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Physiology by Boron

From the picture it can be seen that Bicarbonate is secreted into lumen of duct of salivary glands by exchanging Cl- ions.

So if flow rate of saliva is increased then its obvious that less Cl- is taken in the cells and less bicarbonate is secreted out.

At high flow rates, there is less time for reabsorption and secretion, and therefore the saliva is most like the initial secretion by the acinar cells. Thus, with the increase in flow rate the concentration of ions changes2

But it is also said that when flow rate increases the bicarbonate concentration also increases and when flow rate decreases bicarbonate concentration also decreases.

Doesn't this contradict the former fact?

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  • $\begingroup$ The introduction to this paper clearly states that bicarbonate concentration increases with flow rate. $\endgroup$ – Alan Boyd Jul 26 '17 at 12:08
  • $\begingroup$ When did I denied that? My question is if bicarbonate is exchanged with Cl- as in the figure then higher flow rate should decrease the bicarbonate concentration as per diagram, but its quite opposite. Why so? $\endgroup$ – JM97 Jul 26 '17 at 12:32
  • $\begingroup$ Also if flow rate is slow then bicarbonate concentration n must be higher , since more time for its secretion , bur the final result is opposite again. $\endgroup$ – JM97 Jul 26 '17 at 12:33
  • $\begingroup$ I don't understand why you assert that high flow rate would have that effect. In fact since it is clearly an incorrect inference I think that you need to explain your logic. 'I think this thing but it is wrong, why is it wrong?' Isn't a very good question. $\endgroup$ – Alan Boyd Jul 26 '17 at 12:36
  • $\begingroup$ Let me explain, primary saliva formed isotonic to plasma, then in the duct much of Cl- is taken in and HCO3- excreted out, so this implies that as long as the saliva is in ducts, its concentration of HCO3- should increase and Cl- decrease. if I am correct till here then slow flow rate means higher HCO3-, but the textbook says quite opposite. $\endgroup$ – JM97 Jul 26 '17 at 12:40
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According to this:

Yet another significant finding was the discovery of HCO3− absorbing mechanisms like NHE3 and NBC3 (NBCn1-A) at the luminal membrane of the ducts, which suggested the ducts absorb and scavenge HCO3− in the resting state.

So, initially saliva is isotonic to plasma, it is both reabsorbed and secreted, if its flow is slow, more of bicarbonate is reabsorbed, if the flow is high less bicarbonate is reabsorbed. Therefore when flow rate increases bicarbonate concentration increases.

image Source of image:


At low salivary flow rates, HCO3- is reabsorbed by the striated duct cells and so very little reaches the mouth. At high flow rates, the striated ducts can't keep up and so HCO3- reaches the mouth at high concentration (<25mM)).

This makes perfect sense, from a physiological point of view. HCO3- is valuable to the body and not something to throw into the gut for no reason. The main purpose of unstimulated saliva is to keep the mouth lubricated, help you to talk and so on. Unless you are feeding the plaque bacteria, there is no great amount of acid to neutralise and therefore no need for a high HCO3- saliva. When you feed yourelf, and also your plaque bacteria, you stimulate salivary flow and produce a high HCO3- saliva, with a high buffering capacity, just when you need it.(Source)

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