Page 358 - Biology_F5

Regulation (Homeostasis)

(Figure 7.10). These features enable the loop osmotic gradient which draws water out of
of Henle to create a very high concentration the descending limb into the medulla. This is
gradient between the tissue fluid and blood because the ascending limb is impermeable
in the medulla of the kidney and the urine to water; therefore, water moves out of the
in the collecting ducts. The loop of Henle limb only to the descending limb. When
is connected at one end to the proximal water in the descending limb is pumped out,
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convoluted tubule and at the other end to it causes the fluid in the descending limb
the distal convoluted tubule. It first descends to have a slightly higher salt concentration
deep into the medulla and then bends and compared to the ascending limb. The process
ascends into the cortex again. Throughout continues down the length of the loop so
its length, it is surrounded by fine looped that this concentration effect is multiplied.
blood vessels called the Vasa recta. These The counter current multiplier means that
vessels carry blood from the glomerulus to the fluid in and around the loop of Henle
the renal vein. becomes saltier as it goes down the loop, and
it is saltiest at the bottom end of the loop. In
Salts like sodium and chloride ions, diffuse contrast, it becomes less salty as it goes up
passively out of the thin ascending limb and the ascending limb. Therefore, the final salt
pumped actively out of the thick ascending concentration depends on the length of the
limb into the surrounding tissue fluids. This loop, the longer the loop; the higher the final
pumping of salts out of the limb creates an salt concentration in the tissues becomes.

Descending limb Ascending limb C
O
300 300 100 300 R
T
H O E
2
H O X
2
NaCl
H O H O M
2
2
NaCl 400 Decrease E
600 600 D
H O NaCl H O solute U
2
potential
2
NaCl L
L
H O NaCl
2 A
Interstitial region Interstitial region Interstitial region
NaCl Cl -
H O 1000 Na + 1200
2
1,200 H O
2
1,200
Active transport
of NaCl
Passive transport
of NaCl
Movement of water
Figure 7.10: Counter-current multiplier system of the loop of Henle

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