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==Function==

==Function==

====Thin descending limb====

====Thin descending limb====

[[Image:thickasclimloh.jpg|right|thumb|300px|<small><center>Exchange in the Thick Ascending Limb of the Loop of Henle</center></small>]]

[[Image:thickasclimloh.jpg|right|thumb|275px|<small><center>Exchange in the Thick Ascending Limb of the Loop of Henle</center></small>]]

* Freely permeable to water

* Freely permeable to water

* Therefore passive transfer of water occurs

* Therefore passive transfer of water occurs

==Countercurrent System==

==Countercurrent System==

[[Image:countercsumtri.jpg|right|thumb|300px|<small><center>Movement of ions and water in the counter current system</center></small>]]

[[Image:countercsumtri.jpg|right|thumb|275px|<small><center>Movement of ions and water in the counter current system</center></small>]]

[[Image:vasarectsumtri.jpg|right|thumb|300px|<small><center>Movement of ions and water in the Vasa Recta</center></small>]]

[[Image:vasarectsumtri.jpg|right|thumb|275px|<small><center>Movement of ions and water in the Vasa Recta</center></small>]]

As the thin descending limb makes its way into the medulla the surrounding tissue becomes ever increasingly hypertonic and therefore the osmotic gradient increases the deeper the limb goes.  As the limb is permeable to water it leaves the tubule via [[Osmosis - Physiology| osmosis]].  The tube then bends back on itself and heads back up the cortex.  The thin ascending limb is entered. Here salt enters the tubule passively due to the hypertonicity of the medulla creating a gradient.  This results in a very high salt concentration at the bottom of the loop.  The fluid moves on and enters the thick ascending limb.  This has salt transporters and so salt is pumped into the medulla via active transport causing more water to leave the thin descending limb.  The vasa recta has a similar countercurrent uptake system and only removes what is absorbed maintaining the medulla in a hypertonic state.

As the thin descending limb makes its way into the medulla the surrounding tissue becomes ever increasingly hypertonic and therefore the osmotic gradient increases the deeper the limb goes.  As the limb is permeable to water it leaves the tubule via [[Osmosis and Filtration - Anatomy & Physiology| osmosis]].  The tube then bends back on itself and heads back up the cortex.  The thin ascending limb is entered. Here salt enters the tubule passively due to the hypertonicity of the medulla creating a gradient.  This results in a very high salt concentration at the bottom of the loop.  The fluid moves on and enters the thick ascending limb.  This has salt transporters and so salt is pumped into the medulla via active transport causing more water to leave the thin descending limb.  The vasa recta has a similar countercurrent uptake system and only removes what is absorbed maintaining the medulla in a hypertonic state.

As there is transport of sodium and chloride from the thick ascending limb the concentration of the urine as it passes up this limb decreases back down to around the level it was when it entered the loop.  However the volume is greatly reduced.  The [[Reabsorption and Secretion Along the Distal Tubule and Collecting Duct - Anatomy & Physiology#Collecting Duct| collecting duct]] is where most concentration happens however it is only possible thanks to the incredibly high concentrations of NaCl at the bottom of the loop.  This returning of the concentration back to the same level as when it entered the loop is important for retaining salt and also allows the concentration to be finely controlled by the collecting ducts without the loss of salt.

Due to the transport of sodium and chloride from the thick ascending limb, the concentration of the urine as it passes up this limb decreases back down to around the level it was when it entered the loop.  However the volume is greatly reduced.  The [[Reabsorption and Secretion Along the Distal Tubule and Collecting Duct - Anatomy & Physiology#Collecting Duct| collecting duct]] is where most concentration happens however it is only possible thanks to the incredibly high concentrations of NaCl at the bottom of the loop.  This returning of the concentration back to the same level as when it entered the loop is important for retaining salt and also allows the concentration to be finely controlled by the collecting ducts without the loss of salt.

When there is excess water in the body the excess fluid passes through the loop of henle because the fluid entering the loop is less concentrated already the solutes only have so much osmotic potential and therefore are unable to draw the excess water from the lumen.  This contributes to allowing the kidneys to produce dilute urine.

When there is excess water in the body, the excess fluid passes through the loop of henle because the fluid entering the loop is less concentrated already.  The solutes only have so much osmotic potential and therefore are unable to draw the excess water from the lumen.  This contributes to allowing the kidneys to produce dilute urine.

==Vasa Recta Physiology==

==Vasa Recta Physiology==

Water salts and urea can pass freely across the capillary walls of the vasa recta and therefore as the vessels reach deeper into the medulla where the interstitum surrounding them becomes more hypertonic the blood inside them in turn becomes more hypertonic.  As the blood ascends back out of the medulla the osmolarity reduces until it is only slightly higher than when it entered.  The conclusion drawn from this is that the solutes which are reabsorbed from the fluid mainly remain in the surrounding tissue and maintain the concentration gradient.  If the system was not arranged in loops then this would not be possible.  Although the blood leaving the medulla is always slighty more concentrated than when it entered upon reaching the cortex, if the urine being produced is very concentrated, it actually becomes hypo osmolar as more water than salt is being reabsorbed in the distal tubules which reside in there.  Overall the osmolarity of the blood decreases when concentrated urine is being produced and increased when dilute urine is being produced.

Water, salts and urea can pass freely across the capillary walls of the vasa recta and therefore as the vessels reach deeper into the medulla where the interstitum surrounding them becomes more hypertonic, the blood inside them in turn becomes more hypertonic.  As the blood ascends back out of the medulla, the osmolarity reduces until it is only slightly higher than when it entered.  The conclusion drawn from this is that the solutes which are reabsorbed from the fluid mainly remain in the surrounding tissue and maintain the concentration gradient.  If the system was not arranged in loops then this would not be possible.  Although the blood leaving the medulla is always slighty more concentrated than when it entered upon reaching the cortex, if the urine being produced is very concentrated, it actually becomes hypo osmolar as more water than salt is being reabsorbed in the distal tubules which reside in there.  Overall the osmolarity of the blood decreases when concentrated urine is being produced and increased when dilute urine is being produced.

 

 

 

Use the [[Reabsorption and Secretion Along the Loop of Henle - Renal Flash Cards - Anatomy & Physiology|flash card revision resource]] for this section to test yourself.

The anatomy of the vasa recta can be found [[Microscopic Anatomy of the Nephron - Anatomy & Physiology#The Vasa Recta|here]]

[[Category:Urine Production]]

[[Category:Bullet Points]]