# Explain why the loop of Henlé is essential for the production of urine that is more concentrated than blood plasma.

> OCR A-Level Biology A (H420) — 5.2 Excretion · Explain · 4 marks

> The kidneys regulate water balance by producing urine of variable concentration. The medulla of the kidney contains a gradient of solute concentration that is highest at the innermost region. This gradient is established and maintained by the loop of Henlé.

## Mark scheme (4 marks)

1. The ascending limb actively transports sodium and chloride ions out into the medulla, but is impermeable to water, so water does not follow.
2. This creates a region of low water potential / high solute concentration in the medulla, forming a concentration gradient (countercurrent multiplier effect).
3. As filtrate passes along the descending limb, water leaves by osmosis into the surrounding medulla (descending limb is permeable to water).
4. The steep medullary gradient means that when ADH renders the collecting duct permeable to water, water moves out by osmosis, producing concentrated urine.

## Key terms

- [loop of Henlé](https://www.gradenine.co.uk/glossary/loop-of-henl)
- [concentrated](https://www.gradenine.co.uk/glossary/concentrated)

## 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-loop-of-henl-cdba6c28) · Published by Druglandscape Ltd.