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==Introduction to Reabsorption==

==Introduction to Reabsorption==

* The proximal tubule is a major site for reabsorption and some secretion.

* The [[Nephron Microscopic Anatomy #Proximal Tubule|proximal tubule]] is a major site for reabsorption and some secretion.

* Gradients are small across the epithelium so tight regulation is not possible

* Gradients are small across the epithelium so tight regulation is not possible

* This occurs in the [[Distal Tubule - Anatomy & Physiology | distal tubule]]

* This occurs in the [[Reabsorption and Secretion Along the Distal Tubule and Collecting Duct - Anatomy & Physiology#Distal Tubule| distal tubule]]

* 65-80% of the filtrate is reabsorbed

* 65-80% of the filtrate is reabsorbed

* Most reabsorption is coupled to sodium ion movement

* Most reabsorption is coupled to sodium ion movement

* Small proteins and peptide hormones are reabsorbed by endocytosis

* Small proteins and peptide hormones are reabsorbed by endocytosis

* Other substances are secreted or reabsorbed in the tubules passively by '''[[Transport Proteins - Physiology#Facilitated Diffusion |facilitated]]''', none facilitated [[Diffusion - Physiology| '''diffusion''']] or [[Transport Proteins - Physiology#Diffusion Through Water Filled Protein Channels |'''ion channels''']] down chemical or electrical gradients.

* Other substances are secreted or reabsorbed in the tubules passively by '''[[Transport Proteins - Physiology#Facilitated Diffusion |facilitated]]''', non-facilitated [[Diffusion - Physiology| '''diffusion''']] or [[Transport Proteins - Physiology#Diffusion Through Water Filled Protein Channels |'''ion channels''']] down chemical or electrical gradients.

* Other substances are secreted or reabsorbed by [[Active Transport - Physiology| '''active transport''']] against such gradients

* Other substances are secreted or reabsorbed by [[Active Transport - Physiology| '''active transport''']] against such gradients

** Movement is via [[Transport Proteins - Physiology#ATPases |'''ATPases''']].

** Movement is via [[Transport Proteins - Physiology#ATPases |'''ATPases''']].

Transport capacity is well above what is needed for normal plasma concentrations to ensure that adequate absorption occurs and that there is little/no wastage.  As you are getting transport of sodium, chloride and water simultaneously the concentration does not increase along the proximal tubule. However the volume of filtrate does decrease

Transport capacity is well above what is needed for normal plasma concentrations to ensure that adequate absorption occurs and that there is little/no wastage.  As sodium, chloride and water are reabsorbed at the same rate, the filtrate concentrations remains the same along the proximal tubule. Only the volume of the filtrate decreases.

==Ions and Compounds==

==Ions and Compounds==

 

The following ions and compounds are reabsorbed or secreted partly or completely in the proximal tubule:

The following ions and compounds are reabsorbed or secreted partly or completely in the proximal tubule:

===Sodium===

===Sodium===

The majority (70%) of sodium is reabsorbed in the [[Proximal Tubule - Anatomy & Physiology| proximal tubule.]]  It is reabsorbed into the cytosol of the epithelial cells either alone by [[Diffusion - Physiology| Diffusion]] through [[Transport Proteins - Physiology#Diffusion Through Water Filled Protein Channels|ion channels]] followed by water and chlorine or together with another product using a [[Transport Proteins - Physiology#Co-Transporters|co-transporter]].

The majority (70%) of sodium is reabsorbed in the [[Reabsorption and Secretion Along the Proximal Tubule - Anatomy & Physiology| proximal tubule.]]  It is reabsorbed into the cytosol of the epithelial cells either alone by [[Diffusion - Physiology| diffusion]] through [[Transport Proteins - Physiology#Diffusion Through Water Filled Protein Channels|ion channels]] followed by water and chloride or together with another product such as glucose or AA using a [[Transport Proteins - Physiology#Co-Transporters|co-transporter]] by secondary active co-transport.

To maintain the concentration gradients and allow the diffusion to continue it is essential that sodium is not allowed to build up within the cell.  This is the job of the sodium/potassium [[Transport Proteins - Physiology#ATPases|ATPase Pump]] and is an example of [[Active Transport - Physiology#Primary Active Transport|primary active transport]].  This pump removes sodium from the cell and puts potassium in.  This creates a high concentration of potassium within the cell but this is corrected because their are also potassium ion channels in the basolateral membrane which allow potassium to diffuse back into the interstitium.  Because both sodium and potassium are leaving the cell the net effect is that the tubular cells are negatively charged.  This creates an electro gradient which further increases sodium uptake from the cells.  The combined electrochemical gradient is very large allowing for great amounts of sodium to be reabsorbed.

To maintain the concentration gradients and allow the diffusion to continue it is essential that sodium is not allowed to build up within the cell.  This is the job of the sodium/potassium [[Transport Proteins - Physiology#ATPases|ATPase Pump]] and is an example of [[Active Transport - Physiology#Primary Active Transport|primary active transport]].  This pump removes three sodium ions from the cell and pumps two potassium ions back in.  This creates a high concentration of potassium within the cell but this is corrected by potassium ion channels in the basolateral membrane which allow potassium to diffuse back into the interstitium.  Because both sodium and potassium are leaving the cell the net effect is that the tubular cells are negatively charged.  This creates an electro gradient which further increases sodium uptake from the cells.  The combined electrochemical gradient is very large allowing for great amounts of sodium to be reabsorbed.

