counter current system-2
DESCRIPTION
Counter Current system-2. Before the vertical osmotic gradient is established, the medullary interstitial fluid concentration is uniformly 300 m Osm/L as in the remainder of the body fluids. Step- 1. - PowerPoint PPT PresentationTRANSCRIPT
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Counter Current system-2
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• Before the vertical osmotic gradient is
established, the medullary interstitial fluid
concentration is uniformly 300 m Osm/L as in
the remainder of the body fluids
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Step- 1
• The active salt pump in the thick ascending
limb is able to transport NaCl out of the lumen
until the surrounding interstitial fluid is 200
mOsm/l more concentrated than the tubular
fluid in this limb
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• When the thick ascending limb pump starts
actively extruding salt the medullary
interstitial fluid becomes hypertonic
• Passive diffusion of sodium chloride from the
thin ascending limb (impermeable to water)
also adds to the increase solute conc
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• Descending limb is highly permeable to water,
net diffusion of water by osmosis from
descending limb into the more concentrated
interstitial fluid
• Passive movement of water continues until the
osmolarities of the fluid in the descending
limb and interstitial fluid become equilibrated
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• Tubular fluid entering the loop of Henle
immediately starts to become more
concentrated as it loses water
• At equilibirum, the osmolarity of the
ascending limb fluid is 200 mOsm/L and the
osmolarities of the interstitial fluid and
descending limb fluid are equal at 400
mOsm/liter
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Step- 2
• 200 mOsm/L fluid exits from the top of the
ascending limb into the distal tubule
• New mass of isotonic fluid at 300 mOsm/L
enters the top of the descending limb from the
proximal tubule
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• At the bottom of the loop, mass of 400
mOsm/L fluid from the descending limb
moves forward around the tip into the
ascending limb
• The 200 mOsm/L concentration difference has
been lost at both the top and the bottom of the
loop
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Step - 3
• The ascending limb pumps again NaCl out
while water passively leaves the descending
limb until a 200 mOsm/liter difference is re-
established between the ascending limb and
both the interstitial fluid and descending limb
at each horizontal level
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• The concentration of tubular fluid is
progressively increasing in the descending
limb and progressively decreasing in the
ascending limb
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Step- 4
• As the tubular fluid advances still further, the
200 mOsm/L concentration gradient is
disrupted once again at all horizontal levels
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Step- 5
• Again active extrusion of NaCl from the
ascending limb coupled with the net diffusion
of water out of the descending limb re-
establishes the 200 mOsm/L gradient at each
horizontal level
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Step- 6
• Tubular fluid flows slightly forward again and
the stepwise process continues
• Fluid in the descending limb becomes
progressively more hypertonic until it reaches
a maximum concentration of 1,200 mOsm/L at
the bottom of the loop
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• Because the interstitial fluid always achieves
equilibrium with the descending limb vertical
concentration gradient ranging from 300 to
1,200 mOsm/L is established in the medullary
interstitial fluid
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• Concentration of the tubular fluid
progressively decreases in the ascending limb
as salt is pumped out
• Tubular fluid even becomes hypotonic as it
leaves the ascending limb to enter the distal
tubule at a concentration of 100 mOsm/L
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• Sodium ions are repeatedly recycled within the
medullary interstitium.
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• The loop of Henle multiplies the sodium
concentration within medulla by retaining the
new sodium ions coming from the glomerular
filtrate. It is called Counter Current Multiplier
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Role Of Distal Tubule
• Fluid leaves the loop of Henle and enters the
distal convoluted tubule in the renal cortex
• This fluid has an osmolarity of 100 mOsm/L.
• NaCl is transported out of the tubule but this
part is relatively impermeable to water.
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• End Of Todays Lecture!!