Explain how the double circulatory system in mammals is suited to their needs, and describe two features of this system that make it more efficient than a single circulatory system.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Mammals such as humans have a double circulatory system, meaning blood passes through the heart twice in each full circuit of the body.
Model answer (5 marks)
1. In a double circulatory system blood passes through the heart twice per full circuit – once in the pulmonary circuit (lungs) and once in the systemic circuit (body). 2. Oxygenated and deoxygenated blood are kept separate, so cells receive fully oxygenated blood. 3. The left ventricle re‑pressurises blood before it enters the systemic circulation, maintaining a high pressure that allows rapid delivery of oxygen and nutrients. 4. Mammals are larger and have a higher metabolic demand; a double system can meet this demand by supplying more oxygen per unit time. 5. The separation of circuits prevents a drop in pressure that would otherwise slow the rate of delivery of substances.
Examiner tips
- Use the term ‘double circulatory system’ and mention pulmonary vs systemic circuits; show separation of oxygenated/deoxygenated blood; explain high pressure and rapid delivery; link to mammalian size and metabolic demand; keep answer concise and use UK spelling.
Common mistakes
- Confusing single and double circulatory systems; not mentioning the role of the left ventricle; failing to link to mammalian size/metabolic demand; using vague terms like ‘better’ instead of ‘higher pressure’ or ‘separate blood’.
Mark scheme (5 marks)
- In a double circulatory system, blood passes through the heart twice per full circuit (once through pulmonary circulation and once through systemic circulation).
- Oxygenated and deoxygenated blood are kept separate, so cells receive fully oxygenated blood.
- A double circulatory system can maintain a higher blood pressure, allowing oxygen and nutrients to be delivered to cells more quickly / efficiently.
- Mammals are larger organisms with a greater demand / need for oxygen for respiration, so a double circulatory system is needed to meet this demand.
- Blood returning from the lungs is re-pressurised by the left ventricle before being sent to the body, preventing a drop in pressure that would reduce the rate of delivery of substances.
Key terms in this question
Related
- All WJEC A-Level Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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