Explain why mammals have a double circulatory system rather than a single circulatory system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. In a double circulatory system the blood passes through the heart twice per circuit – once to the lungs and once to the body.
2. Blood returning from the lungs is re‑pressurised by the heart before being sent to the body.
3. This re‑pressurisation maintains a higher blood pressure and faster flow of blood to body tissues.
4. The higher pressure and flow mean oxygen and nutrients are delivered to tissues more efficiently, meeting the demands of a multicellular organism.
2. Blood returning from the lungs is re‑pressurised by the heart before being sent to the body.
3. This re‑pressurisation maintains a higher blood pressure and faster flow of blood to body tissues.
4. The higher pressure and flow mean oxygen and nutrients are delivered to tissues more efficiently, meeting the demands of a multicellular organism.
Examiner tips
- Use the exact terms ‘double circulatory system’, ‘lungs’, ‘body’, ‘re‑pressurised’, ‘higher blood pressure’, ‘efficient delivery’.
- Show the two separate circulations (pulmonary and systemic) in your explanation.
Common mistakes
- Confusing single and double circulation, or saying the blood only passes once.
- Omitting the role of higher pressure or efficient delivery to tissues.
Mark scheme (4 marks)
- In a double circulatory system, blood passes through the heart twice per circuit (once to the lungs and once to the body)
- Blood returning from the lungs is re-pressurised by the heart before being sent to the body
- This maintains a higher blood pressure / faster flow of blood to body tissues
- This ensures that oxygen and nutrients are delivered to tissues more efficiently / meets the demands of a multicellular organism
Key terms in this question
double circulatory system · circulatory system
Related
- All Edexcel GCSE Biology (1BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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