Counter Current system-2
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Transcript of Counter Current system-2
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
• 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
• 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
• 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
• 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
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
• 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
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
• The concentration of tubular fluid is
progressively increasing in the descending
limb and progressively decreasing in the
ascending limb
Step- 4
• As the tubular fluid advances still further, the
200 mOsm/L concentration gradient is
disrupted once again at all horizontal levels
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
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
• 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
• 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
• Sodium ions are repeatedly recycled within the
medullary interstitium.
• 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
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.
• End Of Todays Lecture!!