defect
Contraction Little to none Significant
Scarring Thin, linear scar Larger, irregular scar
Healing time Faster (days to weeks) Slower (weeks to months)
Example Surgical incision Pressure ulcer, deep burn

Factors That Affect Healing

Nutrition
Vitamin C for collagen synthesis, protein for cell proliferation, zinc for immune function
Blood Supply
Oxygen and nutrients require adequate perfusion; ischemia impairs healing
Age
Older adults heal more slowly due to reduced cell division and thinner skin
Infection
Bacterial contamination prolongs inflammation and delays proliferation
Medications
Corticosteroids suppress inflammation; NSAIDs may impair early phases
Chronic Disease
Diabetes impairs neutrophil function and microvascular perfusion

How to Assess a Healing Wound

1
Identify wound type and depth: Is it superficial (epidermis), partial-thickness (into dermis), or full-thickness (through to hypodermis)?
2
Determine current phase: Look for signs: active bleeding (hemostasis), redness and swelling (inflammation), pink granulation tissue (proliferation), or pale mature scar (remodeling).
3
Evaluate healing trajectory: Is the wound progressing through phases on expected timelines? Stalling in inflammation \( \gt 5 \) days suggests complication.
4
Check for complications: Infection signs (purulent drainage, increased pain, fever), dehiscence (wound reopening), or excessive scarring (keloid/hypertrophic).
5
Identify modifiable risk factors: Nutrition, blood supply, medication use, chronic disease management.

Example 1: Paper Cut

A shallow cut on a finger from a sheet of paper. It bleeds briefly, stings, then stops. Within a day the area is slightly red and swollen. After 3-4 days the redness fades. By 7-10 days the skin looks nearly normal.

H 0 min Bleeding stops I 1-2 days Redness, stinging P 3-5 days New skin forms R 7-10 days Healed, no scar

Example 2: Surgical Incision

A clean, sutured abdominal incision after surgery. The wound edges are closely approximated with sutures. The incision passes through epidermis and dermis.

H
Hemostasis: Minimal bleeding controlled during surgery. Clot fills the narrow gap between wound edges.
I
Inflammation (1-3 days): Mild redness around sutures. Neutrophils and macrophages clean the incision line.
P
Proliferation (3-14 days): Fibroblasts bridge the narrow gap with collagen. Epidermis resurfaces quickly. Sutures may be removed at days 7-10.
R
Remodeling (weeks to months): Thin, linear scar matures. Collagen cross-links strengthen. Scar fades from red to pale.

Example 3: Pressure Ulcer

A Stage III pressure ulcer on the sacrum of a bedridden patient with diabetes. The wound extends through dermis into subcutaneous tissue, with a large tissue defect.

Epidermis Dermis Hypodermis Wound Crater Granulation fills from base up Edges contract inward

Common Mistakes

✕ Confusing phases as sequential
The four phases overlap. Inflammation can persist while proliferation begins. They are not strictly one-after-another.
✕ Mixing up collagen types
Type III collagen appears first (weak, thin). During remodeling it is replaced by type I (strong, thick). Not the reverse.
✕ Assuming scars equal normal skin
Scar tissue reaches only \( \sim 80\% \) of original tensile strength. It also lacks hair follicles, sweat glands, and normal pigmentation.

Phase Comparison Summary

Phase Timing Key Cells Main Event
Hemostasis 0 - 30 min Platelets Clot formation
Inflammation 0 - 5 days Neutrophils, macrophages Debris clearance, defense
Proliferation 3 - 21 days Fibroblasts, keratinocytes Granulation, re-epithelialization
Remodeling 21 days - 2 yr Fibroblasts Collagen III → I, scar maturation

Question 1 of 5

Which cell type is considered the master regulator of wound healing due to its role in debris clearance and growth factor release?

Question 2 of 5

During the remodeling phase, which type of collagen replaces the initial wound matrix to increase tensile strength?

Question 3 of 5

A patient has a large, open wound that is filling with pink, granular tissue from the base up. The wound edges are slowly contracting inward. This wound is healing by:

Question 4 of 5

A patient with a sutured surgical wound is at post-operative day 2. The wound edges are red, warm, and slightly swollen. Which phase best describes the wound's current state?

Question 5 of 5

Which nutrient is essential for collagen synthesis, and its deficiency leads to impaired wound healing (as seen in scurvy)?