Explain why the mammalian circulatory system is described as a double circulation, and outline one advantage this arrangement provides over a single circulation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The mammalian heart functions as a dual pump, moving blood through two separate circuits simultaneously. Fish, by contrast, possess a single circulation in which blood passes through the heart only once during each complete circuit of the body.
Model answer (4 marks)
Mammals have a double circulation because the heart has two separate chambers that pump blood in two distinct circuits.
1. The right side of the heart pumps de‑oxygenated blood to the lungs (pulmonary circulation) and the left side pumps oxygenated blood to the rest of the body (systemic circulation).
2. Blood therefore passes through the heart twice in one complete circuit.
3. The systemic circuit is re‑pressurised by the left ventricle, giving a higher pressure than the pulmonary circuit.
4. This higher pressure allows oxygen and nutrients to be delivered more rapidly and efficiently to tissues.
One advantage of this arrangement is that it maintains a high systemic blood pressure, enabling faster and more efficient delivery of oxygen and nutrients to respiring tissues.
1. The right side of the heart pumps de‑oxygenated blood to the lungs (pulmonary circulation) and the left side pumps oxygenated blood to the rest of the body (systemic circulation).
2. Blood therefore passes through the heart twice in one complete circuit.
3. The systemic circuit is re‑pressurised by the left ventricle, giving a higher pressure than the pulmonary circuit.
4. This higher pressure allows oxygen and nutrients to be delivered more rapidly and efficiently to tissues.
One advantage of this arrangement is that it maintains a high systemic blood pressure, enabling faster and more efficient delivery of oxygen and nutrients to respiring tissues.
Examiner tips
- Use the terms pulmonary and systemic circulation; mention the heart chambers. Explain the re‑pressurisation of the systemic circuit. Show the link between high pressure and efficient delivery of oxygen.
- Keep the answer concise – 4 points can be covered in 4–5 sentences.
- Use UK spelling (e.g. re‑pressurised).
Common mistakes
- Confusing the order of chambers (right → left) or omitting the double passage through the heart. Failing to mention the higher pressure in the systemic circuit. Using vague phrases like "better circulation" without explaining the pressure advantage.
Mark scheme (4 marks)
- Blood passes through the heart twice in one complete circuit of the body
- One circuit carries blood to the lungs (pulmonary circulation) and the other carries blood to the rest of the body (systemic circulation)
- Blood returning from the lungs is repressurised before being pumped to the body, maintaining high blood pressure in the systemic circuit
- Higher blood pressure in the systemic circuit allows faster / more efficient delivery of oxygen and nutrients to respiring tissues
Key terms in this question
Related
- All OCR A-Level Biology A (H420) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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