Tubular secretion: Lecture Notes


Tubular secretion:

Secretion of certain substances from peritubular capillary to the tubular lumen, it’s considered as additional mechanism for elimination with filtration

K+ secretion:

* K+ plasma level is very important to be kept in narrow range because abnormal plasma levels will affect heart and CNS functions

* K is filtered freely by filtration in glomerulus and actively reabsorbed in the proximal convoluted tubule in constant, unregulated fashion and its actively secreted from distal and collecting tubules and this secretion is variable and subjected to hormonal regulation (aldosteron) according to the body needs

* If there is K depletion, the amount filtered is reabsorbed in the proximal convoluted tubule except for small percentage, which escapes reabsorption and in the same time K secretion is reduced to minimum so that excreted amount in urine is very little to conserve K in plasma

* If there is excess of K, the K secretion in distal and collecting duct will add enough amount to that escaped from reabsorption and by this, increasing k excretion in urine to reduce plasma K

Mechanism of K secretion:

* K secretion occur only in distal and collecting duct from the tubular cells to the lumen

* K secretion is dependent on Na _ K pump present on the basolateral membrane side of tubular cells, as follow:

1) When K is actively pumped from interstitial fluid of basolateral side to inside of tubular cells by Na _ K pump, its level will decrease in the interstitial fluid which will lead to passive diffusion of K from peritubular capillary plasma to interstitial fluid

2) In the same time Na is pumped from inside cells to interstitial fluid the it will diffuse to peritubular capillaries in the process of Na reabsorption, as to say, the K secretion is linked to Na reabsorption, because the pump will not work unless the both of them is available (Na available inside to be pumped to out side, and K is available in the interstitial fluid to be pumped to inside,) and remember that Na reabsorption from the lumen to the cells are passive diffusion process, and this diffusion that supply Na to Na _K pump to work

3) K will be high inside cells and it will pass to tubular lumen through passive K channels (from high to low) and these channels are present only in the distal and collecting duct

4) It is very important to realize that other tubular segments (other than distal and collecting duct) doesn’t have these K channels on their luminal side, that’s why K secretion doest occur through these segments although there is similar Na _ K pump there, but, instead of that they have K channels in the basolateral side which will allow k to pass to interstitial fluid (to the same place that they originally pumped from it by Na _ K pump), as to say, that K is recycling between tubular cells and interstitial fluid on basolateral side and the benefit from that is just to help Na_K pump to accomplish its work so that Na reabsorption is possible

Effect of aldosteron:

1) K secretion is under aldosteron control in the distal and collecting duct, it stimulates K secretion and Na reabsorption in these segments

2) A rise in plasma K level will stimulate directly the adrenal cortex to secret aldosteron which will increase urinary K excretion so that returning plasma K level back to normal range and vise versa when K level decrease in plasma it will affect on adrenal cortex to decrease the aldosteron secretion

3) Decrease in plasma Na also will stimulate aldosteron secretion indirectly through activation of Renin-Angiotensin- Aldosteron system (RAAS), which will result in aldosteron secretion, which will increase Na reabsorption to maintain normal levels of plasma Na


1- Na reabsorption is important for normal working of Na_K pump, and as a result and as result it’s important for K secretion

2- K secretion doesn’t occur in other segments than distal and collecting duct because they don’t have K channels on their luminal side

3- Aldosteron regulate K secretion and Na reabsorption, it is stimulated by either (RAAS) or by a decrease in the K plasma level

H+ secretion:

1) H secretion is important in acid base balance of body fluids

2) H is secreted in proximal, distal and collecting tubules and the extent of secretion is according to body fluids acidity

Urea reabsorption and excretion:

* Urea is a waste product from protein break down

* Half of filtered urea is reabsorbed passively from proximal convoluted tubule and half is excreted in urine, this excreted amount is adequate to keeps normal levels of urea in blood

* Urea reabsorption is indirectly linked to Na reabsorption as follow:

1) Concentration of urea in the filtrate in the beginning of proximal tubule is similar to that in plasma

2) When Na is actively reabsorbed from proximal tubule, water is passively follow it by osmosis

3) As we know that this water reabsorption in proximal tubule is about 65 % of filtered and this will reduce the volume of filtrate significantly at the end of proximal tubule (e.g. if the filtrate is 125 ml in the beginning it will be 44 ml in the end of proximal tubule)

4) Urea is not reabsorbed in the beginning of the proximal tubule, so, its concentration will be increased to 3 folds in the end of the proximal tubule because of this volume reduction, and by this there will be concentration difference between tubular filtrate urea and that in peritubular capillary plasma urea

5) As a result of above, urea will pass passively from high to low (to peritubular capillary plasma) and this amount of diffusion is about half of that filtered originally

Note: notice the importance of Na reabsorption for urea reabsorption

6) By this, each time plasma pass through kidneys, half of filtered urea is excreted in urine, and this amount is adequate to remove excess urea produced from protein break down

7) Urea may be recycled between tubules, medulla and Vasa recta and excretion may vary according to the amount of urea in plasma

8) Urea reabsorption and recycling participate in Medullary osmotic gradient

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