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Counter Current Mechanism Kidney Regulation Class 11

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BiologyCBSE Class 11 · Excretory Products and their Elimination

Counter Current Mechanism, Regulation of Kidney Function and Disorders

Updated 17 October 2026 · Fact-checked

The counter current mechanism in Henle's loop and vasa recta builds a medullary osmolarity gradient from about 300 mOsmol per litre in the cortex to about 1200 in the inner medulla, allowing concentrated urine. ADH, the renin-angiotensin-aldosterone system and ANF regulate kidney function; disorders include uremia, kidney stones and glomerulonephritis, treated by haemodialysis and transplant.

Understand Counter Current Mechanism and Kidney Regulation

Concentrating the urine. Mammals produce urine hyperosmotic to blood (up to 1200 mOsmol per litre; blood is about 300). Henle's loop and vasa recta work by a counter current mechanism.

  • The two limbs of Henle's loop and the two limbs of the vasa recta run in opposite directions (counter current) and are close together.
  • The descending limb is permeable to water but almost impermeable to electrolytes; as filtrate descends, water leaves and the filtrate becomes more concentrated.
  • The ascending limb is impermeable to water but allows transport of electrolytes (NaCl) actively or passively to the medullary interstitium; so the filtrate becomes dilute as it ascends.
  • NaCl is transported by the ascending limb to the interstitium, and exchanged with the descending limb of the vasa recta; urea enters the thin segment of the ascending limb and is carried back via the collecting duct to the interstitium. The vasa recta takes back NaCl and urea at the same time.
  • Result: an osmolarity gradient in the medulla from about 300 mOsmol per litre in the cortex to about 1200 in the inner medulla, caused by NaCl and urea. This gradient helps water pass out of the collecting duct, producing concentrated urine. Human kidneys can produce urine nearly four times as concentrated as the initial filtrate.

Regulation of kidney function.

  • Hypothalamus, osmoreceptors and ADH: a rise in body fluid osmolarity or a fall in volume activates osmoreceptors, which stimulate the hypothalamus to release ADH (vasopressin) from the neurohypophysis. ADH acts on the distal tubule and collecting duct, increasing water reabsorption, so less urine is formed (prevents diuresis). A fall in osmolarity switches the mechanism off.
  • JGA and the renin-angiotensin-aldosterone system (RAAS): the JGA (specialised cells of the distal tubule and the afferent arteriole) releases renin when glomerular blood flow or pressure falls. Renin converts angiotensinogen to angiotensin I and then angiotensin II, a vasoconstrictor that raises blood pressure and GFR and stimulates the adrenal cortex to release aldosterone, which promotes reabsorption of Na⁺ and water and loss of K⁺.
  • ANF (atrial natriuretic factor): released by the atrial wall when blood pressure rises; causes vasodilation and lowers blood pressure, acting as a check on RAAS.

Micturition. Urine is stored in the urinary bladder until stretch receptors send signals to the CNS; the CNS sends motor messages to contract the smooth muscle of the bladder and relax the urethral sphincter, releasing urine. This is the micturition reflex, under voluntary control in adults.

Other organs in excretion. Lungs (CO₂, water), liver (bile pigments, cholesterol, drugs), skin (sweat: NaCl, urea, lactic acid; sebaceous glands: sterols, wax).

Disorders. Uremia (high blood urea), treated by haemodialysis (blood passed through a dialyser; waste diffuses out, then blood returned with anticoagulant heparin; no reabsorption); kidney transplant is the ultimate method. Renal calculi (stones of calcium oxalate). Glomerulonephritis (inflammation of glomeruli).

Draw and label: concentration gradient diagram of the loop of Henle and vasa recta with osmolarities (300, 600, 900, 1200).

Counter Current Mechanism and Kidney Regulation: key facts

Gradient
Cortex 300 | inner medulla 1200 mOsmol per L
Four-fold
ADH
  • Acts on DCT and collecting duct
  • more water reabsorbed
  • less urine
From neurohypophysis
RAAS
Renin -> angiotensin II -> aldosterone -> Na+ and water reabsorbed
Raises BP
ANF
  • Atrial release
  • vasodilation
  • opposes RAAS
Lowers BP

How to answer Counter Current Mechanism and Kidney Regulation questions

Questions ask for the role of a structure or the effect of a change.

  1. 1State the permeability of each limb and what moves.
  2. 2Link the gradient to water movement from the collecting duct.
  3. 3For regulation, trace stimulus, hormone, target and effect.
  4. 4Compare RAAS and ANF as opposites.
  5. 5For dialysis, mention diffusion and heparin.

Counter Current Mechanism and Kidney Regulation in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Thirsty after salty food

Suppose a person eats very salty food and urine becomes scanty and dark.

Explain with hormones.

Try it first, then show the solutionSolution
  1. Plasma osmolarity rises and osmoreceptors fire.
  2. The hypothalamus triggers ADH release from the neurohypophysis.
  3. ADH increases water reabsorption at the collecting duct, so less, concentrated urine forms.

Answer: ADH conserves water.

Memory hook: Salty means ADH up.

Tricks to remember Counter Current Mechanism and Kidney Regulation

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • Descending: water out

    Descending limb loses water; ascending loses salt.

    Use it when: Counter current

  • ADH = Anti-Diuresis

    ADH prevents diuresis.

    Use it when: ADH

  • RAAS up, ANF down

    RAAS raises BP; ANF lowers it.

    Use it when: Regulation

Counter Current Mechanism and Kidney Regulation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Limb permeability · 3 marks · Short answerBoard level

State the permeability of the two limbs of Henle's loop and the effect on the filtrate.

