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Selective reabsorption
It seems like in every book I read it, something different is said about selective reabsorption in the proximal convoluted tubule (in the nephron). I don't understand the movement of sodium, glucose, amino acids and water. Are they reabsorbed through diffusion or active transport. Help 
Reply 1
11
Water is absorbed by passive transport (osmosis in the case of water). Since plasma proteins are not filtrated by the glomerulus the blood in the capillaries surrounding the nephron (called the peritubular capillaries) have a high capacity to influence reabsorption. Water therefore moves down the concentration gradient back into the plasma.
Sodium moves with the aid of sodium channels in the basolateral membrane of the epithelial cells in the wall of the proximal tubule into the peritubular capillaries once again. This is ATP fuelled but once a sodium gradient has been created, sodium will begin to move passively as well as actively through the membrane. It is this transport which is key to other materials transport across the member as I discuss below. You can read about the sodium potassium pump here if you want further information on how this takes place, I don't know if this is for A-level or degree stuff in which case you may or may not need to know this.
Glucose, amino acids and other organic molecules are moved out of the PCT by co-transport, aided by the sodium gradient which carries the mentioned materials along with it while it passes back into the peritubular capillaries.
PS: Remember that as sodium is reabsorbed it increases the osmotic gradient, so water is reabsorbed at a greater rate. This is a great example of how transport systems work together to produce the desired result, in this case the ultra-filtration and concentration of urine.
Sodium moves with the aid of sodium channels in the basolateral membrane of the epithelial cells in the wall of the proximal tubule into the peritubular capillaries once again. This is ATP fuelled but once a sodium gradient has been created, sodium will begin to move passively as well as actively through the membrane. It is this transport which is key to other materials transport across the member as I discuss below. You can read about the sodium potassium pump here if you want further information on how this takes place, I don't know if this is for A-level or degree stuff in which case you may or may not need to know this.
Glucose, amino acids and other organic molecules are moved out of the PCT by co-transport, aided by the sodium gradient which carries the mentioned materials along with it while it passes back into the peritubular capillaries.
PS: Remember that as sodium is reabsorbed it increases the osmotic gradient, so water is reabsorbed at a greater rate. This is a great example of how transport systems work together to produce the desired result, in this case the ultra-filtration and concentration of urine.
Reply 2
There is active transport involved in reabsorption thou. It's actually one of the main factors in establishing a concentration gradient.
1) PCT cells (epithelial cells) contain sodium (like every other cell in the body). They also contain Na+ pumps on their basolateral surface (i.e. the surface closest to the peritubular capillaries). These pumps actively transport sodium out of the cell and into the peritubular capillaries.
2) The sodium concentration, [Na+], thus decreases in the PCT cells. Sodium in the prox. tubule therefore diffuses down it's concentration gradient into the cells (where they will be again actively transported into the peritubular capillaries).
3) Water molecules then diffuse into the cells due to the the increased solute conc. in the cells.
4) As water is flowing out of the prox. tubule, it increases the conc. of all other solutes (glucose, amino acids, urea etc.) in the tubule, hence making it easier for them to diffuse down their conc. gradient into the PCT cells and then into the peritubular capillaries.
Regarding glucose and amino acids: Both share the same carrier molecule as sodium (as it enters the PCT cells, meaning passively..check point 2), hence are dependent on its concentration.
So, as a little summary: It is the active transport of sodium ions that eastablishes the gradients down which other ions, water and solutes pass passively
1) PCT cells (epithelial cells) contain sodium (like every other cell in the body). They also contain Na+ pumps on their basolateral surface (i.e. the surface closest to the peritubular capillaries). These pumps actively transport sodium out of the cell and into the peritubular capillaries.
2) The sodium concentration, [Na+], thus decreases in the PCT cells. Sodium in the prox. tubule therefore diffuses down it's concentration gradient into the cells (where they will be again actively transported into the peritubular capillaries).
3) Water molecules then diffuse into the cells due to the the increased solute conc. in the cells.
4) As water is flowing out of the prox. tubule, it increases the conc. of all other solutes (glucose, amino acids, urea etc.) in the tubule, hence making it easier for them to diffuse down their conc. gradient into the PCT cells and then into the peritubular capillaries.
Regarding glucose and amino acids: Both share the same carrier molecule as sodium (as it enters the PCT cells, meaning passively..check point 2), hence are dependent on its concentration.
So, as a little summary: It is the active transport of sodium ions that eastablishes the gradients down which other ions, water and solutes pass passively
Reply 3
10
Once the amino acids, glucose etc are in the epithelial cells, do they diffuse out of them too or move by active transport?