Sodium is then able to move from the interstial fluid into the blood thanks to a combination of the blood having a low hydrostatic pressure and a high protein osmotic pressure.  These conditions exist thanks to the selective filtration of water, ions and glucose but the selective obstruction of proteins and promote the reabsorption of water and the associated dissolved ions within it back into the blood.

Sodium is then able to move from the interstial fluid into the blood due to the low hydrostatic pressure within the capillaries and a high protein osmotic pressure.  These conditions are caused by the [[Glomerular Apparatus and Filtration - Anatomy & Physiology#Factors Which Determine Selective Filtration|selective filtration]] of water, ions and glucose but the selective obstruction of proteins and promote the reabsorption of water and the associated dissolved ions within it back into the blood.

===[[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology#The Ability of the Kidney To Alter the Water Content of the Body| Water]]===

===[[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology#The Ability of the Kidney To Alter the Water Content of the Body| Water]]===

50% of filtered urea is reabsorbed in the proximal tubule.  However the concentration of urea actually increases thanks to the reabsorption of 70% of the filtered water in the same portion of the nephron.  Urea is not able to be reabsorbed from this point until it reaches the lower portion of the collecting duct therefore its concentration further increases with the reabsorption of water.

50% of filtered urea is reabsorbed in the proximal tubule.  However the concentration of urea actually increases thanks to the reabsorption of 70% of the filtered water in the same portion of the nephron.  Urea is not able to be reabsorbed from this point until it reaches the lower portion of the collecting duct therefore its concentration further increases with the reabsorption of water.

===Glucose===

===Glucose===

Glucose is a small molecule and so it is filtered in the same concentrations as are found in plasma which is approximately 5mmol/l.  Reabsorption of glucose can only occur in the proximal tubule and occurs regardless of the concentration gradient as it is completed via [[Active Transport - Physiology#Secondary Active Transport|secondary active transport]]. It is reabsorbed using a [[Transport Proteins - Physiology#Co-Transporters|co-transporter]] with [[Sodium Homeostasis - Physiology|sodium]].  The realisation of the [[Active Transport - Physiology#Development of Potential Energy|potential energy]] produced from sodium moving from an area of high concentration to an area of low concentration is enough energy to transport glucose across the membrane into the epithelial cells.  The energy technically comes from the utilisation of ATP by the [http://w01.rvcwiki.wf.ulcc.ac.uk/images/e/ef/NaKATPaseA%2BP.jpg sodium/potassium] [[Transport Proteins - Physiology#ATPases|ATPase]] which keeps sodium concentrations within the epithelial cells low this giving the sodium in the lumen a high potential energy.

Glucose is a small molecule and so it is filtered in the same concentrations as are found in plasma which is approximately 5mmol/l.  Reabsorption of glucose can only occur in the proximal tubule and occurs regardless of the concentration gradient as it is completed via [[Active Transport - Physiology#Secondary Active Transport|secondary active transport]]. It is reabsorbed using a [[Transport Proteins - Physiology#Co-Transporters|co-transporter]] with [[Essential Ion and Compound Balance and Homeostasis - Anatomy & Physiology#Sodium|sodium]].  The realisation of the [[Active Transport - Physiology#Development of Potential Energy|potential energy]] produced from sodium moving from an area of high concentration to an area of low concentration is enough energy to transport glucose across the membrane into the epithelial cells.  The energy technically comes from the utilisation of ATP by the [http://w01.rvcwiki.wf.ulcc.ac.uk/images/e/ef/NaKATPaseA%2BP.jpg sodium/potassium] [[Transport Proteins - Physiology#ATPases|ATPase]] which keeps sodium concentrations within the epithelial cells low this giving the sodium in the lumen a high potential energy.

Glucose is then passively transported out of the epithelial cells across the basolateral membrane.

Glucose is then passively transported out of the epithelial cells across the basolateral membrane.

The kidneys do not really regulate plasma glucose but their main aim is to preserve this valuable nutrient.  It is only at very high levels which the kidneys play a role in helping to prevent any further rises in glucose via excretion.

The kidneys do not really regulate plasma glucose but their main aim is to preserve this valuable nutrient.  It is only at very high levels which the kidneys play a role in helping to prevent any further rises in glucose via excretion.

===The Secretion of H⁺ and the Reabsorption of HCO₃^-===

===The Secretion of H⁺ and the Reabsorption of HCO₃^-===

** In the tubule it works in reverse forming CO₂ and H₂O from the intermediate H₂CO₃ which forms from HCO₃^- and H⁺

** In the tubule it works in reverse forming CO₂ and H₂O from the intermediate H₂CO₃ which forms from HCO₃^- and H⁺

** Allows continuous H⁺ secretion and HCO₃^- reabsorption

** Allows continuous H⁺ secretion and HCO₃^- reabsorption