A2 1 kidney extended response — the single most repeated A2 1 question style · 6 marks
Explain how the loop of Henle enables the production of urine that is more concentrated than the blood plasma.
Marked instantly, no account needed
A2 · A2 1 Physiology, Co-ordination and Control, and Ecosystems
The loop of Henle and the counter-current multiplier
The short answer
The loop of Henle sets up a water potential gradient in the medulla. The ascending limb actively pumps sodium and chloride ions out but is impermeable to water; the descending limb is permeable to water, which leaves by osmosis. This counter-current arrangement multiplies the gradient so the collecting duct can reabsorb water.
Limb by limb
- Descending limb: permeable to water, impermeable to ions. Water leaves by osmosis into the increasingly negative medulla, so filtrate becomes more concentrated towards the tip.
- Ascending limb: impermeable to water. Sodium and chloride ions are actively transported out into the medulla, so filtrate becomes dilute as it rises.
- Medulla: the ions pumped out lower the water potential of the tissue fluid, most steeply at the base of the medulla.
- Collecting duct: filtrate passes back down through that gradient, so water can leave by osmosis all the way down if the walls are permeable.
Why counter-current matters
Because the two limbs run in opposite directions, filtrate flowing down always meets tissue fluid slightly more concentrated than itself. That small difference is maintained along the whole length of the loop and multiplies into a much larger overall gradient than a single pass could produce. Animals adapted to dry habitats, such as the kangaroo rat, have much longer loops and correspondingly more concentrated urine.
ADH and osmoregulation
- Osmoreceptors in the hypothalamus detect a fall in blood water potential.
- The posterior pituitary releases ADH into the blood.
- ADH makes the collecting duct walls more permeable by inserting aquaporins into the membrane.
- More water is reabsorbed into the blood, so a small volume of concentrated urine is produced — a negative feedback loop.
Phrases that earn the marks
- active transport of sodium and chloride ions from the ascending limb
- ascending limb is impermeable to water
- lowers the water potential of the medulla
- counter-current multiplier maintains the gradient along the loop
- ADH increases permeability of the collecting duct by inserting aquaporins
- water is reabsorbed by osmosis, producing concentrated urine
Where marks get lost
- Saying water is pumped out. Water always moves by osmosis; only ions are actively transported.
- Forgetting to state that the ascending limb is impermeable to water.
- Describing ADH as making 'more water go to the kidney' rather than changing collecting duct permeability.
- Omitting the direction of the medullary water potential gradient.
Try these loop of henle a level questions yourself
Write your answer, then see it marked against every mark scheme point before the model answer is revealed.
1/2
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Exam questions on loop of henle a level, marked
Two A2-standard questions in CCEA paper style, each with the full mark scheme, a full-mark answer written the way you should write it, and the point where most candidates drop marks.
A2 1 kidney extended response — the single most repeated A2 1 question style · 6 marks
Explain how the loop of Henle enables the production of urine that is more concentrated than the blood plasma.
Command word: Explain — how to answer itMark scheme — 6 creditworthy points
- The ascending limb is impermeable to water
- Sodium and chloride ions are actively transported out of the ascending limb into the medulla tissue fluid
- This lowers the water potential of the medulla tissue fluid
- The descending limb is permeable to water, so water leaves it by osmosis, concentrating the filtrate as it descends
- The counter-current multiplier maintains a water potential gradient down the length of the medulla
- The collecting duct passes back through the medulla, so water leaves it by osmosis (through aquaporins, under ADH control), producing concentrated urine
Full-mark answer
The ascending limb of the loop of Henle is impermeable to water, and sodium and chloride ions are actively transported out of it into the tissue fluid of the medulla. This lowers the water potential of the medulla tissue fluid. The descending limb is permeable to water, so as filtrate flows down it water leaves by osmosis into the medulla and the filtrate becomes more concentrated towards the hairpin. Because the two limbs run in opposite directions, a counter-current multiplier is set up that maintains a water potential gradient that becomes increasingly negative deeper into the medulla. The collecting duct passes back down through this medulla, so water moves out of the filtrate by osmosis along the gradient, through aquaporins whose number is controlled by ADH, and the urine leaving is more concentrated than the blood plasma.
Examiner insight: 'Counter-current multiplier' alone is one mark; the mark for the gradient down the medulla is separate and needs the word gradient. Longer loops in desert mammals is a common follow-up — link loop length to how negative the medulla water potential can become.
A2 1 osmoregulation and negative feedback · 5 marks
Describe how the body responds to a fall in the water potential of the blood.
Command word: Describe — how to answer itMark scheme — 5 creditworthy points
- Osmoreceptors in the hypothalamus detect the fall in water potential
- The posterior pituitary releases more ADH into the blood
- ADH increases the permeability of the collecting duct (and distal tubule) to water
- More aquaporins are inserted into the cell surface membrane
- More water is reabsorbed by osmosis, so a smaller volume of more concentrated urine is produced and blood water potential rises — negative feedback
Full-mark answer
Osmoreceptors in the hypothalamus detect the fall in the water potential of the blood. This stimulates the posterior pituitary gland to release more ADH into the blood. ADH increases the permeability of the collecting duct and distal convoluted tubule to water by causing more aquaporins to be inserted into the cell surface membranes. More water is therefore reabsorbed by osmosis into the blood, and a smaller volume of more concentrated urine is produced. The water potential of the blood rises back towards normal, which is negative feedback.
Examiner insight: The pituitary releases — it does not produce — ADH, and the aquaporin mark is now expected at A2. Finish with the words 'negative feedback' to secure the final mark.
These questions are written in the style of the CCEA GCE Biology (2016) papers and are not reproduced from any live paper. CCEA owns the copyright in its question papers and mark schemes — always download the official paper and mark scheme from CCEA and use these worked answers alongside them. BioCCEA is an independent study tool and is not endorsed by CCEA.
Download the official CCEA paper and mark schemeCommon questions
- What is the function of the loop of Henle?
- To create a water potential gradient in the medulla so that water can be reabsorbed from the collecting duct and concentrated urine produced.
- Why is a longer loop of Henle an advantage in dry habitats?
- A longer loop maintains a steeper, deeper water potential gradient in the medulla, so more water is reabsorbed and urine is more concentrated.
- What does ADH actually do?
- It makes the collecting duct and distal tubule walls more permeable to water by inserting aquaporin channels, increasing water reabsorption.
Test yourself on this topic
Generate 10 spec-worded questions on loop of henle a level, or photograph your written answer and see it marked line by line against the mark scheme.
Open loop of henle a level in the app
These open straight into BioCCEA with the right unit and topic already picked — no setup, no searching.