Biology · Excretory Products and Their Elimination · NEET
Because fluid flows in OPPOSITE directions in the two limbs that lie side by side. In Henle's loop, filtrate goes DOWN the descending limb and UP the ascending limb. In the vasa recta, blood also flows down one arm and up the other. Two flows moving against each other = counter current. This opposite flow traps salt in the medulla instead of washing it away.
The counter current keeps the medulla very salty (high osmolarity). When the filtrate in the collecting duct passes through this salty medulla, water moves OUT of the collecting duct into the interstitium by osmosis. Losing water leaves behind a small volume of concentrated urine. So the mechanism does not remove water directly from urine; it builds the gradient that pulls water out.
The vasa recta is a U-shaped blood vessel that runs parallel to Henle's loop. Its counter current flow lets it carry away the water without washing out the salt gradient. NaCl returned to the interstitium by the ascending limb of Henle's loop is exchanged with the descending vasa recta, and NaCl is returned to the interstitium by the ascending vasa recta. So the vasa recta PRESERVES the gradient rather than destroying it.
NaCl and urea. NaCl is transported out by the ascending limb of Henle's loop and exchanged with the descending limb of the vasa recta. Urea enters the thin ascending limb of Henle's loop and is transported back to the interstitium by the collecting duct. Together NaCl and urea raise osmolarity from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla.
It works strongly only in juxtamedullary nephrons, which have a long loop of Henle running deep into the medulla and a well-developed vasa recta. Cortical nephrons have a short loop and the vasa recta is absent or highly reduced, so they contribute little to concentrating urine. This is why animals that make very concentrated urine have more long-looped nephrons.
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 is the arrangement of Henle's loop and vasa recta, with fluids flowing in opposite directions, that maintains a high osmolarity gradient in the medulla to concentrate urine.
About 300 mOsmol/L in the cortex rising to about 1200 mOsmol/L in the inner medulla, mainly caused by NaCl and urea.
Human kidneys can produce urine that is nearly four times more concentrated than the initial filtrate.
Its counter current blood flow removes reabsorbed water without washing out the salt gradient, so the medulla stays hyperosmotic.
Juxtamedullary nephrons, because they have a long loop of Henle and a well-developed vasa recta reaching deep into the medulla.
