Page 447 - Anatomy and Physiology of Farm Animals, 8th Edition

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(hypertonic) or more dilute than plasma factor in generation of hypertonic intersti-
tial fluid in the medulla (Fig. 23‐10), and
(hypotonic). The ability of the kidneys to
VetBooks.ir generate hypertonic or hypotonic urine this hypertonic fluid has an essential role
in the ability to generate hypertonic urine.
depends on the functional and anatomic
characteristics of both the loop of Henle The transport of particles without water
and the collecting duct. The excretion of from the lumen of the loops also creates
hypertonic urine also requires antidiuretic hypotonic fluid in the tubule, and this is an
hormone (ADH, or arginine vasopressin) essential step in the ability to generate
to alter the transport characteristics of the hypotonic urine. However, regardless of
collecting duct. the tonicity of the final urine, the transport
characteristics of the thick ascending limbs
of the loops of Henle remain the same, so
Sodium Chloride and Water that hypertonic fluid is generated within
Reabsorption by the Loop of Henle the renal medulla and hypotonic fluid is
generated within the loop of Henle.
Loops of Henle are the nephron segments The descending limbs of the loops of
found in the renal medulla. The U‐shaped Henle are relatively permeable to water but
loops extend to variable depths in the relatively impermeable to particles. As
medulla, and the terms descending and fluid flows into and through the descend-
ascending limbs are applied to the differ- ing limbs, water is removed because of the
ent parts of the loops (Fig. 23‐3). osmotic gradient between the tubular
The ascending limbs of the loops of lumen and the interstitial fluids of the renal
Henle are relatively impermeable to water medulla (Fig. 23‐10).
and have a thick portion that is the site of Because the ascending and descending
a great deal of sodium and chloride reab- limbs of the loops of Henle are relatively
sorption. Sodium and chloride transport close together in the medulla and because
by the thick ascending limb uses a unique the tubular flows move in opposite direc-
membrane transporter that cotransports tions, the combined effect of transport by
sodium, chloride, and potassium into the ascending and descending limbs produces
cell from the lumen. This transport is an osmotic gradient in the interstitial flu-
sodium‐linked in that the Na ‐K ‐ATPase ids of the renal medulla. Interstitial fluid
+
+
pump on the opposite side of the cell osmolality increases from the outer zones
maintains the low intracellular sodium to the inner zones of the renal medulla
concentration that permits the cotrans- (Fig. 23‐10).
porter to function. The net effect of this The countercurrent mechanism is any
cellular transport is continuous addition mechanism that depends on streams of
of sodium and chloride to the interstitial flow moving in opposite directions, and
fluid of the medulla without any accompa- these are usually close to each other. The
nying water (Fig. 23‐10). Sodium chloride ascending and descending loops of Henle
is also reabsorbed from the thin ascending form a countercurrent mechanism that
limb of the loop (Fig. 23‐10), but the amplifies the osmolyte (sodium chloride)
mechanism responsible for this transport transport properties of the ascending limb
is controversial. The fluid entering the of the loop of Henle. This countercurrent
loop of Henle is isotonic, and the fluid mechanism generates the osmotic gradient
exiting is hypotonic (Fig. 23‐10). This in the interstitial fluids of the renal medulla.
change in tubular fluid shows that the net The ascending and descending vasa recta
effect of loop of Henle transport is to add also form a countercurrent exchange mech-
more particles than water to the intersti- anism by permitting free exchange of sol-
tial fluids in the renal medulla. utes and water between the ascending and
Transport of sodium and chloride into descending blood vessels. This exchange
the interstitium without water is the key allows for blood flow into and out of the