Explain why mammals have a double circulatory system rather than a single circulatory system.

Cambridge International IGCSE Biology (0610) — 9.2 Heart · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. In a double circulatory system blood passes through the heart twice, so pressure is restored after pulmonary passage.
2. Oxygenated and deoxygenated blood remain separate, ensuring tissues receive fully oxygenated blood.
3. Mammals are larger and more active, requiring a higher oxygen supply for respiration.
4. The system is more efficient at delivering oxygen and nutrients, supporting a higher metabolic rate.

Examiner tips

Use the exact points from the mark scheme; number each point to show structure.
Show the link between separate circuits and higher pressure/oxygen delivery.
Mention size/activity as a reason for higher demand.
Keep sentences concise and use UK spelling.

Common mistakes

Confusing a single circulatory system with a double one; not explaining the pressure restoration.
Failing to state that oxygenated and deoxygenated blood are kept separate.
Omitting the link between size/activity and oxygen demand.

Mark scheme (4 marks)

In a double circulatory system, blood passes through the heart twice per circuit, allowing pressure to be restored after passing through the lungs.
Oxygenated and deoxygenated blood are kept separate, so tissues receive fully oxygenated blood.
Mammals are larger / more active and have a greater demand for oxygen for respiration.
A double circulatory system is more efficient at delivering oxygen / nutrients to tissues, allowing a higher metabolic rate to be sustained.

Key terms in this question

double circulatory system · single circulatory system