 salivary secretion is not essential for life but it is highly important for oral hygiene. So how does this salivary secretion occur? There are three main salivary glands, parietate, submandibular and sublingual. So these three are major salivary glands but apart from them there are many minor salivary glands located throughout oral mucosa. Now each salivary gland opens with a duct into the mouth. So this is the common duct which opens into the mouth. If we follow this duct we will see the duct branches into smaller ducts which finally ends into blind sac asinus. So this red one year short is the asinus. Each asinus is lined by epithelial cells which in turn are surrounded by myopithelial cells. So these myopithelial cells can actually contract and they help in emptying the secretions which are present in the asinus into the duct. Now ducts are also lined by epithelial cells. These asinus cells form a primary salivary secretion which is very similar in composition to plasma. If we see the electrolyte composition of the salivary secretion will be almost same as that of plasma. But as formed salivary secretion travels through the duct. These ductal epithelial cells make some changes in the composition of the salivary secretion. So we will see at the cell level how these asinus cells are secreting the salivary secretion and how these ductal cells are modifying this secretion. First we will see how asinus cells form salivary secretion. So right side shows the basolateral side and the left side is the luminal or apical side. So this area actually forms the lumen what we saw here and we have magnified this particular cell. So this portion is the basolateral side here is the luminal side of the cell. So we will see how various transporters are working to bring about salivary secretion. Now whenever we are seeing the role of the transporters first thing is the role of the sodium potassium adiabase which as a rule of thumb is always present on the basolateral membrane. So a sodium potassium adiabase is throwing out three sodium and bringing into potassium inside the cell. Because of this there develops a concentration gradient for sodium to move inside the cell. However the concentration gradient of potassium will drive it outside the cell. Now because of the function of the sodium potassium adiabase sodium moves into the cell along its concentration gradient and along with it chloride iron also moves. So you see the concentration of two ions increases in the cell potassium and chloride. These potassium and chloride move along their concentration into the lumen. So potassium moves from cell into the lumen via potassium channel and chloride also moves from cell into the lumen. But along with chloride another ion also moves from inside the cell to outside and that is bicarbonate ion. Now from where is this bicarbonate coming? It is generated when carbon dioxide inside the cell binds with water in presence of another enzyme carbonic anhydrase and produces two ions bicarbonate and H plus ions. So bicarbonate ions move from inside the cell to the lumen. But H plus ions move from inside the cell to the basolateral side and this occurs in exchange of sodium. Sodium via paracellar root into the lumen. So in asinous cell sodium potassium chloride bicarbonate all are coming into the lumen. But ductal lipithelial cells has another set of transporters by which it is able to modify the salivary secretion which is passing through the duct. But again as a rule of thumb basolateral membrane is having sodium potassium at base. Now again you see that it is creating a concentration gradient for sodium to move inside the cell. But here instead of sodium moving from basolateral membrane into the ductal cell it moves from lumen into the cell. So there are other transporters and compared to asinous cell there is sodium hydrogen exchanger which causes sodium to move inside the cell and hydrogen to move outside the cell. Plus there is potassium hydrogen exchanger but if you see potassium is high in the cell so potassium goes out and hydrogen moves in. So there is a recycling of hydrogen which is going on it is moving out and in and with that it is moving sodium in and potassium out. Now there is another transporter chloride bicarbonate exchanger this is different from which we saw in the asinous cell. Here chloride and bicarbonate move in opposite direction thus causing chloride movement into the cell and this chloride starts accumulating in the cell thus it has to move out towards the basolateral site. But if you see what are the changes made in the composition of the salivary secretion. In the lumen the concentration of electrolytes was same as that in plasma but the ductal cell has caused the reabsorption of sodium the reabsorption of chloride and there is more secretion of potassium and bicarbonate out of the cell. In addition more ions are absorbed than they are secreted but water is not able to pass from basolateral membrane to the lumen. So more absorption of ions leads to the salivary secretion becoming hypotonic. Initially in the asinous cell it is isotonic but later on in the duct it becomes hypotonic with more potassium in bicarbonate and less sodium and chloride. But it has been found that there are differences in concentration of these electrolytes in saliva at different flow rates means that different stimulation of salivary glands if there is very high stimulation the electrolyte concentration is different. Now what is happening is that at very high flow rates these ductal cells are not able to act upon the saliva which is crossing through the ducts. So they are not able to reabsorb sodium they are not able to secret potassium into the saliva. But bicarbonate is an exception. What happens that whenever these glands and ductal cells are stimulated there is a stimulation of bicarbonate secretion also that means with high flow rate the salivary secretion will become more and more alkaline. It makes sense also because one of the stimulus which causes increase in salivary secretion is acidic sore foods. So it is important that salivary secretion is alkaline and utilizes these in our mouth. So it is having some protective effect it is buffering the excess acid present in mouth. So now what are the stimulants for salivary secretion? When salivary secretion wholly is controlled by nerves there are very very few hormones from blood and the effect which they are having on salivary secretion. So these nerves which control salivary secretion are parasympathetic and sympathetic nerves that is it is having autonomic control. Now in salivary secretions both act hand in hand they are the best friends in there that parasympathetic secretion also stimulates salivary secretion and sympathetic stimulation also stimulates salivary secretion. However the composition of saliva differs a bit. Parasympathetic mostly causes increase in watery secretion and sympathetic stimulation causes increase in mucin and enzymatic secretion is increased. Now one very interesting fact about salivary secretion is that volume of saliva which is secreted per gram of tissue is largest secretion of any of body's exocrine glands. So you can understand how it is so essential for oral hygiene and the resting flow of saliva is 0.5 ml per minute which can increase to a maximum of 4 ml per minute. So if so much saliva is secreted what are the functions of the saliva? Saliva has basically three functions lubrication protection and digestion. Now as you eat food there is increase in salivary secretion and what it does it helps in making formation of a bolus out of the food and thus it also helps in swallowing of the food because round bolus is formed and which is a smoother because of lubrication. Then it also protects the oral cavity there are certain proteins in salivary secretion like lysozyme lysozyme breaks down bacterial cell wall there is a lactoferrin as a name suggests this lactoferrin chelates the ferrous iron and it deprives the microbes of the essential iron content. Then there is also secretory IgA antibodies which are present in the saliva. Not only that there are certain minerals also present in saliva like fluoride, calcium, phosphate some demineralization is always occurring from our tooth because of bacterial decay but if salivary secretion is normal these minerals present in the saliva will replenish the minerals which are depleted due to bacterial decay. Then there are enzymes for digestion for carbohydrate digestion there is alpha amylase and for lipid digestion there is lipase but in normal scenario especially alpha amylase contributes a lot to carbohydrate digestion but even if it is absent no abnormalities have been found in digestion. Now with these functions in mind you can very well predict what will happen if salivary secretion decreases this condition is known as zero stomia that is dry mouth there will be problem in lubrication and bolus formation so swallowing will be affected plus if proper lubrication is not there person will not be able to speak also the speech will also be affected and you see the protection provided by our salivary secretion will not be there and there may be development of dental caries. So this condition zero stomia that is dry mouth occurs very commonly with the certain medications since we know that parasympathetic and sympathetic nerve supply increases salivary secretion anticholinergics which block parasympathetic secretion and beta blockers which block sympathetic stimulation will lead to zero stomia it is most commonly due to medications but there are multiple other reasons also