Explain why the mammalian circulatory system is described as a double circulatory system, and outline one advantage this arrangement provides over a single circulatory system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mammals, such as humans, have a heart divided into four chambers. Blood passes through the heart twice for each complete circuit of the body.
Model answer (4 marks)
A double circulatory system means blood passes through the heart twice for each complete circuit of the body.
1. One circuit carries de‑oxygenated blood from the body to the lungs (pulmonary circulation).
2. The second circuit carries oxygenated blood from the lungs to the rest of the body (systemic circulation).
3. The two circuits keep oxygenated and de‑oxygenated blood separate, so they do not mix.
4. This separation allows the systemic circuit to maintain a higher pressure, ensuring efficient delivery of oxygen to tissues.
1. One circuit carries de‑oxygenated blood from the body to the lungs (pulmonary circulation).
2. The second circuit carries oxygenated blood from the lungs to the rest of the body (systemic circulation).
3. The two circuits keep oxygenated and de‑oxygenated blood separate, so they do not mix.
4. This separation allows the systemic circuit to maintain a higher pressure, ensuring efficient delivery of oxygen to tissues.
Examiner tips
- Use the phrase "double circulatory system" and state the two circuits explicitly. Mention that the circuits keep oxygenated and de‑oxygenated blood separate. Explain the pressure advantage in the systemic circuit.
- common_mistakes
- :
- Failing to mention that blood passes through the heart twice. Confusing the two circuits or saying they mix. Not linking the separation to the pressure advantage.
Mark scheme (4 marks)
- Blood passes through the heart twice per 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)
- Oxygenated and deoxygenated blood are kept separate / do not mix
- Advantage: blood pressure can be raised/maintained in the systemic circuit after returning from the lungs, ensuring efficient delivery of oxygen 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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