Explain how blood clotting protects the body when a blood vessel is damaged.

Eduqas A-Level Biology — 2.1 Classification and biodiversity · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

When a blood vessel is broken due to injury, a series of events takes place to prevent blood loss and stop pathogens from entering the body.

Model answer (5 marks)

Platelets are the key cells in blood clotting.
1. When a vessel is damaged, platelets are activated.
2. They stick to the exposed collagen of the vessel wall and to each other.
3. This adhesion forms a platelet plug that blocks the break.
4. The plug prevents blood loss and also stops pathogens from entering the bloodstream.
5. By sealing the wound, clotting allows the vessel wall to repair itself and the infection to be avoided.

Examiner tips

Use the word ‘platelets’ first, then describe their activation and adhesion.
Show the sequence: activation → adhesion → plug formation → protection of vessel and prevention of infection.
Mention that the plug allows healing to occur.

Common mistakes

Confusing platelets with red blood cells or fibrin. Misstating that clotting only stops bleeding but not infection. Omitting the step that the plug allows healing.

Mark scheme (5 marks)

Platelets are involved in/responsible for blood clotting
Platelets stick to the broken vessel wall / clump together at the site of injury
This blocks the cut / forms a scab
Blood clotting prevents pathogens from entering the blood vessel / prevents infection
This allows the blood vessel to heal

Key terms in this question

blood clotting · blood vessel