Biology · Excretory Products and Their Elimination · NEET
As you go deeper into the medulla, the interstitial fluid around the tubules holds more and more solute. NCERT gives the exact numbers: about 300 mOsmol/L in the cortex rising to about 1200 mOsmol/L in the inner medulla. This steady rise is why the filtrate passing through the collecting duct keeps losing water to the interstitium, ending up concentrated.
NaCl and urea. NCERT says clearly: 'This gradient is mainly caused by NaCl and urea.' NaCl is transported out by the ascending limb of the loop of Henle and handed to the descending limb of the vasa recta, then returned to the interstitium by the ascending limb of the vasa recta. A small amount of urea moves out of the collecting duct into the interstitium and back into the ascending limb of the loop. Both are retained (trapped) by this looped arrangement.
The cortex is the outer region and stays near normal blood osmolarity, about 300 mOsmol/L. The inner medulla is the deep region and reaches about 1200 mOsmol/L. So the medulla is about four times more concentrated than the cortex. This four-times figure matches how NCERT says the DCT and collecting duct concentrate the filtrate about four times.
By the counter current mechanism. The loop of Henle and the vasa recta both have two limbs where fluid flows in opposite directions (a 'U' shape). Because flow is opposite, solutes leaving one limb are picked up by the neighbouring limb instead of being carried away in the blood. This traps NaCl and urea in the medulla, so the gradient stays stable.
Not much. The strong gradient needs a long loop of Henle that runs deep into the medulla and a well-developed vasa recta. Juxtamedullary nephrons have long loops and good vasa recta, so they build the gradient. Cortical nephrons have short loops and a reduced or absent vasa recta, so they contribute little to the deep 1200 value.
Which of the following factors is responsible for the formation of concentrated urine?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It rises from about 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla. Remember these two numbers exactly for NEET.
Mainly NaCl and urea. NaCl comes from the ascending limb of the loop of Henle; urea is added from the collecting duct into the interstitium.
About four times, from 300 to 1200 mOsmol/L, done chiefly by the DCT and collecting duct with help from the medullary gradient.
The vasa recta runs parallel to the loop of Henle in a counter current pattern, so it removes water but leaves NaCl and urea behind, preventing the gradient from being washed away.
The juxtamedullary nephron, because it has a long loop of Henle deep in the medulla and a well-developed vasa recta.
