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==Introduction to Reabsorption==

==Introduction to Reabsorption==

* The [[Microscopic Anatomy of the Nephron - Anatomy & Physiology#Proximal Tubule|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

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==

===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 there 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 [[The Formation of the Filtrate by the Glomerular Apparatus- 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.

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]]===

===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.

===Protein===

===Protein===

Peptide hormones and small amounts of albumin make it through the [[Glomerulus and Bowmans Capsule - Anatomy & Physiology#Function of Renal Corpuscle|glomerular filtration barrier]] and these need to be reabsorbed.  The reabsorption occurs via '''endocytosis''' in the proximal tubules.  They are then broken down to amino acids in the epithelial cell cytoplasm and move via facilitated diffusion into the interstial fluid.  The reabsorption of protein is usually complete though it is normal to detect small quantities of protein in the urine of some mammals e.g. the dog

Peptide hormones and small amounts of albumin make it through the [[Glomerular Apparatus and Filtration - Anatomy & Physiology#Function of Renal Corpuscle|glomerular filtration barrier]] and these need to be reabsorbed.  The reabsorption occurs via '''endocytosis''' in the proximal tubules.  They are then broken down to amino acids in the epithelial cell cytoplasm and move via facilitated diffusion into the interstial fluid.  The reabsorption of protein is usually complete though it is normal to detect small quantities of protein in the urine of some mammals e.g. the dog

===Primary Active Secretion - Organic Acids and Bases===

===Primary Active Secretion - Organic Acids and Bases===

The secretion of these compounds occurs '''only in the proximal tubules'''.  These molecules are mainly bound to plasma proteins with a small amount free in an active ionised form.  It is only the free ions which are able to be transported.  As the ionised molecules are transported out of the blood more molecules are released from the plasma proteins to take their place.  These can then be secreted etc etc.  This allows a large amount of the substance to be secreted at one time.  The mechanisms are not very selective and so many different substances are secreted at the same time.  Secretion mechanisms are responsible for the secretion of drugs, hormones and things like food additives.  Many unwanted or toxic organic molecules which enter the body are unionized.  They therefore cannot be secreted so it falls to the liver to alter them into ionized forms to allow them to be disposed of.

The secretion of these compounds occurs '''only in the proximal tubules'''.  These molecules are mainly bound to plasma proteins with a small amount free in an active ionised form.  It is only the free ions which are able to be transported.  As the ionised molecules are transported out of the blood more molecules are ionised from the plasma proteins to take their place.  These new ionised molecules are then able to be excreted thus releasing more and so it goes on.  This allows a large amount of the substance to be secreted at one time.  The mechanisms are not very selective and so many different substances are secreted at the same time.  Secretion mechanisms are responsible for the secretion of drugs, hormones and things like food additives.  Many unwanted or toxic organic molecules which enter the body are unionized.  They therefore cannot be secreted so it falls to the liver to alter them into ionized forms to allow them to be disposed of.

===[[Calcium Homeostasis - Anatomy & Physiology|Calcium]]===

===[[Calcium|Calcium]]===

Half the plasma calcium is bound to proteins so it is only the ionised form which is available for filtration.  Reabsorption of calcium occurs in the proximal tubule paracellulary however the regulation of how much is reabsorbed occurs in the ascending limb of the loop of henle, the distal tubule and collecting ducts.

Half the plasma calcium is bound to proteins so it is only the ionised form which is available for filtration.  Reabsorption of calcium occurs in the proximal tubule paracellulary however the regulation of how much is reabsorbed occurs in the ascending limb of the loop of henle, the distal tubule and collecting ducts.