Platelets and blood clottingSpec 2.64B
In short
Blood clotting is how the blood seals a cut, preventing blood loss and stopping micro-organisms entering the body. Platelets collect at the wound and release chemicals that change the soluble plasma protein fibrinogen into insoluble fibrin. Fibrin forms a mesh of fibres that traps red blood cells to make a clot, which dries to form a scab.
When a blood vessel is cut, the blood must clot. Platelets are small fragments of cells that take part in this. Clotting prevents blood loss and stops micro-organisms entering the body through the wound.
- Platelets collect at the cut and release chemicals.
- These chemicals cause the soluble protein fibrinogen in the plasma to change into insoluble fibrin.
- Fibrin forms a mesh of fibres across the wound.
- The mesh traps red blood cells and forms a clot.
- The clot dries to form a scab, which seals the wound while the skin underneath repairs.
(opens full size in a new tab)Give two reasons why clotting matters: it stops blood loss and it stops pathogens (micro-organisms) entering the body.
Quick check
Name the four components of blood.
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Red blood cells, white blood cells, platelets and plasma.
Give three adaptations of a red blood cell for carrying oxygen.
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Biconcave shape, no nucleus, contains haemoglobin.
How do phagocytes destroy pathogens?
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They ingest (engulf) them.
What do lymphocytes release?
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Antibodies that are specific to the pathogen.
Biology only What do memory cells allow after vaccination?
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Antibody production that is sooner, faster and in greater quantity if the pathogen enters again.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026