Water reabsorption (The Kidney).

Around 125cm3 of fluid is filtered from the blood per minute and enters the nephron tubules. After selective reabsorption there is around 45cm3 left.

The role of the Loop Of Henle is to create a LOW, NEGATIVE water potential in the TISSUE of the medulla. This ensures even more water can be reabsorbed from the fluid in the collecting duct.

Loop Of Henle:

• Consists of a DESCENDING LIMB that descends into the medulla and an ASCENDING limb that ascends back out to the cortex. 
• Arrangement of the loop of Henle allows salts (SODIUM and CHLORIDE ions) to be transferred from the ASCENDING limb to the DESCENDING limb. 
• The overall effect is to INCREASE the concentration of SALTS in the tubule fluid and they DIFFUSE out from the thin-walled ASCENDING limb into the MEDULLA TISSUE, giving the tissue fluid in the medulla a very LOW/NEGATIVE water potential. 

How is this achieved? 

Water potential in fluid in tubule becomes lower, because:

• LOSS of WATER by OSMOSIS to the surrounding tissue fluid.
• DIFFUSION of SODIUM and CHLORIDE ions into the tubule from the surrounding tissue fluid.

As the fluid ascends back up towards the cortex, its water potential becomes HIGHER and less negative. This is because:

• At the base of the tubule, sodium and chloride ions DIFFUSE OUT of the tubule into the tissue fluid.
• Higher up the tubule, sodium and chloride ions are ACTIVELY TRANSPORTED OUT into the tissue fluid. 
• The wall of the ASCENDING limb is IMPERMEABLE to water, so water CANNOT LEAVE the tubule. 
• The fluid LOSES SALTS but NOT water as it moves up the ascending limb. 

Arrangement of the loop of Henle is known as the Hairpin COUNTERCURRENT MULTIPLIER system. 

• The overall effect of this arranement is to INCREASE the efficiency of SALT TRANSFER from the ascending limb to the descending limb. 
• This causes a BUILD-UP of SALT CONCENTRATION in surrounding TISSUE FLUID.

Purpose of loop of Henle is to REABSORB WATER from the FLITRATE. 

It does this by:

• Creating a gradient of HYPERTONCITY (INCREASING SALT concentration) within the INTERSTITIAL TISSUE of the MEDULLA. 
• This becomes increasingly concentrated, moving DOWN the medulla from the CORTEX end towards the PELVIS. 
• This gradient is achieved through the operation of a HAIRPIN COUNTERCURRENT MULTIPLIER MECHANISM. 

• The ascending limb of the loop of Henle is always 200 mOsm less. 
• In the ASCENDING limb, SODIUM and CHLORIDE ions are ACTIIVELY TRANSPORTED from the ascending limb into the INTERSTITIAL TISSUE of the MEDULLA. 
• The THICK portion of the ASCENDING limb is IMPERMEABLE to WATER. 
• The walls of the THIN part of the DESCENDING limb are relatively IMPERMEABLE to SODIUM and CHLORIDE ions, but are PERMEABLE to WATER. 
• Fluid tries to ACHIEVE a CONC GRADIENT as it moves around. 
• MAXIMUM concentration = 1200 mOsm due to the LENGTH of the LOOP.

Function of loop of Henle:

• To create a HIGH concentration of SODIUM and CHLORIDE ions in the TISSUE FLUID of the MEDULLA. 

Why does it need to do this?

• This allows WATER to be REABORBED from the contents of the NEPHRON as it passes through the COLLECTING DUCT. 

What are the survival advantages of reabsorbing water?

• Very CONCENTRATED URINE can be produced.
• CONSERVES WATER and PREVENTS DEHYDRATION as much as possible.