Four phases, and they overlap
Textbooks split wound healing into four steps — hemostasis, inflammation, proliferation and remodelling — but that tidy list hides the real story: the phases overlap. While one patch of the wound is still fighting bacteria, another is already laying down collagen. Healing is less like a relay race with clean hand-offs and more like a construction site where the demolition crew, the framers and the finishers are all on the job at once. What the animation shows as a smooth slide from red to pink to a pale scar is, up close, dozens of these processes running in parallel.
1. Hemostasis — stop the bleeding (seconds to minutes)
The instant a blood vessel is cut, the wall constricts and platelets stick to the exposed edges, changing shape and clumping into a soft platelet plug within a few minutes. At the same time the clotting cascade converts a blood protein called fibrinogen into sticky threads of fibrin that weave through the plug like rebar through concrete, trapping red cells into a firm clot. That fibrin mesh does double duty: it seals the leak and becomes the temporary scaffold that every later repair cell will crawl along. The platelets aren't just a cork — as they activate they dump out growth factors (PDGF, TGF-β, VEGF) that ring the alarm bell for the next phase.
2. Inflammation — the clean-up crew (0–3 days)
Now the wound turns red, warm, swollen and sore — and that is not the injury getting worse, it is the repair working. Vessels widen and get leaky so that immune cells and fluid can flood the area. First to arrive are neutrophils, which peak around 24–48 hours and kill bacteria with a burst of oxygen-based chemistry (one reason oxygen matters so much). Then macrophages take over from about day 2–3: they are the site foreman, eating dead tissue and spent neutrophils while releasing the growth factors that summon fibroblasts and new blood vessels. A wound that stays red, hot and pus-filled past a few days isn't “still healing” — it may be stuck in inflammation because of infection, which is exactly what the Infected scenario shows.
3. Proliferation — rebuild the gap (about 3–21 days)
This is where the hole actually fills in, and three things happen together. Fibroblasts move in and spin out collagen — at first the quick, disorganised type III kind — laying the structural mesh of the new tissue. New capillaries sprout into the wound (angiogenesis), and because they are so numerous the fresh tissue looks beefy-red and bumpy: this is granulation tissue, and it is a good sign. Special contractile fibroblasts called myofibroblasts grab the wound edges and slowly pull them together (contraction), while a thin sheet of new skin cells (keratinocytes) migrates across the surface to re-cover it (epithelialisation). Two ingredients are non-negotiable here: collagen-building depends on vitamin C, and both collagen and new vessels depend on oxygen.
4. Remodelling — the long rebuild (weeks to ~2 years)
The wound looks “healed” once the surface is closed — but under the surface the work goes on for months, even a couple of years. Enzymes called matrix metalloproteinases gradually dismantle the hasty type III collagen and fibroblasts replace it with stronger, cross-linked type I collagen, which is re-aligned along the lines of mechanical stress — the scar literally reorganises itself to take load. Strength climbs slowly: roughly 3% of normal skin at one week, ~20% at three weeks, and it keeps rising for a year toward a plateau of about 80%. That is the ceiling of scar tissue, which is why the animation's strength bar stops short of the 80% mark.
What stalls a wound — the teaching list
Most wounds that won't heal are stuck for one of a handful of reasons, and nearly all of them hit the proliferation phase. Poor blood supply and low oxygen starve the collagen machinery and cripple bacteria-killing. Diabetes is the classic culprit: chronically high blood glucose damages small vessels and blunts the immune cells, so proliferation grinds to a halt — the result is the diabetic foot ulcer, a wound frozen half-open for months. Infection traps the wound in inflammation. Smoking constricts vessels and the carbon monoxide in smoke steals oxygen off red blood cells, so a smoker's wounds heal slowly and split more often. And malnutrition — too little protein, vitamin C or zinc — or long-term steroid use removes the raw materials and the signals the rebuild depends on.
Why vitamin C and zinc keep showing up
Collagen is not just protein thrown together — it has to be chemically stiffened to hold a triple-helix shape, and the enzymes that do that stiffening, prolyl hydroxylase and lysyl hydroxylase, cannot work without vitamin C (ascorbate) as their cofactor. Starve the body of vitamin C and you get scurvy, whose hallmark is genuinely bizarre: old, long-healed wounds break open again, because the collagen holding them shut is no longer being maintained. Zinc is a cofactor for the enzymes that copy DNA and remodel the matrix (the matrix metalloproteinases), so zinc-deficient tissue can't proliferate or reorganise properly. Neither is a magic booster if you already have enough — correcting a real deficiency restores healing; megadoses on top of a normal diet do not speed it up.
What actually helps a wound heal
The practical list is refreshingly ordinary. Eat enough protein and get adequate vitamin C and zinc from food (citrus, peppers, meat, shellfish, legumes) — a wound is a construction project and it needs materials. Control blood sugar if you're diabetic; it is the single biggest lever for a chronic ulcer. Stop smoking — even a few weeks off cigarettes before planned surgery measurably improves healing. Keep the wound clean and covered rather than baking it dry, and don't pick the scab. For pressure and foot ulcers, take the weight off the area (offloading) so new tissue isn't crushed as fast as it forms. And give it time — the surface may close in a week or two, but the strength you feel underneath keeps building for a year.
What healing well looks like — and what doesn't
A wound that is on track goes through a recognisable sequence. In the first day or two the edges are a little pink, puffy and tender — that is the deliberate inflammation, not a problem. Over the next week or two the base turns a healthy beefy pink or red and slightly bumpy (that's the good granulation tissue and its new capillaries), the wound gets visibly smaller and shallower, and a thin sheen of new skin creeps in from the rim. Some clear or slightly yellow fluid is normal. As weeks pass the closed wound flattens and fades from red or purple toward a paler, softer scar — a colour change that can take many months. Itching during healing is common and usually just means the nerves and skin are knitting back together.
When a wound needs a professional — the red flags
Some changes mean the repair has gone off the rails and it is time to get medical help rather than wait. Watch for spreading redness, warmth or a red streak moving away from the wound; increasing pain, swelling or throbbing after the first couple of days rather than less; thick, cloudy or foul-smelling pus; a fever; or a wound that is simply not smaller after two to three weeks. A wound that has essentially stalled — the same size, same depth, week after week — is a chronic wound, and behind most of them sits one of the culprits above: poor circulation, uncontrolled diabetes, unrelieved pressure, or a smouldering infection. Deep cuts, anything gaping enough to need stitches, animal or human bites, wounds from something dirty or rusty, and any wound in someone with diabetes or poor circulation deserve prompt attention. Getting the underlying problem treated is what un-sticks the wound — no dressing or supplement fixes a wound that is starved of blood, oxygen or blood-sugar control.