Page 110 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Abnormalities of Urinary Concentration
Depending on requirements, the kidney can Up to 20 L of hypotonic urine are excreted per
+
normally excrete hypotonic (< 100 mosm/L) or day. The excretion of Na and urea can also be
hypertonic (> 1200 mosmol/L) urine. Concen- increased.
tration and dilution of urine are in the first in- If reabsorption in the loop of Henle is inhib-
stance the result of processes in the thick as- ited, the hyperosmolality of the renal medulla
cending loop of Henle (pars ascendens) which dissipates. Therapeutic loop diuretics inhibit
+
+
transports NaCl (→ A, red arrow) to the inter- Na -K -2 Cl – cotransport. Hypercalcemia in-
stitial space of the renal medulla (see also hibits the reabsorption via a Ca 2+ receptor at
Kidney, Salt and Water Balance ascending part, while the interstitial space be- hibit the recirculation of K and thus indirectly
the tubule and by inhibiting paracellular reab-
p. 96) without water (blue arrow) being able
+
sorption. Hypokalemia or defective K chan-
to follow. The tubular fluid becomes hypotonic
+
nels (ROMK, an inward rectifier K channel) in-
(50–100 mosmol/L) by the time it passes the
+
+
–
+
Na -K -2 Cl cotransport (→ p. 97 B).
comes hypertonic. The hyperosmolar intersti-
tial space takes more water (blue arrow) than
Raised perfusion through the renal medulla
electrolytes (red arrow) from the descending
washes
medullary
out
hyperosmolality
part of the loop of Henle so that osmolality
(→ A3). Mediators (e.g., kinins, prostaglan-
rises in the descending tubular fluid on its
dins) released during inflammation therefore
The arrangement of the renal medullary
urinary concentration. Caffeine, too, acts as a
dilator of the vasa recta. Raised blood pressure
vessels (vasa recta) in the loop prevents dilu-
tion of the medullary hyperosmolality.
can also increase perfusion of the vasa recta
5 way to the apex of the loop. lower medullary osmolality and thus reduce
Urea (violet arrow) only partly follows the and thus wash out the medulla (pressure di-
reabsorbed water in the proximal and distal uresis).
tubules and the loop of Henle, so that the lumi- The reabsorption of water can also be re-
nal urea concentration increases up to the col- duced if tubular fluid contains poorly absorb-
lecting duct. The medullary collecting duct is able or nonabsorbable substances. These sub-
highly permeable to urea, which diffuses into stances are then concentrated by fluid reab-
the interstitial space. The high urea concentra- sorption and hold back water (→ A4). Osmotic
tions in the renal medulla draw water out of diuresis occurs. Secondarily, the impaired wa-
the descending part of the loop of Henle. ter reabsorption leads to reduced reabsorption
Some of the urea diffuses into the tubular lu- of NaCl and urea. As a result, osmolality in the
men and reaches the collecting duct via the renal medulla is reduced and urinary concen-
loop of Henle and the distal tubule. tration compromized. Osmotic diuresis is trig-
ADH stimulates the insertion of water chan- gered therapeutically with mannitol, a poorly
nels (aquaporins) into the apical cell mem- absorbed sugar. Furthermore, osmotic diuresis
brane in the distal tubule and collecting duct, also occurs when increased amounts of glu-
and thus allows water reabsorption following cose, bicarbonate, urea, and phosphate are ex-
the osmotic gradient. The tubular fluid in the creted.
distal tubule is at first hypotonic (see above), A protein-low diet impairs the concentrat-
but toward the end of the distal tubule it at- ing ability of the kidney because of reduced
tains the osmolality of the blood. More water contribution of urea to the concentrating
is taken from the collecting duct in the renal mechanism (→ A5).
medulla, until the osmolality of the luminal Impaired concentrating ability becomes ap-
fluid in the collecting duct approaches that in parent through nocturnal diuresis (nycturia),
the renal medulla. thirst, and large, unconcentrated volumes of
In ADH deficiency (central diabetes insipi- urine.
dus) or in insensitivity of the distal nephron
and the collecting duct for ADH (renal diabetes
100 insipidus) the water permeability of the distal
tubule and the collecting duct is low (→ A1).
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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