Osmolality of interstitial fluid (mOsm)
300
300
400
600
400
900
700
1200
100
100
Nephron loop
Active transport
Passive transport
Water impermeable
400
600
900
1200
300
300
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
(a)
(continued)
As water and solutes are reabsorbed, the loop first concentrates the filtrate, then dilutes it.
Cortex
Outer
medulla
Inner
medulla
(c) Collecting ducts use the gradient.
Under the control of antidiuretic hormone, the collecting
ducts determine the final concentration and volume of
urine. This process is fully described in Figure 25.17.
(b) Vasa recta preserve the gradient.
The entire length of the vasa recta is highly permeable to water
and solutes. Due to countercurrent exchanges between each
section of the vasa recta and its surrounding interstitial fluid, the
blood within the vasa recta remains nearly isosmotic to the
surrounding fluid. As a result, the vasa recta do not undo the
osmotic gradient as they remove reabsorbed water and solutes.
NaCI
NaCI
NaCI
NaCI
NaCI
NaCI
NaCI
NaCI
1200
300
300
400
400
600
600
900
900
325
Vasa recta
To vein
Blood from
efferent
arteriole
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
The countercurrent
flow of fluid moves
through two adjacent
parallel sections of
the vasa recta.
Osmolality of interstitial fluid (mOsm)
400
600
900
1200
300
Urine
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
H
2
O
Collecting duct
200
NaCI
NaCI
NaCI
NaCI
NaCI
1
Filtrate entering the
nephron loop is isosmotic to
both blood plasma and cortical
interstitial fluid.
2
Water moves out of the
filtrate in the descending limb
down its osmotic gradient.
This concentrates the filtrate.
3
Filtrate reaches its highest
concentration at the bend of the
loop.
4
Na
+
and Cl
-
are pumped out
of the filtrate. This increases the
interstitial fluid osmolality.
5
Filtrate is at its most dilute
as it leaves the nephron loop. At
100 mOsm, it is hypo-osmotic to
the interstitial fluid.
975
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