Describe the roles of the four main components of blood in the human body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Blood is a specialised tissue that transports substances, protects the body against infection and helps repair damage to blood vessels.
Model answer (5 marks)
Plasma carries substances such as nutrients, hormones, urea or antibodies through the bloodstream.
Red blood cells contain haemoglobin, which binds oxygen and transports it to tissues.
White blood cells defend the body by recognising and destroying invading pathogens.
Platelets are small cell fragments that aggregate at sites of vascular injury to form a clot.
The platelet clot stops bleeding and also acts as a barrier, preventing pathogens from entering the bloodstream through the damaged vessel.
Red blood cells contain haemoglobin, which binds oxygen and transports it to tissues.
White blood cells defend the body by recognising and destroying invading pathogens.
Platelets are small cell fragments that aggregate at sites of vascular injury to form a clot.
The platelet clot stops bleeding and also acts as a barrier, preventing pathogens from entering the bloodstream through the damaged vessel.
Examiner tips
- Use the exact terms: plasma, red blood cells, white blood cells, platelets. Mention haemoglobin for RBCs. Show that platelets form a clot that both stops bleeding and blocks infection. Keep each point brief and separate with a line break.
Common mistakes
- Confusing plasma with whole blood. Forgetting to mention haemoglobin in RBCs. Saying platelets only stop bleeding, omitting the infection barrier.
Mark scheme (5 marks)
- Plasma transports substances such as nutrients, hormones, urea or antibodies (any one acceptable example)
- Red blood cells contain haemoglobin which binds to / carries oxygen around the body
- White blood cells defend against infection / fight pathogens
- Platelets are involved in blood clotting when a blood vessel is damaged
- Blood clotting (by platelets) allows the vessel to heal AND/OR prevents pathogens entering the bloodstream through the wound
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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