This MCQ module is based on: Tubule Function Counter Current
TOPIC 73 OF 90
Tubule Function Counter Current
🎓 Class 11
Biology
CBSE
Theory
Ch 16 – Excretory Products and their Elimination
⏱ ~14 min
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Function of the Tubules and the Counter Current Mechanism
Part 2 left 180 litres of filtrate a day entering the tubules, of which only a litre and a half may leave as urine. This part shows exactly which segment recovers what — and then explains the elegant piece of engineering that allows a mammal to produce urine four times more concentrated than the fluid it started from.
16.3 Function of the Tubules
Proximal Convoluted Tubule (PCT)
- PCT is lined by simple cuboidal brush border epithelium, which increases the surface area for reabsorption.
- Nearly all of the essential nutrients, and 70–80 per cent of electrolytes and water, are reabsorbed by this segment.
- PCT also helps to maintain the pH and ionic balance of the body fluids by selective secretion of hydrogen ions and ammonia into the filtrate, and by absorption of HCO₃⁻ from it.
Why the PCT is the workhorse. The bulk of the recovery happens here — nearly all the nutrients and up to four fifths of the electrolytes and water. The brush border is the structural reason: dense microvilli multiply the absorbing surface, exactly as the villi of the intestine do. Note also that the PCT does both jobs at once — it reabsorbs useful material and secretes H⁺ and ammonia — so traffic crosses the tubule wall in both directions simultaneously.
Henle's Loop
Reabsorption is minimum in its ascending limb. However, this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid.
The two limbs have opposite permeabilities — this is the key to the whole chapter.
• The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down.
• The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes to the medullary fluid.
• The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down.
• The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes to the medullary fluid.
Distal Convoluted Tubule (DCT)
- Conditional reabsorption of Na⁺ and water takes place in this segment.
- DCT is also capable of reabsorption of HCO₃⁻ and selective secretion of hydrogen and potassium ions and NH₃, to maintain the pH and sodium-potassium balance in blood.
The word “conditional” is doing real work. Reabsorption in the PCT happens regardless — it is bulk recovery. In the DCT it happens only if the body needs it, under hormonal instruction: ADH for water and aldosterone for Na⁺, both taken up in Part 4. That is why the DCT, not the PCT, is where the fine tuning of urine composition is done.
Collecting Duct
- This long duct extends from the cortex of the kidney to the inner parts of the medulla.
- Large amounts of water could be reabsorbed from this region to produce a concentrated urine.
- This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity.
- It also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of H⁺ and K⁺ ions.
| Segment | Reabsorbed | Secreted | Special feature |
|---|---|---|---|
| PCT | Nearly all essential nutrients; 70–80% of electrolytes and water; HCO₃⁻ | H⁺ and ammonia | Brush border epithelium increases surface area |
| Descending limb of Henle's loop | Water (permeable to water, almost impermeable to electrolytes) | — | Concentrates the filtrate as it moves down |
| Ascending limb of Henle's loop | Minimum reabsorption; electrolytes pass out actively or passively | — | Impermeable to water; the filtrate gets diluted; maintains high osmolarity of medullary interstitial fluid |
| DCT | Conditional reabsorption of Na⁺ and water; HCO₃⁻ | H⁺, K⁺ and NH₃ | Maintains pH and Na⁺–K⁺ balance in blood |
| Collecting duct | Large amounts of water | H⁺ and K⁺ | Lets small amounts of urea into the medullary interstitium; produces concentrated urine |
16.4 Mechanism of Concentration of the Filtrate
Mammals have the ability to produce a concentrated urine. The Henle's loop and vasa recta play a significant role in this.
What makes it a counter current. The flow of filtrate in the two limbs of Henle's loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern.
The gradient that results
The proximity between the Henle's loop and vasa recta, as well as the counter current in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium — i.e., from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
What the gradient is made of, and how it is kept up. This gradient is mainly caused by NaCl and urea.
• NaCl is transported by the ascending limb of Henle's loop, which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.
• Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop, which is transported back to the interstitium by the collecting tubule.
• NaCl is transported by the ascending limb of Henle's loop, which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta.
• Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle's loop, which is transported back to the interstitium by the collecting tubule.
