Describe the roles of the four main components of blood in the human body.

WJEC A-Level Biology (Wales) — 2.1 Classification and biodiversity · Describe · 5 marks · View as Markdown

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

Blood is a vital tissue that carries out several important functions in the human body. It is made up of four main components, each with a specific role.

Model answer (5 marks)

Plasma carries substances such as nutrients, hormones, urea or antibodies around the body.
Red blood cells contain haemoglobin which binds to and carries oxygen to the body's cells.
White blood cells protect against infection and fight pathogens.
Platelets are involved in blood clotting, sticking together to block a cut or wound, preventing pathogens from entering the blood vessel and allowing the vessel to heal.

Examiner tips

  • Use the exact terminology (plasma, red blood cells, white blood cells, platelets).
  • Show each component’s main function in one sentence.
  • Include the clotting role of platelets as part of the fourth point.

Common mistakes

  • Mixing up the functions of plasma and platelets; e.g., saying platelets transport nutrients.
  • Failing to mention haemoglobin in red blood cells or the immune role of white blood cells.

Mark scheme (5 marks)

  1. Plasma transports substances such as nutrients, hormones, urea or antibodies around the body (accept any one valid example)
  2. Red blood cells contain haemoglobin which binds to / carries oxygen to the body's cells
  3. White blood cells protect against infection / fight pathogens
  4. Platelets are involved in blood clotting / stick together to block a cut/wound
  5. Blood clotting (by platelets) prevents pathogens from entering the blood vessel / allows the vessel to heal

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