The big idea: When a blood vessel is cut, the blood does not keep flowing out forever — it forms a clot that plugs the wound.
A clot is a mesh of fibres that traps blood cells, seals the broken vessel and dries into a scab.
This does two jobs at once: it stops blood loss, and it forms a barrier that keeps pathogens out. So clotting is part of the body's first line of defence against infection.
- Blood clot
- A plug of trapped blood cells held together by a mesh of fibrin fibres, which seals a damaged blood vessel.
- Platelet
- A tiny cell fragment in the blood that sticks to a wound, clumps together and releases the clotting factors that start a clot forming.
- Clotting factor
- A chemical released at a wound that switches on the cascade of reactions leading to a clot.
- Fibrinogen
- A soluble protein dissolved in the blood plasma; it is the raw material that fibrin is made from.
- Fibrin
- An insoluble protein that forms long fibres; these tangle into a mesh that traps blood cells and forms the clot.
Two jobs of a clot: A clot is not only about stopping bleeding.
By sealing the broken skin and vessel, the clot also blocks the way in for pathogens — so it is both a plug and a defensive barrier.
Clotting happens as a cascade — a chain of steps where each step switches on the next, like a row of dominoes.
It is triggered by damage: clotting only starts where a blood vessel has actually been cut, so the body never forms clots in healthy, undamaged vessels.
The clotting cascade, in order
- A blood vessel is cut, exposing its rough inner surface to the blood (this is the trigger).
- Platelets touch the exposed surface, stick to it, change shape and clump together to form a temporary plug.
- The activated platelets release clotting factors, which switch on a cascade of reactions.
- The clotting factors convert the inactive protein prothrombin into the active enzyme thrombin.
- Thrombin converts soluble fibrinogen into insoluble fibrin, which forms a tangled mesh of fibres.
- The fibrin mesh traps platelets and red blood cells, forming a clot that dries into a scab.
| Step | What happens | Why it matters |
|---|---|---|
| 1 — Vessel is cut | A blood vessel is damaged, exposing its rough inner surface and tissues to the blood | This is the trigger — clotting only starts where a vessel is broken |
| 2 — Platelets stick & activate | Platelets touch the exposed surface, stick to it, change shape and clump together to form a temporary plug | Plugs the gap quickly and starts the chemical cascade |
| 3 — Clotting factors released | Activated platelets (and damaged tissue) release clotting factors that switch on a cascade of reactions | Sets off the chain that will build the permanent clot |
| 4 — Thrombin is made | The clotting factors convert the inactive protein prothrombin into the active enzyme thrombin | Thrombin is the enzyme that builds the fibre meshwork |
| 5 — Fibrin mesh forms | Thrombin converts soluble fibrinogen into insoluble fibrin, which forms a tangled mesh of fibres across the wound | The mesh is the scaffolding of the clot |
| 6 — Clot and scab | The fibrin mesh traps platelets and red blood cells, forming a clot that dries into a scab | Seals the wound, stops blood loss and blocks pathogen entry |
The key chemical change: The heart of the cascade is one change: soluble fibrinogen → insoluble fibrin, carried out by the enzyme thrombin.
Because fibrin is insoluble, it does not dissolve away — it stays in place as a solid mesh that holds the clot together.
Before this can happen, thrombin itself has to be switched on: clotting factors convert inactive prothrombin into active thrombin.
| Feature | Fibrinogen | Fibrin |
|---|---|---|
| State | Soluble — dissolved in the plasma | Insoluble — does not dissolve |
| Form | Floats freely as separate molecules | Long fibres that tangle into a mesh |
| When present | Always circulating in the blood | Only made at a wound, when needed |
| Made from / by | Already in plasma | Made from fibrinogen by the enzyme thrombin |
| Role | The raw material for the clot | The mesh that traps cells and seals the wound |
Why the trigger matters: Clotting must happen only at a wound.
If clots formed inside healthy vessels they could block blood flow and cause serious harm. That is why the cascade is started by the damaged vessel surface itself — no damage means no clot.
This is why the answer to 'what causes a clot to form?' is a cut / damaged blood vessel (which activates platelets).
What starts clotting
- A cut / damaged blood vessel
- Platelets contact the exposed surface
- Platelets activate and release clotting factors
- Only happens where there is damage
What the clot is made of
- A mesh of insoluble fibrin fibres
- Trapped platelets and red blood cells
- Built from fibrinogen by the enzyme thrombin
- Dries into a protective scab
A memory hook: Walk it as a chain: cut → platelets stick → clotting factors → thrombin → fibrin mesh → clot/scab.
And the one-line version of the chemistry: thrombin turns soluble fibrinogen into insoluble fibrin.
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How this is tested: On Paper 2, a 3-mark Explain question can ask how platelets help prevent infection from a skin cut — you must link the platelets to a clot and the clot to a barrier that keeps pathogens out (not just 'stops bleeding').
On Paper 1, a 1-mark question may ask you to put the blood components / events in the order they form a clot — so you have to know the cascade sequence.
Another Paper 1 item simply asks you to identify what causes a clot to form — the answer is a damaged / cut blood vessel activating platelets.
IB-style question — explain how platelets help prevent infection
A gardener gets a deep cut on her hand from a thorn. Explain how platelets in her blood help to prevent infection through the cut. [3]
How to score all three marks
- Platelets start a clot. Platelets stick to the damaged vessel, clump together and release clotting factors, which trigger the cascade that forms a clot.
- The clot seals the wound. The clotting factors lead to a fibrin mesh that traps blood cells and seals the cut, drying into a scab.
- The seal blocks pathogens. The clot / scab acts as a physical barrier, so bacteria and other pathogens cannot enter the tissues through the cut. (Award 1 mark for each distinct point, up to 3.)
Final answer
Platelets stick to the wound and release clotting factors, which form a fibrin clot that seals the cut; the clot / scab is a physical barrier, so pathogens cannot enter through the broken skin.
✓ Why this scores full marks: It links the chain all the way through: platelets → clotting factors / clot → barrier → pathogens kept out.
An answer that only says 'platelets stop bleeding' would miss the infection point — the question is about preventing infection, so you must mention the clot acting as a barrier to pathogens.
| How the clot helps | What it does | Why it protects against infection |
|---|---|---|
| Seals the cut | Plugs the broken skin and vessel with a fibrin mesh and scab | Closes the gap pathogens would enter through |
| Acts as a barrier | The scab is a physical barrier over the wound | Bacteria and other pathogens cannot get through to the tissues below |
| Stops blood loss | Traps blood cells so blood stops leaking out | Keeps the body's defences (and blood) inside, where they work |
| Buys time to heal | Holds the wound shut while new skin grows underneath | By the time the scab falls off, the intact skin barrier is restored |