Biology · Excretory Products and Their Elimination · NEET
The Loop of Henle is a hairpin-shaped part of the nephron that dips into the medulla. Its main job is not to reabsorb a lot, but to build a salt gradient in the medulla (300 to 1200 mOsmol/L) that lets the collecting duct make concentrated urine. Memory hook: "Descending Drinks water, Ascending Adds salt out" — descending limb loses water, ascending limb pushes out electrolytes.
The hairpin loop of Henle: the descending limb loses water while the ascending limb pumps out salt (impermeable to water). Together this counter current builds the medullary gradient (300 to 1200 mOsmol/L) that the collecting duct uses to concentrate urine.
Your doubts, answered
Is the descending limb permeable to water or to electrolytes?
The descending limb is permeable to WATER but almost impermeable to electrolytes. So as filtrate moves down, water leaves into the salty medulla and the filtrate becomes more concentrated at the bottom of the loop. This is the exact opposite of the ascending limb.
Why is the ascending limb impermeable to water?
The ascending limb is impermeable to water but allows transport of electrolytes (Na+, Cl-) out of the filtrate, actively or passively. Because water cannot follow the salt out, the filtrate gets DILUTED as it moves up, while the salt pumped out makes the surrounding medulla more concentrated. This one-way salt removal is what builds the medullary gradient.
Does the loop of Henle actually concentrate the urine?
No — this is the most common trap. The loop of Henle SETS UP the concentration gradient in the medulla (300 to 1200 mOsmol/L). The urine is finally concentrated in the COLLECTING DUCT, where water is drawn out into that gradient (under ADH control). NCERT even says the loop has minimal direct reabsorption; its real role is building the gradient.
What is the counter current and why does it matter?
Filtrate flows in opposite directions in the two limbs of the loop (down, then up), forming a counter current. Blood in the two limbs of the vasa recta also flows opposite. This proximity plus counter current maintains the rising osmolarity toward the inner medulla, so the gradient is not washed away. Loop of Henle + vasa recta together run this mechanism.
How is the loop different in cortical vs juxtamedullary nephrons?
Cortical nephrons have a SHORT loop that dips only slightly into the medulla. Juxtamedullary nephrons have a LONG loop that runs deep into the medulla and have a well-developed vasa recta — these are the ones mainly responsible for concentrating urine. NEET reverses this in trick questions, so watch the direction.
⚠️ The NEET trap ✗ The loop of Henle concentrates the urine. ✓ The loop of Henle only BUILDS the medullary osmotic gradient (300 to 1200 mOsmol/L). The concentrated urine is finally formed in the COLLECTING DUCT under ADH. 🧠 Loop builds the gradient, duct makes the urine. If an option says the loop concentrates urine, it is testing this exact confusion.
Real NEET questions
NEET 2017-2023 (repeated)
Which of the following statements is correct?
A · The ascending limb of loop of Henle is impermeable to water ✓
B · The descending limb of loop of Henle is impermeable to water
C · The ascending limb of loop of Henle is permeable to water
D · The descending limb of loop of Henle is permeable to electrolytes
Solution: The ascending limb is impermeable to water but transports electrolytes out, so the filtrate gets diluted going up. The descending limb is the reverse — permeable to water but nearly impermeable to electrolytes. Only option A is correct. This exact statement has been asked in NEET 2017, 2018, 2019, 2020, 2021 and 2023, making it one of the most repeated facts in the chapter.
NEET 2019
Which of the following factors is responsible for the formation of concentrated urine?
A · Low levels of antidiuretic hormone
B · Maintaining hyperosmolarity towards inner medullary interstitium in the kidneys ✓
C · Secretion of erythropoietin by Juxtaglomerular complex
D · Hydrostatic pressure during glomerular filtration
Solution: Concentrated urine is produced by the counter current mechanism between the loop of Henle and vasa recta, which maintains rising osmolarity (300 to 1200 mOsmol/L) toward the inner medulla. This gradient pulls water out of the collecting duct. Low ADH gives dilute urine; erythropoietin relates to RBCs; hydrostatic pressure drives filtration, not concentration.
NEET 2018
Match Column I (Function) with Column II (Part): a. Ultrafiltration b. Concentration of urine c. Transport of urine d. Storage of urine | i. Henle's loop ii. Ureter iii. Urinary bladder iv. Malpighian corpuscle
A · a-v, b-iv, c-i, d-ii
B · a-iv, b-i, c-ii, d-iii ✓
C · a-iv, b-v, c-ii, d-iii
D · a-v, b-iv, c-i, d-iii
Solution: Ultrafiltration happens at the Malpighian (renal) corpuscle (iv). Concentration of urine involves Henle's loop (i). Transport of urine is by the ureter (ii) and storage is in the urinary bladder (iii). So a-iv, b-i, c-ii, d-iii — option B.
Solved Excretory Products and Their Elimination NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
To build the osmotic (salt) gradient in the medulla, rising from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. This gradient later lets the collecting duct reabsorb water and form concentrated urine.
Which limb is permeable to water?
The descending limb is permeable to water and nearly impermeable to electrolytes. The ascending limb is impermeable to water but moves electrolytes out.
Does the loop of Henle concentrate urine directly?
No. NCERT states the loop has minimal direct reabsorption. It sets up the gradient; the collecting duct does the final concentration under ADH.
What role does the vasa recta play with the loop?
The vasa recta is a U-shaped blood vessel running parallel to the loop. Its counter current flow removes reabsorbed water without washing away the medullary salt gradient, so the gradient stays intact.
Which nephron has a longer loop of Henle?
Juxtamedullary nephrons have a long loop that dips deep into the medulla and a well-developed vasa recta, making them mainly responsible for concentrating urine. Cortical nephrons have a short loop.
