Difference between revisions of "Reabsorption and Secretion Along the Distal Tubule and Collecting Duct - Anatomy & Physiology"

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[[Image:disttubexch.jpg|right|thumb|250px|<small><center>Exchange in the Principal Cells of the Distal Tubule</center></small>]]
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|linkpage =Reabsorption and Secretion Along the Nephron - Anatomy & Physiology
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|linktext =REABSORPTION AND SECRETION ALONG THE NEPHRON
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|maplink = Urinary System (Content Map) - Anatomy & Physiology
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==Distal Tubule==
 
==Distal Tubule==
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[[Image:juxtaapp.jpg|right|thumb|250px|<small><center>Histology section showing the juxtaglomerular apparatus (© RVC 2008)</center></small>]]
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[[Image:disttubexch.jpg|right|thumb|300px|<small><center>Exchange in the Principal Cells of the Distal Tubule</center></small>]]
 
* Important site of regulation of ions and water
 
* Important site of regulation of ions and water
 
* Less emphasis on bulk transport compared with proximal tubule
 
* Less emphasis on bulk transport compared with proximal tubule
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* It is able to do this as it has high resistance epithelia.  Allowing it to maintain substantial gradients across it
 
* It is able to do this as it has high resistance epithelia.  Allowing it to maintain substantial gradients across it
 
* Very important for the homeostasis of:
 
* Very important for the homeostasis of:
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** Sodium
+
** [[Sodium Homeostasis - Physiology#Distal Tubule and Collecting Ducts| Sodium]]
−
** Potassium
+
** [[Potassium Homeostasis - Physiology#Distal Tubule| Potassium]]
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** Acid / Base
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** [[Acid Base Balance By The Kidney - Anatomy & Physiology#Secretion of H+ in the Distal Tubule and Collecting Ducts| Acid Base]]
 +
 
 +
 
 
* There are two cell types present each with different functions.  They are similar to the cells of the collecting ducts
 
* There are two cell types present each with different functions.  They are similar to the cells of the collecting ducts
 
** Principal cells
 
** Principal cells
 
*** Absorb sodium
 
*** Absorb sodium
 
*** Excrete potassium and hydrogen
 
*** Excrete potassium and hydrogen
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*** Site of action of [[Aldosterone|Aldosterone]]
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*** Site of action of [[Aldosterone]]
 
** Intercalated cells
 
** Intercalated cells
 
*** ATP driven proton secretion
 
*** ATP driven proton secretion
  
 
===Juxtaglomerular Apparatus===
 
===Juxtaglomerular Apparatus===
 +
[[Image:juxtaapp.jpg|right|thumb|200px|<small><center>Histology section showing the juxtaglomerular apparatus (© RVC 2008)</center></small>]]
 
* The terminal portion of the straight distal tubule contacts the afferent and efferent vessels supplying its own glomerulus
 
* The terminal portion of the straight distal tubule contacts the afferent and efferent vessels supplying its own glomerulus
 
* These vessels are said to embrace the distal tubule
 
* These vessels are said to embrace the distal tubule
 
* Here a special apparatus called the Juxtaglomerular Apparatus has 3 different structures:
 
* Here a special apparatus called the Juxtaglomerular Apparatus has 3 different structures:
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** The tubular epithelial cells of the distal tubule which are in contact with the arterioles supplying the glomerulus of that nephron are called the '''macula densa'''.  They play a vital role in the [[Glomerular Filtration Rate#Autoregulation|regulation of the GFR]].
+
** The tubular epithelial cells of the distal tubule which are in contact with the arterioles supplying the glomerulus of that nephron are called the '''macula densa'''.  They play a vital role in the [[Autoregulation of GFR - Anatomy and Physiology#Tubuloglomerular Feedback (TGF)|regulation of the GFR]].
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** The Juxtaglomerular Cells are smooth muscle cells which adjoin the macula densa in the capillary wall.
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** The [[Juxtaglomerular Cells of The Distal Tubule - Renal Physiology | Juxtaglomerular Cells]] are smooth muscle cells which adjoin the macula densa in the capillary wall.
 
** The Extraglomerular Mesangium has an unclear function
 
** The Extraglomerular Mesangium has an unclear function
  
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====Juxtaglomerular Cells====
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===Developmental===
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* Smooth muscle cells
 
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* Synthesise [[Kidney Endocrine Function - Anatomy & Physiology#Renin|renin]]
 
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* From the zymogen prerenin
 
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* Described as intra-renal baroreceptors
 
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** Respond to stretch
 
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* The secretion of renin from these cells depends on the stretch
 
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** '''If stretch decreases - blood pressure has decreased - renin secretion increases'''
 
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* Also respond to the sympathetic nervous system
 
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** Increases renin output
 
  
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===Developmental===
 
 
Develops from metanephric tubule
 
Develops from metanephric tubule
  
 
==Collecting Duct==
 
==Collecting Duct==
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[[Image:collductloh.jpg|right|thumb|250px|<small><center>Histology section of the collecting duct showing the close proximity of the loop of henle (© RVC 2008)</center></small>]]
 
