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 · View as Markdown

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

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.

Model answer (4 marks)

The loop of Henlé creates a counter‑current multiplier that establishes a high solute concentration in the medulla. In the ascending limb Na⁺ and Cl⁻ are actively pumped out into the interstitium while the limb is impermeable to water, so water does not leave and the filtrate becomes dilute. This creates a region of low water potential (high solute concentration) in the medulla. In the descending limb the tubule is permeable to water, so as filtrate passes downwards water follows the osmotic gradient and leaves the tubule, concentrating the filtrate. The steep medullary gradient therefore allows the collecting duct, when ADH makes it water‑permeable, to re‑absorb water by osmosis, producing urine that is more concentrated than blood plasma.

Examiner tips

  • Mention the counter‑current multiplier and the impermeability of the ascending limb to water
  • Explain the role of ADH in making the collecting duct water‑permeable
  • Show the sequence: ascending → descending → collecting duct

Common mistakes

  • Confusing the direction of ion transport in the ascending limb
  • Forgetting that the ascending limb is impermeable to water
  • Not linking ADH action to the final concentration of urine

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 in this question

loop of Henlé · concentrated

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