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, a 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 pump 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.

Therefore, 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.