[[Image:collductexch.jpg|right|thumb|250px|<small><center>Exchange in the Principal Cells of the Collecting Duct</center></small>]]
 
[[Image:collductexch.jpg|right|thumb|250px|<small><center>Exchange in the Principal Cells of the Collecting Duct</center></small>]]
 
This part of the nephron has two cell types
 
This part of the nephron has two cell types
−
[[Image:collductloh.jpg|right|thumb|250px|<small><center>Histology section of the collecting duct showing the close proximity of the loop of henle (© RVC 2008)</center></small>]]
 
  
 
===Principal Cells===
 
===Principal Cells===
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* [[Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland | ADH]] acts on these cells inserting [[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology#What are Aquaporins|aquaporins]] into the cell membranes  
+
 
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* It is released from the [[Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland | posterior pituitary gland]]
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* [[Endocrine System - Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland | ADH]] acts on these cells inserting [[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology#What are Aquaporins|aquaporins]] into the cell membranes  
 +
* It is released from the [[Endocrine System - Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland | posterior pituitary gland]]
  
 
===Intercalated cells===
 
===Intercalated cells===
 +
 
* The intercalated cells can be subdivided further to:
 
* The intercalated cells can be subdivided further to:
 
** Alpha intercalated cells secrete H<sup>+</sup>
 
** Alpha intercalated cells secrete H<sup>+</sup>
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===Developmental===
 
===Developmental===
 +
 
* Develops from branched ureteric bud
 
* Develops from branched ureteric bud
  
 
===The Concentrating Mechanism, Aquaporins and ADH===
 
===The Concentrating Mechanism, Aquaporins and ADH===
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Water is drawn from the lumen of the tubule by the increasing hypertonicity of the surrounding tissue as the duct makes its way deeper into the medulla.  However this reabsorption is only possible thanks to [[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology|ADH]] inserting [[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology#What are Aquaporins|aquaporins]] into the apical membrane.  These channels are always present on the basolateral membrane of the epithelial cells but not on the apical membrane.  The reabsorption would not be possible if the urine did not go back up the [[Reabsorption and Secretion Along the Loop of Henle - Anatomy & Physiology#Thick ascending limb| thick ascending limb]] of the loop of henle, where its concentration was decreased by the reabsorption of salt, but instead went straight into the collecting ducts.  Although this would mean very concentrated urine it would result in massive salt losses.  Thus the collecting duct allows for very concentrated urine with minimal salt loss. It also allows the concentration of the urine to vary from dilute to concentrated under the control of the hypothalamus and ADH concentrations.
 
−
 
−
==Ions and Compounds==
 
−
The following ions and compounds are reabsorbed or secreted partly or completely in the distal tubules and collecting ducts:
 
−
 
−
===Sodium===
 
−
The remaining 10% is reabsorbed in the distal tubule.  Although the transport capacity here is minimal the high resistance epithelia mean that very little back flow into the lumen occurs allowing the remaining sodium to be easily reabsorbed.  Some of this reabsorption takes place using a sodium chloride co-transporter. The reabsorption of sodium from here and the collecting ducts is under hormonal control allowing "fine tuning" to occur.
 
−
 
−
'''See:'''
 
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* [[Aldosterone|Aldosterone]]
 
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* [[Atrial Natriuretic Peptide|Atrial Natriuretic Peptide]]
 
−
 
−
===Potassium===
 
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[[Image:potcollductexch.jpg|right|thumb|250px|<small><center>Potassium Transport in the Principal Cells of the Collecting Duct</center></small>]]
 
−
 
−
=====Distal Tubule=====
 
−
* Here the potassium and chloride symporter actually excretes the two ions into the lumen of the tubule
 
−
* Na<sup>+</sup> / K<sup>+</sup> ATPases in the basolateral membrane move potassium into epithelial cells from the intercellular spaces in order to remove sodium.  The potassium then diffuses into the lumen via channels
 
−
* The ability to secrete potassium here is vital in animals with high potassium diets such as herbivores where excreting only the amount filtered would not be suffice
 
−
 
−
=====Principal Cells of Collecting Duct=====
 
−
* Potassium transported into lumen
 
−
* Is both absorbed and secreted via paracellular route
 
−
* Na<sup>+</sup> / K<sup>+</sup> ATPases in the basolateral membrane move potassium into epithelial cells from the intercellular spaces in order to remove sodium and keep potassium concentrations high.
 
−
** Aldosterone works here
 
−
 
−
=====Intercalated Cells of Collecting Duct=====
 
−
* Potassium is absorbed here in exchange for hydrogen
 
−
 
−
===H<sup>+</sup> and HCO<sub>3</sub><sup>-</sup>===
 
−
* H<sup>+</sup> ions are secreted in the cortical and medullary collecting tubules
 
−
* This is an active secretion using a H<sup>+</sup> ATPase
 
−
* Also a H<sup>+</sup>/K<sup>+</sup> ATPase antiporter
 
−
* The main function of this is maintaining potassium
 
−
* HCO<sub>3</sub><sup>-</sup> is reabsorbed here
 
−
 
−
===Urea===
 
−
In the lower portion of the collecting duct urea freely diffuses across the epithelia.  The permeability is also increased with ADH.  This adds to the hypertonicity of the medulla and increases water reabsorption.
 
