# Explain why the mammalian circulatory system is described as a double circulation, and outline one advantage this arrangement provides over a single circulation.

> OCR A-Level Biology A (H420) — 3.2 Transport in animals · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Blood passes through the heart twice in one complete circuit of the body
2. One circuit carries blood to the lungs (pulmonary circulation) and the other carries blood to the rest of the body (systemic circulation)
3. Blood returning from the lungs is repressurised before being pumped to the body, maintaining high blood pressure in the systemic circuit
4. Higher blood pressure in the systemic circuit allows faster / more efficient delivery of oxygen and nutrients to respiring tissues

## Key terms

- [double circulation](https://www.gradenine.co.uk/glossary/double-circulation)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-mammalian-circulatory-system-7ca908c2) · Published by Druglandscape Ltd.