1.2.6Unit 1: Lifestyle, Transport, Genes and Health
Blood Clotting — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 18 of 29 Unit quiz
Section 1 of 4The Clotting Cascade
1
When a blood vessel is damaged, platelets stick to the damaged region and release a clotting factor called thromboplastin.
2
Thromboplastin, together with calcium ions, catalyses the conversion of the soluble plasma protein prothrombin into the enzyme 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
This meshwork dries to form a clot or scab, which seals the wound and prevents blood loss and entry of pathogens.
| Step | Substance | Change |
|---|---|---|
| 1 | Prothrombin | Thrombin |
| 2 | Fibrinogen | Fibrin |
Section 2 of 4Why Clotting Matters
Prevents blood loss
- A quick clot avoids dangerous loss of blood and keeps blood pressure steady after an injury.
Infection barrier
- The clot seals the wound against microorganisms while the tissue beneath repairs.
Cascade
- Each enzyme activates the next, so a small signal is amplified into a rapid and large response.
Control
- Clotting normally happens only at a wound, since the factors are inactive in the blood vessel until damage occurs.
Section 3 of 4When Clotting Goes Wrong
When Clotting Goes Wrong
Thrombosis
- A clot forming inside an intact vessel, usually at a damaged or rough atheroma, can block blood flow.
Heart attack
- A clot in a coronary artery cuts the oxygen supply to part of the heart muscle, which dies.
Stroke
- A clot lodged in an artery of the brain starves the brain tissue of oxygen and glucose.
Deep vein thrombosis
- A clot in a deep leg vein may break away as an embolus and be carried to the lungs, where it blocks an artery.
Section 4 of 4Exam Checklist & Tips
1.2.6 – Key Points for the Exam
- Thrombin The enzyme that converts soluble fibrinogen to insoluble fibrin.
- Fibrin An insoluble protein that forms a mesh in a clot.
- Platelet A cell fragment in blood that helps to start clotting.
- Thrombosis Formation of a blood clot inside a blood vessel.
Common Mistakes
- Saying that thrombin is the inactive precursor of prothrombin; prothrombin is the inactive form.
- Stating that fibrin is soluble; the conversion makes it insoluble.
- Forgetting that calcium ions are needed in the cascade.
- Confusing a thrombus, a clot that stays in place, with an embolus, a clot that travels.
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