The counter current mechanism. The above described transport of substances facilitated by the special arrangement of Henle's loop and vasa recta is called the counter current mechanism. This mechanism helps to maintain a concentration gradient in the medullary interstitium.
And what the gradient is for
Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.
Why counter current is the clever part. A single straight tube could never build this gradient: whatever it pumped out would be swept away by the blood at once. The hairpin arrangement solves that in two steps. First, because the two limbs of Henle's loop run in opposite directions and have opposite permeabilities, salt leaving the ascending limb is deposited right beside the descending limb, where it draws water out — so each turn of the loop makes the medulla a little saltier, and the effect multiplies down the length of the loop. Second, the vasa recta runs alongside in counter current too, so blood entering the medulla picks up NaCl in its descending portion and NaCl is returned to the interstitium by the ascending portion before the blood leaves. The salt is therefore trapped rather than washed away. Put the two together and you can see why the chapter stresses the proximity of loop and vasa recta as much as the counter current itself.
📐 Activity 16.3 — Trace the osmolarity of the filtrate
What to do. Draw a long horizontal axis representing the journey of the filtrate: Bowman's capsule → PCT → down the descending limb → the hairpin bend → up the ascending limb → DCT → collecting duct → urine. On a vertical axis mark osmolarity from 0 to 1200 mOsmol L⁻¹. Now sketch how the osmolarity of the fluid inside the tubule changes along this route, marking 300 where the filtrate starts. Answer three questions beside your graph: (i) where does the fluid become most concentrated? (ii) where does it become less concentrated than when it started? (iii) at which point is water finally removed to make concentrated urine?
Predict: is the fluid at the top of the ascending limb more or less concentrated than the plasma it came from?
The curve. It starts at 300 in Bowman's capsule, stays near 300 through the PCT (water and solutes are reabsorbed together there, so the fluid shrinks in volume without changing much in concentration), rises steeply to about 1200 at the hairpin bend, then falls to about 100–200 at the top of the ascending limb — below the starting value — and finally rises again in the collecting duct as water leaves, to as much as 1200 in the urine.
(i) Most concentrated inside the loop: at the bend, because the descending limb is permeable to water but almost impermeable to electrolytes, so as it passes through ever saltier medulla, this concentrates the filtrate as it moves down.
(ii) Less concentrated than the start: at the top of the ascending limb — which is the answer to the prediction, and it surprises most students. The ascending limb is impermeable to water but allows transport of electrolytes, so as the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes to the medullary fluid. Salt leaves and water cannot follow, so the fluid becomes dilute.
(iii) Water is finally removed in the collecting duct: large amounts of water could be reabsorbed from this region to produce a concentrated urine, made possible because presence of such interstitial gradient helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate.
The insight the graph gives. The loop of Henle does not itself concentrate the urine. It dilutes the fluid passing through it, and its real product is a salty medulla. The concentrating of urine then happens later, in the collecting duct, by letting water escape into that salty medulla. This is why the chapter says the loop plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid rather than saying it concentrates the urine directly — and it is why human kidneys can produce urine nearly four times concentrated than the initial filtrate (300 → about 1200).
🎯 Interactive: What happens in each segment?
Effect on the filtrate: Volume falls greatly; concentration barely changes
Lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption. Nearly all the essential nutrients and 70 to 80 per cent of electrolytes and water are reabsorbed here. It also secretes hydrogen ions and ammonia and absorbs bicarbonate to maintain pH and ionic balance.
🎯 Competency-Based Questions
Scenario: Four experiments are done. In A, a drug blocks electrolyte transport out of the ascending limb of Henle's loop. In B, an animal with unusually long loops of Henle is compared with one having only short cortical nephrons. In C, the vasa recta of a kidney is experimentally straightened so that its two limbs no longer run counter current. In D, a person drinks three litres of water rapidly.
Q1. Predict the effect of the drug in experiment A on the ability to concentrate urine. L4 Analyse
The ability to concentrate urine would be largely lost, and a large volume of dilute urine would be produced. The reason is that the medullary gradient is mainly caused by NaCl and urea, and NaCl is transported by the ascending limb of Henle's loop. Block that transport and the interstitium never becomes salty, so osmolarity cannot rise from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla. Without the gradient, the collecting duct loses its driving force, since it is the presence of such interstitial gradient that helps in an easy passage of water from the collecting tubule, thereby concentrating the filtrate. This is in fact how the strongest class of diuretic drugs works.
