Urea is a nitrogenous water product produce in liver during
catabolism of proteins into amino acids.
Urea is filtered through the filtration membrane into the
Bowman's capsule.
The urea is reabsorbed at two regions in the nephron tubule
:
1) Proximal convoluted tubule : About 50% of urea
is passively reabsorbed here. 2) Collecting duct : It is the terminal region of
nephron that has high permeability to urea.
Urea reabsorption, increases its concentration in the inner
medullary region.
This creates an osmotic gradient.
More water follows this osmotic gradient and is reabsorbed from
loop of Henle (descending limb that is permeable to water).
Hence, urea reabsorption occurs to facilitate water
reabsorption and formation of concentrated urine.