Page 12 - LN- EXCRETION

NaCl is returned to the medullary interstitium by the ascending part of vasa recta. But,
contrarily, the water diffuses into the blood of ascending limb of vasa recta and is carried
away into the general.blood circulation.
Permeability to urea is found only in the deeper parts of thin ascending limb of Henle’s
loops and collecting ducts.
Urea diffuses out of the collecting ducts and enters into the thin ascending limb. A certain
amount of urea recycled in this way is trapped in medullary interstitium by the collecting
tubule. Thus, collecting tubule also play a minor role in the process (as shown in the figure
above).
Note:
The counter current mechanism helps in the maintenance of a concentration gradient in the
medullary interstitium.
Presence of such gradient helps in an easy passage of water from the collecting tubule
resulting in the formation of concentrated urine (filtrate), i.e., nearly four times
concentrated than the initial filtrate formed.