Q2. In experiment B, which animal can produce the more concentrated urine, and why? L3 Apply
The animal with the long loops of Henle — that is, with juxta medullary nephrons, in which the loop of Henle is very long and runs deep into the medulla. A longer loop means a longer stretch over which the counter current can multiply the gradient, so a higher osmolarity can be reached in the inner medulla and more water can be drawn out of the collecting duct. The animal with only cortical nephrons, whose loop of Henle is too short and extends only very little into the medulla, is doubly handicapped, since in such nephrons the vasa recta is absent or highly reduced and the salt cannot be trapped either. This is why desert mammals characteristically have very long loops.
Q3. Fill in the blanks: The descending limb is ______ to water but almost ______ to electrolytes, so the filtrate ______. The ascending limb is ______ to water but transports ______, so the filtrate ______. Osmolarity rises from ______ in the cortex to about ______ in the inner medulla. L1 Remember
permeable; impermeable; concentrates as it moves down; impermeable; electrolytes; gets diluted; 300 mOsmol L⁻¹; 1200 mOsmol L⁻¹.
Q4. Why does straightening the vasa recta in experiment C reduce urine concentration, even though Henle's loop is untouched? L4 Analyse
Because the vasa recta's counter current arrangement is what stops the gradient being washed away. The chapter names both vessels: the flow of blood through the two limbs of vasa recta is also in a counter current pattern, and it is the proximity between the Henle's loop and vasa recta, as well as the counter current in them, that maintains the gradient. The mechanism is explicit: NaCl transported by the ascending limb of Henle's loop is exchanged with the descending limb of vasa recta, and then NaCl is returned to the interstitium by the ascending portion of vasa recta. If the vessel ran straight through instead, blood entering the medulla would pick up that NaCl and carry it out of the medulla altogether, so the salt deposited by the loop would be continuously removed. The loop would keep pumping and the gradient would keep collapsing. The lesson is that the counter current mechanism needs both structures — one to build the gradient, the other to preserve it.
Q5. “The loop of Henle concentrates the urine.” Evaluate this common statement. L5 Evaluate
The statement is the right answer to the wrong question, and the chapter's wording is careful to avoid it.
What is true. The loop is indispensable. Without it there is no medullary gradient, and NCERT's exercises themselves state that Henle's loop plays an important role in concentrating the urine. Animals with long loops concentrate urine better than those with short ones.
Why the statement is nevertheless loose. (i) The fluid leaving the loop is dilute, not concentrated. The descending limb concentrates the filtrate on the way down, but the ascending limb is impermeable to water and allows transport of electrolytes, so as the concentrated filtrate passes upward, it gets diluted. At the top of the loop the fluid is less concentrated than the plasma it came from. (ii) The loop's real product is a salty medulla. The chapter says the ascending limb plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid — a statement about the interstitium, not about the urine. (iii) The concentration of urine happens elsewhere. It happens in the collecting duct, from which large amounts of water could be reabsorbed to produce a concentrated urine, and only because the presence of such interstitial gradient helps in an easy passage of water from the collecting tubule. (iv) The loop cannot do it alone. It needs the vasa recta to trap the salt, and the collecting duct both to return urea and to let the water out.
The precise formulation: the loop of Henle, working with the vasa recta, creates the osmotic gradient in the medulla; the collecting duct then uses that gradient to concentrate the urine. Stating it that way explains not only that the kidney concentrates urine but how — and it is the difference between a two-mark answer and a five-mark one.
What is true. The loop is indispensable. Without it there is no medullary gradient, and NCERT's exercises themselves state that Henle's loop plays an important role in concentrating the urine. Animals with long loops concentrate urine better than those with short ones.
Why the statement is nevertheless loose. (i) The fluid leaving the loop is dilute, not concentrated. The descending limb concentrates the filtrate on the way down, but the ascending limb is impermeable to water and allows transport of electrolytes, so as the concentrated filtrate passes upward, it gets diluted. At the top of the loop the fluid is less concentrated than the plasma it came from. (ii) The loop's real product is a salty medulla. The chapter says the ascending limb plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid — a statement about the interstitium, not about the urine. (iii) The concentration of urine happens elsewhere. It happens in the collecting duct, from which large amounts of water could be reabsorbed to produce a concentrated urine, and only because the presence of such interstitial gradient helps in an easy passage of water from the collecting tubule. (iv) The loop cannot do it alone. It needs the vasa recta to trap the salt, and the collecting duct both to return urea and to let the water out.
