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[[Image:disttubexch.jpg|right|thumb|250px|<small><center>Exchange in the Principal Cells of the Distal Tubule</center></small>]]

|linkpage =Reabsorption and Secretion Along the Nephron - Anatomy & Physiology

|linktext =REABSORPTION AND SECRETION ALONG THE NEPHRON

|maplink = Urinary System (Content Map) - Anatomy & Physiology

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==Distal Tubule==

==Distal Tubule==

[[Image:juxtaapp.jpg|right|thumb|250px|<small><center>Histology section showing the juxtaglomerular apparatus (© RVC 2008)</center></small>]]

[[Image:juxtaapp.jpg|right|thumb|250px|<small><center>Histology section showing the juxtaglomerular apparatus (© RVC 2008)</center></small>]]

* Important site of regulation of ions and water

* Important site of regulation of ions and water

** Potassium

** Potassium

** Acid / Base

** 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

*** Site of action of [[Important Hormonal Regulators of the Kidney - Anatomy & Physiology#Aldosterone|Aldosterone]]

*** Site of action of [[Aldosterone|Aldosterone]]

** Intercalated cells

** Intercalated cells

*** ATP driven proton secretion

*** ATP driven proton secretion

===Juxtaglomerular Apparatus===

===Juxtaglomerular Apparatus===

* 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:

** 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]].

** 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 [[Juxtaglomerular Cells of The Distal Tubule - Renal Physiology | Juxtaglomerular Cells]] are smooth muscle cells which adjoin the macula densa in the capillary wall.

** The 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

====Juxtaglomerular Cells====

====Juxtaglomerular Cells====

* Smooth muscle cells

* Smooth muscle cells

* Synthesise [[Useful definitions - Renal Anatomy & Physiology#R| renin]]

* Synthesise [[Kidney Endocrine Function - Anatomy & Physiology#Renin|renin]]

* From the [http://en.wikipedia.org/wiki/Zymogen zymogen] prerenin

* From the zymogen prerenin

* Described as intra-renal baroreceptors

* Described as intra-renal baroreceptors

** Respond to stretch

** Respond to stretch

* Also respond to the sympathetic nervous system

* Also respond to the sympathetic nervous system

** Increases renin output

** Increases renin output

===Developmental===

===Developmental===

Develops from metanephric tubule

Develops from metanephric tubule

===Principal Cells===

===Principal Cells===

* [[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  

* [[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 [[Pituitary Gland - Anatomy & Physiology #Posterior Pituitary Gland | posterior pituitary gland]]

* It is released from the [[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⁺

** Alpha intercalated cells secrete H⁺

===Developmental===

===Developmental===

* Develops from branched ureteric bud

* Develops from branched ureteric bud

===The Concentrating Mechanism, Aquaporins and ADH===

===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 [[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.

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==

==Ions and Compounds==

The following ions and compounds are reabsorbed or secreted partly or completely in the distal tubules and collecting ducts:

The following ions and compounds are reabsorbed or secreted partly or completely in the distal tubules and collecting ducts:

===Sodium===

===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.

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:'''

'''See:'''

* [[Important Hormonal Regulators of the Kidney - Anatomy & Physiology#Aldosterone|Aldosterone]]

* [[Aldosterone|Aldosterone]]

* [[Important Hormonal Regulators of the Kidney - Anatomy & Physiology#Atrial Natriuretic Peptide|Atrial Natriuretic Peptide]]

* [[Atrial Natriuretic Peptide|Atrial Natriuretic Peptide]]

===Potassium===

===Potassium===

* This is an active secretion using a H⁺ ATPase

* This is an active secretion using a H⁺ ATPase

* Also a H⁺/K⁺ ATPase antiporter

* Also a H⁺/K⁺ ATPase antiporter

* The main function of this is maintaining [[Potassium Homeostasis - Physiology| potassium]]

* The main function of this is maintaining potassium

* HCO₃^- is reabsorbed here

* HCO₃^- is reabsorbed here

===Urea===

===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.

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=====

=====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.

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

 

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.

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 Homeostasis - Anatomy & Physiology #Parathyroid Hormone (PTH)| Parathyroid hormone]] stimulates reabsorption.  It is normal for a 1-2% excretion of calcium to occur

[[Category:Urine Production]]

[[Category:Bullet Points]]