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The formation of hypertonic urine in mammals during periods
of water deprivation is provided by the kidney's urine concentrating mechanism:
water is aborbed, in excess of solute, from the collecting ducts into the
vasculature of the renal medulla, thus increasing the osmolality of the urine.
In the outer medulla, this absorption of water is maintained by the active
absorption of NaCl from the water-impermeable thick ascending limbs.
This active transport favors the absorption of water from the water-permeable
descending limbs and collecting ducts, and results in a ``single effect'',
which is then multiplied by the countercurrent arrangement of renal tubules
and blood vessels in the outer medulla.
While the hypothesis of the countercurrent multiplication for the
outer medulla is believed to be well established, the urine concentrating
mechanism in the inner medulla remains controversial.
In the inner medulla, the thin ascending limbs appear to have
no significant active NaCl transport.
The most popular hypothesis for the inner medullary concentrating
mechanism is the passive mechanism, proposed
independently by Stephenson and Kokko and Rector in 1972.
According to the passive mechanism, urea is absorbed from the highly
urea-permeable inner medullary collecting ducts, increasing the urea
concentration and osmolality in the interstitial fluid.
This also results in an transmural gradient favoring the absorption of
water from the water-permeable collecting ducts and descending limbs, thus
increasing the osmolality of the collecting duct fluid (and eventually, urine)
and the NaCl concentration in the descending limbs.
Thus, the fluid entering the thin ascending limbs has a high NaCl
concentration relative to urea;
consequently, NaCl diffuses down the gradient from the ascending limbs
into the interstitium.
The passive mechanism requires low urea and NaCl permeabilities in the
descending limbs, so that the concentration difference in urea and NaCl
in the descending limbs is not dissipated before the fluid reaches the
ascending limbs.
However, experiments have reported moderately large urea and NaCl
permeabilities in the descending limbs.
When these values are used in model studies, no concentration gradient
is generated in the inner medulla.
Thus, the underlying concentrating mechanism in the inner medulla
remains to be elucidated.
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