−
 
−
=====Urea and Water Reabsorption=====
 
−
Water follows urea as it is reabsorbed from the collecting duct.  This actually increases the concentration of urea in the tubular fluid despite some of it being reabsorbed.  This means that urea concentration only drops moderately when the urine concentration falls due to increased reabsorption of water.
 
−
 
−
===[[Calcium|Calcium]]===
 
−
The regulation of how much calcium is reabsorbed occurs in the ascending limb of the loop of henle, the distal tubule and collecting ducts.  [[Calcium #Parathyroid Hormone (PTH)| Parathyroid hormone]] stimulates reabsorption.  It is normal for a 1-2% excretion of calcium to occur
 
−
 
−
==Revision==
 
−
Use the [[Reabsorption and Secretion Along the Distal Tubule and Collecting Duct - Renal Flash Cards - Anatomy & Physiology|flash card revision resource]] for this section to test yourself.
 
  
−
{{OpenPages}}
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Water is drawn from the lumen of the tubule by the increasing hypertonicity of the surrounding tissue as the duct makes its way deeper into the medulla.  However this reabsorption is only possible thanks to [[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology|ADH]] inserting [[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology#What are Aquaporins|aquaporins]] into the apical membrane.  These channels are always present on the basolateral membrane of the epithelial cells but not on the apical membrane.  The reabsorption would not be possible if the urine did not go back up the [[Loop Of Henle - Anatomy & Physiology #Thick ascending limb| thick ascending limb]] of the loop of henle, where its concentration was decreased by the reabsorption of salt, but instead went straight into the collecting ducts.  Although this would mean very concentrated urine it would result in massive salt losses.  Thus the collecting duct allows for very concentrated urine with minimal salt loss. It also allows the concentration of the urine to vary from dilute to concentrated under the control of the hypothalamus and ADH concentrations.
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[[Category:Urine Production]]
 
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[[Category:Bullet Points]]
 

Revision as of 15:10, 3 September 2008

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()Map REABSORPTION AND SECRETION ALONG THE NEPHRON (Map)



Distal Tubule

Exchange in the Principal Cells of the Distal Tubule
  • Important site of regulation of ions and water
  • Less emphasis on bulk transport compared with proximal tubule
  • More emphasis on fine management
  • It is able to do this as it has high resistance epithelia. Allowing it to maintain substantial gradients across it
  • Very important for the homeostasis of:


  • There are two cell types present each with different functions. They are similar to the cells of the collecting ducts
    • Principal cells
      • Absorb sodium
      • Excrete potassium and hydrogen
      • Site of action of Aldosterone
    • Intercalated cells
      • ATP driven proton secretion

Juxtaglomerular Apparatus

Histology section showing the juxtaglomerular apparatus (© RVC 2008)
  • The terminal portion of the straight distal tubule contacts the afferent and efferent vessels supplying its own glomerulus
  • These vessels are said to embrace the distal tubule
  • Here a special apparatus called the Juxtaglomerular Apparatus has 3 different structures:
    • The tubular epithelial cells of the distal tubule which are in contact with the arterioles supplying the glomerulus of that nephron are called the macula densa. They play a vital role in the regulation of the GFR.
    • The Juxtaglomerular Cells are smooth muscle cells which adjoin the macula densa in the capillary wall.
    • The Extraglomerular Mesangium has an unclear function

Developmental

Develops from metanephric tubule

Collecting Duct

Histology section of the collecting duct showing the close proximity of the loop of henle (© RVC 2008)
Exchange in the Principal Cells of the Collecting Duct

This part of the nephron has two cell types

Principal Cells

Intercalated cells

  • The intercalated cells can be subdivided further to:
    • Alpha intercalated cells secrete H+
    • Beta intercalated cells secrete HCO3-

Developmental

  • Develops from branched ureteric bud

The Concentrating Mechanism, Aquaporins and ADH

Water is drawn from the lumen of the tubule by the increasing hypertonicity of the surrounding tissue as the duct makes its way deeper into the medulla. However this reabsorption is only possible thanks to ADH inserting aquaporins into the apical membrane. These channels are always present on the basolateral membrane of the epithelial cells but not on the apical membrane. The reabsorption would not be possible if the urine did not go back up the thick ascending limb of the loop of henle, where its concentration was decreased by the reabsorption of salt, but instead went straight into the collecting ducts. Although this would mean very concentrated urine it would result in massive salt losses. Thus the collecting duct allows for very concentrated urine with minimal salt loss. It also allows the concentration of the urine to vary from dilute to concentrated under the control of the hypothalamus and ADH concentrations.

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