The precise formulation: the loop of Henle, working with the vasa recta, creates the osmotic gradient in the medulla; the collecting duct then uses that gradient to concentrate the urine. Stating it that way explains not only that the kidney concentrates urine but how — and it is the difference between a two-mark answer and a five-mark one.
🧠 Assertion–Reason Questions
For each pair choose: (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.
Assertion (A): The filtrate becomes more concentrated as it passes down the descending limb of Henle's loop.
Reason (R): The descending limb is permeable to water but almost impermeable to electrolytes.
Answer: A. Both are true and the reason is the correct explanation. Water leaves into the increasingly salty medulla while the solutes stay behind, so the fluid inside becomes concentrated.
Assertion (A): Most of the reabsorption in the nephron takes place in the ascending limb of Henle's loop.
Reason (R): The PCT is lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption.
Answer: D. The assertion is false — reabsorption is minimum in the ascending limb; it is the PCT that reabsorbs nearly all the essential nutrients and 70–80 per cent of electrolytes and water. The reason is true and is why the PCT is so effective at it.
Assertion (A): Urea is allowed to pass from the collecting duct into the medullary interstitium.
Reason (R): The medullary osmotic gradient is mainly caused by NaCl and urea, and the collecting duct returns urea to keep up the osmolarity.
Answer: A. Both are true and the reason explains the assertion. A waste product is deliberately recycled because it is osmotically useful — which also explains why some urea is retained in the kidney matrix of ureotelic animals.
Coming next. Part 4 takes up Sections 16.5 to 16.8 — the hormonal regulation of kidney function through ADH, the renin-angiotensin mechanism and ANF, micturition and the micturition reflex, the role of the lungs, liver and skin in excretion, and the disorders of the excretory system with haemodialysis.
Frequently Asked Questions - Function of the Tubules and the Counter Current Mechanism
What does the proximal convoluted tubule do?
The PCT is lined by simple cuboidal brush border epithelium which increases the surface area for reabsorption. Nearly all the essential nutrients and 70 to 80 per cent of electrolytes and water are reabsorbed here. It also maintains pH and ionic balance by selective secretion of hydrogen ions and ammonia into the filtrate and absorption of bicarbonate from it.
How do the two limbs of Henle's loop differ?
The descending limb is permeable to water but almost impermeable to electrolytes, which concentrates the filtrate as it moves down. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively, so the concentrated filtrate gets diluted as it passes upward.
What is the role of the ascending limb of Henle's loop?
Reabsorption is minimum in it, but it plays a significant role in the maintenance of the high osmolarity of the medullary interstitial fluid, because the electrolytes it transports out pass into the medullary fluid.
What happens in the distal convoluted tubule?
Conditional reabsorption of Na+ and water takes place there. The DCT is also capable of reabsorption of bicarbonate and of selective secretion of hydrogen and potassium ions and ammonia, to maintain the pH and sodium-potassium balance in blood.
What is the function of the collecting duct?
It extends from the cortex to the inner parts of the medulla. Large amounts of water can be reabsorbed from it to produce a concentrated urine, it allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity, and it helps maintain pH and ionic balance by selective secretion of H+ and K+.
What is the counter current mechanism?
The flow of filtrate in the two limbs of Henle's loop is in opposite directions, forming a counter current, and the flow of blood through the two limbs of vasa recta is also counter current. The transport of substances facilitated by this special arrangement is called the counter current mechanism, and it maintains the concentration gradient in the medullary interstitium.
What causes the osmotic gradient in the kidney medulla?
Mainly NaCl and urea. NaCl is transported by the ascending limb of Henle's loop and exchanged with the descending limb of vasa recta, then returned to the interstitium by its ascending portion. Small amounts of urea enter the thin segment of the ascending limb and are transported back to the interstitium by the collecting tubule.
What is the range of osmolarity across the kidney?
It increases towards the inner medullary interstitium, from 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla.
How concentrated can human urine be?
Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed. The interstitial gradient allows an easy passage of water out of the collecting tubule, which is what concentrates the filtrate.
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