# Explain why the loop of Henle is essential for the production of concentrated urine in mammals.

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

> Mammals living in arid environments, such as desert rodents, possess exceptionally long loops of Henle compared with mammals living in wetter habitats. This anatomical difference allows them to produce urine far more concentrated than their blood plasma.

## Mark scheme (4 marks)

1. The descending limb of the loop of Henle is permeable to water, so water leaves by osmosis into the medullary interstitium, increasing the solute concentration of the filtrate as it descends.
2. The ascending limb actively transports sodium (and chloride) ions out into the medullary interstitium, but is impermeable to water, so the interstitium becomes increasingly hypertonic towards the apex of the medulla.
3. This establishes a (hairpin) counter-current multiplier mechanism that generates a progressively increasing osmotic gradient (hypertonic medullary interstitium) from the cortex to the medulla.
4. The hypertonic medullary interstitium lowers the water potential around the collecting duct, so when ADH increases the permeability of the collecting duct, water is reabsorbed by osmosis, producing concentrated urine.

## Key terms

- [loop of Henle](https://www.gradenine.co.uk/glossary/loop-of-henle)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-loop-of-henle-ea73e846) · Published by Druglandscape Ltd.