7. Dr. Mohand Hassan Moalla 2017 7
300 mOsm/L
1200 mOsm/L
cortex
papilla
countercurrent system
urea recycling
corticopapillary osmotic gradient
8. Urine concentration
Urine concentration is operated by water
reabsorption
Water reabsorption requires ADH and osmotic
gradient “corticopapillary osmotic gradient”
The major factors that contribute to the
buildup this osmotic gradient into the renal
medulla are as follows:-
1. countercurrent system
2. urea recycling
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Dr. Mohand Hassan Moalla 2017
9. it is a gradient of osmolarity in the interstitial fluid of
the kidney from the cortex to the papilla
The osmolarity of the cortex is approximately 300
mOsm/L
The osmolarity of the papilla is approximately 1200
mOsm/L
corticopapillary osmotic gradient
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Dr. Mohand Hassan Moalla 2017
10. This occurs in:-
loops of Henle
vasa recta
parallel
counter [opposite direction]
close proximity
countercurrent system
inflow outflow
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Dr. Mohand Hassan Moalla 2017
11. The concentrating mechanism depends upon the
maintenance of a gradient of increasing osmolality
along the medullary pyramids
countercurrent system
concentrating
mechanism
countercurrent
exchangers
countercurrent
multipliers
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Dr. Mohand Hassan Moalla 2017
22. countercurrent multipliers
Single effect (First step)
1. NaCl is reabsorbed
in the thick
ascending limb
2. Water is
reabsorbed in the
descending limb
flow of tubular fluid
(second step)
1. inflow of tubular
fluid from the
proximal tubule
2. outflow of tubular
fluid into the distal
tubule.
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Dr. Mohand Hassan Moalla 2017
24. single effect
NaCl is reabsorbed via the Na+-K+-2Cl-
cotransporter out of the ascending limb
NaCl is deposited in the surrounding interstitial fluid
interstitial fluid osmolarity increases to 400 mOsm/L
the fluid in the ascending limb is diluted to 200
mOsm/L.
Fluid in the descending limb equilibrates with the
interstitial fluid, and its osmolarity also becomes 400
mOsm/L.
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Dr. Mohand Hassan Moalla 2017
26. New fluid with an osmolarity of 300 mOsm/L
enters the descending limb from the proximal
tubule
Equal volume of fluid is displaced from the
ascending limb
High osmolarity fluid in the descending limb
(400 mOsm/L) is "pushed down" toward the
bend [apex] of the loop of Henle.
flow of fluid
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Dr. Mohand Hassan Moalla 2017
27. These two basic steps are repeated until the
full corticopapillary gradient is established
each repeat of the two steps increases, or
multiplies, the gradient.
The size of the corticopapillary osmotic
gradient depends on the length of the loop of
Henle.
In humans, the osmolarity of interstitial fluid at
the bend of the loop of Henle is 1200 mOsm/L
countercurrent multipliers
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Dr. Mohand Hassan Moalla 2017
28. vasa recta operate as countercurrent exchanger
countercurrent exchange differs from
countercurrent multiplication as follows:-
countercurrent exchange
countercurrent
multiplication
countercurrent
exchange
active process passive process
establishes the osmotic
gradient
maintain the osmotic
gradient
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Dr. Mohand Hassan Moalla 2017
29. vasa recta are capillaries
vasa recta consisted of:-
1. The descending limb (where Blood entering )
2. The ascending limb (where Blood exit )
Purpose:-
Maintaining of osmotic gradient by keeping
solute (Na+, urea) in pyramid from losing by the
circulation
countercurrent exchange
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Dr. Mohand Hassan Moalla 2017
32. Urea contributes to the establishment of the osmotic
gradient in the medullary pyramids
ability to form a concentrated urine in the collecting
ducts.
Urea transport is mediated by urea transporters (UT-A)
Urea is produced as result of protein catabolism
high-protein diet increases the ability of the kidneys to
concentrate the urine.
Urea Recycling
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Dr. Mohand Hassan Moalla 2017
