3.6.3Additional lessons
Blood Clotting — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 72 of 142
Part 1 of 4Why Blood Clots
Seal wounds
- When the skin is broken, clotting stops blood loss and blocks the entry of microorganisms.
Platelets
- These are small cell fragments in the blood that stick together and release substances that start clotting.
Clotting factors
- These plasma proteins interact in a series of steps called a cascade, which amplifies the response.
Plasma proteins
- Fibrinogen is soluble in plasma, but its product fibrin is insoluble and forms fibres.
Part 2 of 4Steps in Clot Formation
1
A damaged blood vessel exposes tissue, and platelets gather at the site and release clotting factors.
2
The clotting factors, together with calcium ions, convert the inactive enzyme prothrombin into active thrombin.
3
Thrombin catalyses the conversion of soluble fibrinogen into insoluble fibrin.
4
Fibrin forms a mesh of fibres across the wound that traps platelets and red blood cells.
5
The clot dries on exposure to air and hardens into a scab, which protects the tissue as the skin underneath repairs.
Part 3 of 4The Clotting Cascade
The Clotting Cascade
Cascade
- Each activated factor activates the next one, so a small trigger produces a large response.
Positive feedback
- Thrombin also activates more platelets and clotting factors, which speeds up clotting at the wound.
Local action
- Clotting must stay confined to the injury, because clots inside healthy vessels block blood flow.
Healing
- After repair, the clot is gradually broken down and the scab falls away.
| Stage | Substance | Change |
|---|---|---|
| 1 | Prothrombin | Becomes thrombin |
| 2 | Fibrinogen | Becomes fibrin |
| 3 | Fibrin | Forms mesh and traps blood cells |
Part 4 of 4Exam Checklist & Tips
3.6.3 – Key Points for the Exam
- Platelet A cell fragment in blood that helps start clotting.
- Thrombin An enzyme that converts fibrinogen to fibrin.
- Fibrin An insoluble protein that forms the mesh of a clot.
- Fibrinogen A soluble plasma protein that is the precursor of fibrin.
- Cascade A series of reactions in which each product activates the next.
Common Mistakes
- Saying platelets are whole cells with nuclei; they are fragments of cells.
- Mixing up fibrin and fibrinogenfibrinogen is soluble, fibrin is insoluble.
- Writing that thrombin is made from fibrinogen, when it is made from prothrombin.
- Thinking blood clots form only in the air; clotting begins in the blood and air then dries the scab.
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