In addition, further water is reabsorbed in the process of urine
concentration which occurs in the distal nephron.
Dilution of urine is achieved through the removal of NaCl from the
tubular lumen fluid in the segment which is impermeable to H 2 O (thick
part of the ascending loop of Henle, DCT), or from the segment which
becomes impermeable to H 2 O as an effect of ADH (collecting tubule and
duct). The most important of them is the loop of Henle which secretes
more H 2 O and less Nacl in urine making it hypotonic (diluted).
Urine concentration results from the reabsorption of water in
excess of nitrogenous wastes and other solutes. Therefore, in urine the
concentration of urea is about 60 times that in plasma. In states of maximal
urine concentration, urine osmolarity is about 1200 mosmol/liter. Further
increase in urine osmolarity to 1400 mosmol/liter can be achieved with
persistence of the stimulus for urine concentration. Urine concentration,
through excess reabsorption of free water occurs mainly in collecting
tubules.
The mechanism of urine concentration depends on passage of
collecting tubules through the hypertonic renal medulla. The tonicity of
renal medulla is maximum at the tip of renal papillae and decreases
gradually towards the direction of the corticomedullary junction. ADH
when secreted will increase the collecting tubule permeability to water
which gets out to the interstitium leaving tubular contents hypertonic. The
interstitial water is picked up by the vasa recta and renal venules and will
be drained away.