Explain why blood in the double circulatory system returns to the heart between the two circuits, and describe the advantages this provides for delivering oxygen to body tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mammals have a double circulatory system. Blood travels from the heart to the lungs and back to the heart, then from the heart to the body and back to the heart again.
Model answer (5 marks)
Blood returns to the heart after the pulmonary circuit so that it can be re‑pressurised by the heart’s contraction. This allows the oxygenated blood to be pumped into the systemic circulation at a high pressure, which delivers oxygen to tissues more quickly and at a higher rate. Because the double circulatory system keeps oxygenated and deoxygenated blood separate, the body receives fully oxygenated blood at maximum oxygen concentration, supporting efficient aerobic respiration.
Examiner tips
- Use the word ‘re‑pressurised’ to show understanding of the heart’s role.
- Explain that high pressure speeds delivery of oxygen to tissues.
- Mention separation of oxygenated/deoxygenated blood to justify full oxygen delivery.
- Link the faster delivery to aerobic respiration.
Mark scheme (5 marks)
- Blood returning to the heart after the pulmonary circuit is re-pressurised / pumped again by the heart
- Blood can be delivered to body tissues at high pressure
- This means blood / oxygen is delivered to tissues more quickly / at a faster rate
- Oxygenated and deoxygenated blood are kept separate (in the double circulatory system)
- This ensures fully oxygenated blood (at maximum oxygen concentration) is delivered to body tissues, supporting aerobic respiration
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Exchange and Transport questions
- Explain how the structure of capillaries makes them well suited for the exchange…
- Explain why multicellular organisms need both an exchange surface and a mass tra…
- Explain how the structure of alveoli makes them well adapted as an exchange surf…
- Explain how the structure of arteries and veins makes each vessel suited to its …