3.7.3Additional lessons
Blood Clotting — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 2 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Why Clotting Matters
Seals the wound
- Seals the wound A clot plugs a damaged vessel, which reduces blood loss and stops blood pressure from falling dangerously.
Blocks entry
- Blocks entry By closing the break in the skin, the clot prevents bacteria and viruses from entering the bloodstream.
Platelets
- Platelets These are small cell fragments formed in the bone marrow;
- they have no nucleus and circulate in the plasma.
Clotting factors
- Clotting factors Plasma proteins and calcium ions, many made by the liver, that take part in the chain of reactions leading to clot formation.
Scab formation
- Scab formation The clot dries at the surface to form a scab, which protects the tissue below while new cells repair the damage.
Part 2 of 4The Clotting Cascade
Each step in the cascade activates the next, so a small stimulus produces a large and rapid response.
1
Damage to a blood vessel wall exposes collagen and causes platelets to stick to the injured site and form a loose plug.
2
Platelets and damaged tissue cells release clotting factors, which start a cascade of enzyme-controlled reactions in the plasma.
3
The cascade activates the enzyme thrombin from its inactive precursor prothrombin, and this step needs calcium ions.
4
Thrombin catalyses the conversion of soluble fibrinogen into insoluble fibrin, a protein made of long threads.
5
Fibrin threads form a mesh across the wound that traps red blood cells and platelets, producing a solid clot.
6
Later the clot shrinks and dries into a scab, and the tissue underneath is repaired until the scab falls away.
Part 3 of 4Control and Consequences
Control and Consequences
Inactive precursors
- Inactive precursors Prothrombin and fibrinogen circulate as inactive or soluble forms, so blood does not clot inside undamaged vessels.
Positive feedback
- Positive feedback Activated platelets release chemicals that attract more platelets, amplifying the response until the wound is sealed.
Haemophilia
- Haemophilia A genetic condition in which a clotting factor is missing or defective, so even small injuries cause prolonged bleeding.
Thrombosis
- Thrombosis A clot that forms inside an unbroken vessel can block blood flow and may cause a heart attack or stroke.
| Substance | Form before clotting | Role |
|---|---|---|
| Prothrombin | Inactive plasma protein | Converted to thrombin |
| Thrombin | Active enzyme | Converts fibrinogen to fibrin |
| Fibrinogen | Soluble plasma protein | Becomes the fibrin mesh |
| Fibrin | Insoluble fibres | Traps blood cells to form the clot |
Part 4 of 4Exam Checklist & Tips
3.7.3 – Key Points for the Exam
- Platelet A cell fragment that sticks to damaged vessels and releases clotting factors.
- Thrombin The enzyme that converts soluble fibrinogen to insoluble fibrin.
- Fibrin An insoluble fibrous protein that forms the mesh of a blood clot.
- Cascade A series of reactions in which each product activates the next step.
- Clotting factor A plasma protein or ion needed in the clotting sequence.
Common Mistakes
- Saying platelets are whole cells; they are fragments of larger bone marrow cells and have no nucleus.
- Reversing the roles of thrombin and fibrin, when thrombin is the enzyme and fibrin is the product.
- Forgetting that fibrinogen is soluble and fibrin is insoluble, which is what makes the clot solid.
- Claiming that clots form in healthy vessels, when the cascade is triggered only by damage.
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