Explain how blood is transported from the heart to the lungs and back to the heart, and why it is important that this circulation is kept separate from the circulation to the rest of the body.

AQA A-Level Biology (7402) — 3.3.4 Mass Transport · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

The right ventricle pumps deoxygenated blood into the pulmonary artery, which carries it to the lungs. In the lungs, blood exchanges CO₂ for O₂ and becomes oxygenated. The oxygenated blood then returns to the heart via the pulmonary vein, entering the left atrium. The two circulations are kept separate so that oxygenated and deoxygenated blood do not mix. This separation ensures that the blood delivered to body tissues has a high oxygen concentration, providing more oxygen for cellular respiration.

Examiner tips

  • Use the correct vessel names (pulmonary artery, pulmonary vein)
  • Show the sequence: right ventricle → lungs → left atrium
  • Explain the importance of separation for oxygen delivery

Common mistakes

  • Confusing pulmonary artery with systemic arteries
  • Lacking the step that oxygenated blood returns via the pulmonary vein
  • Not stating why separation matters for oxygen concentration

Mark scheme (5 marks)

  1. The right ventricle pumps deoxygenated blood to the lungs (via the pulmonary artery)
  2. Blood picks up oxygen / becomes oxygenated at the lungs
  3. Oxygenated blood returns to the left side of the heart (via the pulmonary vein / into the left atrium)
  4. The two circulations are separate so that oxygenated and deoxygenated blood do not mix
  5. This means oxygenated blood delivered to body tissues / organs has a high oxygen concentration, so more oxygen is available for respiration

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