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!!