Blood Clotting: Platelets, Fibrin and Wound Sealing
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
When a blood vessel is damaged, platelets clump at the wound and trigger the conversion of the soluble plasma protein fibrinogen into insoluble fibrin threads. These threads form a mesh that traps blood cells, creating a clot that seals the wound and prevents pathogen entry.
Blood clotting is assessed as both a function of platelets and as a body defence mechanism. Make sure your answer covers the sequence: wound → platelets → fibrinogen to fibrin → clot formation → wound sealed.
The clotting cascade
When a blood vessel wall is broken, platelets are exposed to damaged tissue. They become sticky and clump together at the wound site, forming a temporary plug. Platelets also release chemicals that initiate a chain of reactions.
These reactions convert the soluble plasma protein fibrinogen into insoluble fibrin. Fibrin forms a network of threads across the wound. Red blood cells and more platelets become trapped in this mesh, forming a solid blood clot that seals the wound.
Why clotting matters
Clotting serves two vital purposes: it stops blood loss (haemorrhage), maintaining blood volume and pressure; and it prevents pathogens from entering the body through the open wound. The dried surface of a clot forms a scab, which protects the healing tissue beneath.
Without effective clotting, even small cuts could lead to life-threatening blood loss. People with haemophilia produce insufficient clotting factors and bleed for much longer than normal.
Clotting versus immunity
Clotting is a physical defence: it seals the wound as a barrier. It is not the same as the immune response, which involves white blood cells recognising and destroying pathogens. Both contribute to the body's defence, but through different mechanisms.
Inappropriate clotting inside blood vessels (thrombosis) can block blood flow. If a clot forms in a coronary artery already narrowed by atherosclerosis, it can cause a heart attack. This links blood clotting to coronary heart disease.
Try a focused question (3 marks)
Describe the role of platelets in the clotting of blood at a wound.
Show the answer and marking guidance
Platelets clump together at the wound site to form a temporary plug.
They release chemicals that trigger the conversion of fibrinogen to fibrin.
Fibrin threads form a mesh that traps blood cells, creating a clot that seals the wound.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Calling platelets cells: they are cell fragments without a nucleus.
Saying platelets produce antibodies: that is the role of lymphocytes.
Confusing fibrin (insoluble threads) with fibrinogen (the soluble precursor).
Describing clotting as the immune system attacking pathogens.