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 · View as Markdown

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

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.

Model answer (4 marks)

The loop of Henle is a counter‑current multiplier that creates a hypertonic medullary interstitium.
1. In the descending limb water is permeable, so water leaves the tubular fluid by osmosis into the interstitium, concentrating the filtrate.
2. In the ascending limb sodium and chloride are actively pumped out, but the limb is impermeable to water, so the interstitium becomes progressively more hypertonic toward the apex.
3. The combination of water loss in the descending limb and ion loss in the ascending limb establishes a hairpin counter‑current system that generates a steep osmotic gradient from cortex to medulla.
4. This hypertonic interstitium lowers the water potential around the collecting duct; when ADH increases ductal permeability, water is reabsorbed by osmosis, producing highly concentrated urine.

Examiner tips

  • Use the word ‘counter‑current multiplier’ – examiners expect this term.
  • Show the sequence of water and ion movement in each limb.
  • Explain how the gradient affects the collecting duct.
  • Mention ADH only if you have space – it’s a bonus point.

Common mistakes

  • Confusing the permeability of the limbs – the ascending limb is impermeable to water.
  • Failing to mention the role of ADH or the collecting duct.
  • Giving a generic ‘water is reabsorbed’ without linking it to the osmotic gradient.

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 in this question

loop of Henle

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