Page 179 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
vides an additional driving force for water re- Final adjustment of the excreted urine volume
absorption. The more water filtered at the glo- occurs in the collecting duct. In the presence of
merulus, the higher this oncotic pressure. antidiuretic hormone, ADH (which binds to ba-
Thus, the reabsorption of water at the proxi- solateral V 2 receptors, named after vasopres-
mal tubule is, to a certain extent, adjusted in sin, the synonym for ADH), aquaporins (AQP2)
accordance with the GFR (glomerulotubular in the (otherwise water-impermeable) lumi-
balance). nal membrane of principal cells are used to ex-
Because the descending limb of the loop of tract enough water from the urine passing
Henle has aquaporins (AQP1) that make it per- through the increasingly hypertonic renal
Kidneys, Salt, and Water Balance ingly hypertonic as it approaches the papillae U osm/P osm can drop to ! 0.3. The U osm can even
medulla. Thereby, the U osm rises about four
meable to water, the urine in it is largely in
times higher than the P osm (U osm/P osm ! 4), cor-
osmotic balance with the hypertonic inter-
stitium, the content of which becomes increas-
responding to maximum antidiuresis. The ab-
sence of ADH results in water diuresis, where
(! A5). The urine therefore becomes increas-
fall below the osmolality at the end of TAL,
ingly concentrated as it flows in this direction.
since reabsorption of Na and Cl is continued
+
–
In the thin descending limb, which is only
in the distal convoluted tubule and collecting
sparingly permeable to salt, this increases the
–
+
duct (! p. 162) but water can hardly follow.
conc. of Na and Cl . Most water drawn into the
interstitium is carried off by the vasa recta
Urea also plays an important role in the for-
of the loop of Henle are largely impermeable to
diet leads to increased urea production, thus
water, Na and Cl passively diffuses (thin
increased the urine-concentrating capacity of
+
–
7 (! B). Since the thin and thick ascending limbs mation of concentrated urine. A protein-rich
limb) and is actively transported (thick limb)
the kidney. About 50% of the filtered urea
out into the interstitium (! B). Since water leaves the proximal tubule by diffusion (! C).
cannot escape, the urine leaving the loop of Since the ascending limb of the loop of Henle,
Henle is hypotonic. the distal convoluted tubule, and the cortical
–
+
Active reabsorption of Na and Cl from the and outer medullary sections of the collecting
thick ascending limb of the loop of Henle duct are only sparingly permeable to urea, its
(TAL; ! p. 162) creates a local gradient (ca. conc. increases downstream in these parts of
200 mOsm/kg H 2O; ! A5) at all points be- the nephron (! C). ADH can (via V 2 receptors)
tween the TAL on the one side and the de- introduce urea carriers (urea transporter type
scending limb and the medullary interstitium 1, UT1) in the luminal membrane, thereby
on the other. Since the high osmolality of fluid making the inner medullary collecting duct
in the medullary interstice is the reason why permeable to urea. Urea now diffuses back into
water is extracted from the collecting duct (see the interstitium (where urea is responsible for
below), active NaCl transport is the ATP-con- half of the high osmolality there) via UT1 and is
suming “motor” for the kidney’s urine-concen- then transported by UT2 carriers back into the
trating mechanism and is up-regulated by sus- descending limb of the loop of Henle, compris-
tained stimulation of ADH secretion. ing the recirculation of urea (! C). The non-re-
absorbed fraction of urea is excreted: FE urea !
Along the course of the distal convoluted tubule
and, at the latest, at the connecting tubule, which 40%. Urea excretion increases in water diuresis
and decreases in antidiuresis, presumably due
contains aquaporins and ADH receptors of type V 2
(explained below), the fluid in the tubule will again to up-regulation of the UT2 carrier.
become isotonic (in osmotic balance with the
isotonic interstice of the renal cortex) if ADH is pres- Urine concentration disorders primarily occur due
to (a) excessive medullary blood flow (washing out
ent (! p. 168), i.e., when antidiuresis occurs. Al- Na , Cl and urea); (b) osmotic diuresis; (c) loop di-
+
–
though Na + and Cl – are still reabsorbed here
(! p. 162), the osmolality does not change signifi- uretics (! p. 172); (d) deficient secretion or effec-
cantly because H 2O is reabsorbed (ca. 5% of the GFR) tiveness of ADH, as seen in central or peripheral dia-
into the interstitial space due to osmotic forces and betes insipidus, respectively.
urea increasingly determines the osmolality of the
166
tubular fluid.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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