Bicarbonate concentration in saliva
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.

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 (&lt;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)