Counter Current system-2

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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!!