# Explain why the mammalian circulatory system is described as a double circulatory system, and outline the advantage this arrangement provides for the delivery of oxygen to respiring tissues.

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

> Mammals maintain high metabolic rates and require a continuous, efficient supply of oxygen to all body tissues. The arrangement of their circulatory system is closely linked to this demand.

## Mark scheme (4 marks)

1. Blood passes through the heart twice in one complete circuit of the body
2. Oxygenated and deoxygenated blood are kept separate / do not mix
3. Blood returning from the lungs is repressurised by the left side of the heart before entering the systemic circuit
4. This maintains a steep / high concentration gradient / high pressure so oxygen is delivered rapidly to respiring tissues

## Key terms

- [double circulatory system](https://www.gradenine.co.uk/glossary/double-circulatory-system)
- [respiring tissues](https://www.gradenine.co.uk/glossary/respiring-tissues)

## 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-d332acbc) · Published by Druglandscape Ltd.