Figure 25.16
Juxtamedullary nephrons create an osmotic gradient within the
renal medulla that allows the kidney to produce urine of varying concentration.
The countercurrent multiplier depends on three properties
of the nephron loop to establish the osmotic gradient.
Fluid flows in the
opposite direction
(countercurrent)
through two
adjacent parallel
sections of a
nephron loop.
The
descending
limb
is permeable
to water, but not
to salt.
The
ascending limb
is impermeable to
water, and pumps
out salt.
(a) Long nephron loops of juxtamedullary nephrons create the gradient.
These properties establish a positive feedback cycle that
uses the flow of fluid to multiply the power of the salt pumps.
The osmolality of the medullary
interstitial fluid progressively
increases from the 300 mOsm of
normal body fluid to 1200 mOsm
at the deepest part of the medulla.
400
300
300
600
900
1200
(a)
The
long nephron loops
of
juxtamedullary nephrons create
the gradient. They act as
countercurrent multipliers.
(b)
The
vasa recta
preserve the
gradient. They act as
countercurrent exchangers.
(c)
The
collecting ducts
of
all nephrons use the gradient
to adjust urine osmolality.
The three key players and their
orientation in the osmotic gradient:
Osmolality of filtrate
in descending limb
Start
here
Salt is pumped out
of the ascending limb
Interstitial fluid
osmolality
Osmolality of filtrate
entering the ascending
limb
Water leaves the
descending limb
NaCI
H
2
O
NaCI
H
2
O
FOCUS
Medullary Osmotic Gradient
 
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