Thanks for the rest of the info
Thanks for the rest of the info
Reply 4
They diffuse out. The only time active transport is involved is when the sodium ions are being pumped into the peritubular capillaries
Reply 5
15
Iscariot
Sodium moves with the aid of sodium channels in the basolateral membrane of the epithelial cells in the wall of the proximal tubule into the peritubular capillaries once again. This is ATP fuelled but once a sodium gradient has been created, sodium will begin to move passively as well as actively through the membrane. It is this transport which is key to other materials transport across the member as I discuss below. You can read about the sodium potassium pump here if you want further information on how this takes place, I don't know if this is for A-level or degree stuff in which case you may or may not need to know this.
There are many different types of sodium pumps in the proximal convoluted tubule but they have little importance that I know about... Saying that the Na+/K+ pump is responsible for the absorption of sodium is misleading...
but Na+/K+ pumps are very important in the distal convoluted and collecting ducts... You may have heard of the hormone aldosterone... It's action is to increase the activity of Na+/K+ pumps, which is why it causes increased Na+ levels and decreased levels of K+...
Drugs, such as spironolactone, can act as aldosterone antagnosists... they prevent the action of aldosterone, which reduces the activity of Na+/K+ pumps... The major side effect of this is that too high levels of potassium - hyperkalemia, which can be a very serious problem... You get a similar effect when you take ACE inhibitors for blood pressure (since these also inhibit the action of aldosterone)... It is because of the risk of hyperkalemia that ACE inhibitors must never be taken with aldosterone antagnosists...
Reply 6
16
Whats the function of aldosterone? to increase the efficency of nutriant reabsorbtion? or to help with muscles and nerves?
Reply 7
15
It is involved with regulation of salt levels and in regulation of blood pressure...
Increased Na+ uptake -> increases the absorption of water -> increases blood pressure...
Increased Na+ uptake -> increases the absorption of water -> increases blood pressure...
Reply 8
10
Ok, I managed to dig this thread up again!
Is the same mechanism used for absorption in the ileum?
Is the same mechanism used for absorption in the ileum?
Reply 9
bit late now but u never no...
basic version
Lets just say sodium for now
Sodium is activly transported out of the cells lining the Proximal convulated tubial, this then allows solidum to diffuse into the cells.
thats a basic version thou
basic version
Lets just say sodium for now
Sodium is activly transported out of the cells lining the Proximal convulated tubial, this then allows solidum to diffuse into the cells.
thats a basic version thou
Reply 10
Revenged
There are many different types of sodium pumps in the proximal convoluted tubule but they have little importance that I know about... Saying that the Na+/K+ pump is responsible for the absorption of sodium is misleading...
but Na+/K+ pumps are very important in the distal convoluted and collecting ducts... You may have heard of the hormone aldosterone... It's action is to increase the activity of Na+/K+ pumps, which is why it causes increased Na+ levels and decreased levels of K+...
Drugs, such as spironolactone, can act as aldosterone antagnosists... they prevent the action of aldosterone, which reduces the activity of Na+/K+ pumps... The major side effect of this is that too high levels of potassium - hyperkalemia, which can be a very serious problem... You get a similar effect when you take ACE inhibitors for blood pressure (since these also inhibit the action of aldosterone)... It is because of the risk of hyperkalemia that ACE inhibitors must never be taken with aldosterone antagnosists...
but Na+/K+ pumps are very important in the distal convoluted and collecting ducts... You may have heard of the hormone aldosterone... It's action is to increase the activity of Na+/K+ pumps, which is why it causes increased Na+ levels and decreased levels of K+...
Drugs, such as spironolactone, can act as aldosterone antagnosists... they prevent the action of aldosterone, which reduces the activity of Na+/K+ pumps... The major side effect of this is that too high levels of potassium - hyperkalemia, which can be a very serious problem... You get a similar effect when you take ACE inhibitors for blood pressure (since these also inhibit the action of aldosterone)... It is because of the risk of hyperkalemia that ACE inhibitors must never be taken with aldosterone antagnosists...
no - essentially there is isosmotic reabsorption in the proximal tubule. The sodium-potassium ATPase is ultimately responsible for the reabsorption of most substances. The pumping of sodium into the interstitium results in a transcellular electrical and concentration gradient for sodium. So sodium enters the proximal cells to be reabsorbed. Glucose is co-absorbed with the sodium-glucose symporter, amino acids are absorbed via a symporter too, whereas chloride is absorbed using an antiport (which is actually a tertiary chloride / organic anion antiporter which relies on the secondary sodium-hydrogen antiporter, which in turn depends on the primary ATPase). Water is then reabsorbed paracellularly (leaky junctions), and transcellularly using the aquaporin subtype 1.
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