Counter Current Mechanism in Kidneys — NEET Biology
✅ Asked in NEET 2019Which of the following factors is responsible for the formation of concentrated urine?
Which of the following factors is responsible for the formation of concentrated urine?
Answer & NCERT explanation
Correct answer: C — Maintaining hyperosmolarity towards the medullary interstitium
Concentrated urine formation requires maintaining hyperosmolarity towards the medullary interstitium through counter-current mechanism. This osmotic gradient allows maximum water reabsorption from collecting duct. High ADH levels (not low), proper JGA function, and the established gradient - not hydrostatic pressure - are responsible for concentration.
Read more NCERT concept on the PYQ
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 mOsmolL-1 in the cortex to about 1200 mOsmolL-1 in the inner medulla. 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. 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
The counter current mechanism is how the kidney maintains an osmolarity gradient (300 to 1200 mOsmol/L) from cortex to inner medulla using Henle's loop and vasa recta. Students often confuse which limb of Henle's loop transports NaCl (ascending limb does) and which limb of vasa recta reabsorbs it (ascending limb does). The key is remembering that NaCl moves out of the ascending limb into interstitium, while urea enters the thin ascending limb and is transported back by collecting duct. This gradient is essential for water reabsorption and concentration of urine—a fundamental kidney function tested through scenario-based questions in NEET.
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