First slide

Mechanism of Concentration of the Filtrate

Question

All of the following help in maintaining an increasing osmolarity towards the inner medullary interstitium, except

Moderate

A

Transport of urea from collecting duct into the medullary interstitium
B

Selective secretion of $H^{+}$ and $K^{+}$ ions in the collecting duct
C

Active reabsorption of NaCl from the ascending limb
D

The counter current flow of the filtrate in the two limbs of Henle’s loop

Solution

Selective secretion of $H^{+}$ and $K^{+}$ ions in the collecting duct helps in the maintenance of ionic and acid base balance of body fluids but not in maintaining the osmolarity of medullary interstitium. 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. 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.

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