Try it first, then show the solutionSolution
  1. Descending limb permeable to water, nearly impermeable to electrolytes (1 mark).
  2. Filtrate loses water and becomes concentrated (1 mark).
  3. Ascending limb impermeable to water but transports NaCl out, so filtrate becomes dilute (1 mark).

Answer: Descending concentrates; ascending dilutes.

Example 2: ADH · 3 marks · Short answerBoard level

Explain how ADH regulates urine volume.

Try it first, then show the solutionSolution
  1. Fall in blood volume or rise in osmolarity stimulates osmoreceptors and hypothalamus (1 mark).
  2. ADH from the neurohypophysis acts on DCT and collecting duct (1 mark).
  3. More water reabsorbed, so urine volume falls (1 mark).

Answer: Osmoreceptors, ADH, water reabsorption.

Example 3: RAAS · 4 marks · Short answerHarder board level

Trace the sequence from low glomerular pressure to restoration of blood pressure.

Try it first, then show the solutionSolution
  1. JGA releases renin when pressure falls (1 mark).
  2. Renin converts angiotensinogen to angiotensin I, then II (1 mark).
  3. Angiotensin II is a vasoconstrictor and stimulates aldosterone release (1 mark).
  4. Aldosterone increases Na+ and water reabsorption, raising blood volume and pressure (1 mark).

Answer: Renin - angiotensin II - aldosterone.

Example 4: Dialysis · 3 marks · Short answerHarder board level

Explain haemodialysis and why heparin is added.

Try it first, then show the solutionSolution
  1. Patient's blood passes through a dialyser with a cellophane membrane in dialysing fluid (1 mark).
  2. Nitrogenous waste diffuses out down the gradient while proteins stay (1 mark).
  3. Heparin prevents clotting during the process (1 mark).

Answer: Diffusion of wastes across a membrane; heparin stops clots.

Example 5: Four-fold gradient · 5 marks · Long answerHOTS / competency

Suppose the medullary interstitium were 300 mOsmol per L throughout. Predict the urine concentration and the water balance, relating to the counter current gradient and ADH.

Try it first, then show the solutionSolution
  1. No gradient means water cannot be drawn from the collecting duct (1 mark).
  2. Urine would be isosmotic to blood, at most about 300 (1 mark).
  3. Large volumes of dilute urine would be lost (1 mark).
  4. ADH would have little effect since the osmotic drive is missing (1 mark).
  5. The body would risk dehydration (1 mark).

Answer: No gradient: dilute large volumes of urine.

Where students lose marks in Counter Current Mechanism and Kidney Regulation

The slips examiners see most often, and what to write instead.

  • Mistake: ADH from adenohypophysis

    Source mixed.

    Wrong working

    ADH from the anterior pituitary.

    Right working

    ADH from the neurohypophysis.

    Do this: Posterior.

    Usually costs: 1 mark.

  • Mistake: Renin as hormone lowering BP

    Role reversed.

    Wrong working

    Renin lowers BP.

    Right working

    Renin starts the pathway that raises BP.

    Do this: RAAS raises.

    Usually costs: 1 mark.

  • Mistake: Ascending limb permeable to water

    Limb confusion.

    Wrong working

    Ascending limb is water permeable.

    Right working

    Descending limb is water permeable.

    Do this: Descending water.

    Usually costs: 1 mark.

Exam tips

  • Describe limb permeability precisely.
  • Give the osmolarity numbers.
  • Trace hormone paths in order.
  • Draw the gradient figure.

Practice questions on Counter Current Mechanism and Kidney Regulation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    The hormone that increases water reabsorption in collecting ducts is
    (a) aldosterone (b) ANF (c) ADH (d) renin

    Hint

    Neurohypophysis.

    Answer

    (c).

  2. Q2 · 1 mark · Assertion-reasonHarder board level

    Assertion (A): ANF lowers blood pressure.
    Reason (R): ANF causes vasodilation.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Opposes RAAS.

    Answer

    (a).

  3. Q3 · 2 marks · Very short answerBoard level

    What is micturition? Which muscles are involved?

    Hint

    Release of urine.

    Answer

    Release of urine from the bladder by a reflex: bladder smooth muscle contracts and the urethral sphincter relaxes.

  4. Q4 · 4 marks · Case-basedHarder board level

    Read and answer.
    Suppose a patient has high blood urea and swollen feet.
    (a) Name the condition. (1)
    (b) Name the treatment for temporary relief. (1)
    (c) Name the ultimate treatment. (2)

    Hint

    Uremia.

    Answer

    (a) Uremia (kidney failure). (b) Haemodialysis. (c) Kidney transplant.

  5. Q5 · 3 marks · Short answerHOTS / competency

    Suppose a person drinks 2 litres of water quickly. Predict the changes in plasma osmolarity, ADH and urine volume, and justify.

    Hint

    Osmoreceptors.

    Answer

    Plasma osmolarity falls, osmoreceptors reduce ADH release, less water is reabsorbed in the collecting duct, and a large volume of dilute urine is produced to restore balance.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.Max urine concentration?Show answer

    About 1200 mOsmol per L.

  2. 2.Hormone from atria?Show answer

    ANF.

  3. 3.What does JGA release?Show answer

    Renin.

  4. 4.Dialysis anticoagulant?Show answer

    Heparin.

  5. 5.What is ADH also called?Show answer

    Vasopressin.

  6. 6.Kidney stones made of?Show answer

    Calcium oxalate.

Counter Current Mechanism and Kidney Regulation: frequently asked questions

What is the counter current mechanism?

Opposite flows in the loop and vasa recta that build a gradient.

What does aldosterone do?

Increases Na+ and water reabsorption.

What is uremia?

Accumulation of urea in blood.

What is ANF?

A hormone that lowers blood pressure.

What is JGA?

Juxtaglomerular apparatus.