See also: pH of saliva
Saliva is a hyposmotic liquid secreted by the salivary glands located in the oral cavity. Like all secretions, saliva also consists mainly of water (99%), while only 1% is represented by inorganic and organic substances.
Among the inorganic substances, we find above all mineral salts, in particular sodium, potassium and calcium chlorides and bicarbonates. The organic fraction is instead represented by enzymes (amylase, mucin, lysozyme) and immunoglobulins.
Salivary secretion is entrusted to various glands: 60% is produced by the submandibular glands, 30% by the parotid glands and 5% by the sublinguals. There are also minor salivary glands (5%).
The amount of saliva produced by the parotids increases strongly following a strong stimulation.
The liquid secreted by the salivary glands does not always have the same characteristics: the parotids secrete more fluid saliva rich in ptyalin; the submandibulars secrete mixed saliva, while the sublinguals produce a viscous liquid, because it is rich in mucin.
Saliva covers numerous and important functions, let's see the main ones.
The digestion of food begins in the mouth, thanks to a mechanical system (chewing) assisted by chemical reactions, made possible by the presence of saliva.
This liquid transforms food into bolus (an almost uniform mixture of chopped and salivated food), protecting the pharynx and esophagus from any sharp or oversized food fragments.
In addition to mechanical means, saliva exerts its digestive properties through enzymes, such as lipase and salivary amylase or ptyalin. The latter begins to digest cooked starch (starch is a polysaccharide, present in bread, pasta, potatoes, chestnuts and other plant foods, consisting of many glucose units linked together in a linear and branched way). Amylase manages to partially break the internal bonds to the starch molecule, leading to the formation of maltose (disaccharide formed by the union of two glucose units), maltotriose (this time there are three glucose molecules) and dextrins (7-9 units glucose, with the presence of a branch).
Due to the reduced residence time of food in the mouth, amylase cannot digest all the starch. However, if we voluntarily chew a piece of bread for a long time, the effective digestive action of saliva will be evidenced by the onset of a sweetish taste.
Once in the stomach, the amylase associated with the bolus is inactivated by the strongly acidic environment, losing its functions. This enzyme is in fact active only in conditions of neutrality (pH 7), guaranteed by the presence in the saliva of bicarbonates, substances capable of maintaining the salivary pH close to neutrality (buffer system). The pH of saliva is below 7 when secretion is scarce and shifts towards alkalinity as salivary secretion increases.
Amylase digests only cooked starch, since raw starch comes in the form of granules surrounded by an indigestible wall, made up of cellulose. Cooking, on the other hand, succeeds in eliminating this membrane, releasing the starch.
Saliva also has a hygienic function for the oral cavity, especially due to the presence of water and mineral salts, which pass between the teeth, removing any food residues.
Saliva also has a lubricating function for the oral cavity, thanks to which it facilitates swallowing and phonation (the act of speaking). This property is linked to its content of mucin, a protein which, mixing with the water present in saliva , takes on a sticky consistency.
The mucin builds up along the walls of the oral cavity, protecting it from abrasions from food fragments. This protein also has a protective action against the larynx and, by surrounding and lubricating the bolus, facilitates swallowing.
The mucin contained in saliva also facilitates phonation: if salivation is eliminated, we find it difficult to speak precisely because the lubricating action of this liquid fails. In Ancient China this assumption was used to test the good faith of people suspected of crimes : by forcing the unfortunate to chew dry rice during the interrogation, he was considered innocent who could produce enough saliva to swallow it and guilty who, getting nervous and eliminating salivation, could not swallow it and spoke with difficulty.
Saliva protects the body from microorganisms introduced with food, thanks to an antibacterial agent called lysozyme, whose protective action is enhanced by the simultaneous presence of immunoglobulins (antibodies).
The salivary glands work in a continuous cycle and saliva is secreted continuously, although varying in quantity (1000-1500 ml per day). During sleep, about 0.3 ml of saliva per minute is secreted, while when awake this amount rises to 0.5 ml per minute. Following stimulation, salivary secretion can reach 3-4 ml / minute.
The secretory stimulus is mediated by cellular mechanoreceptors, present on the walls of the oral cavity and sensitive to the presence of food (biting a pen), and by chemoreceptors activated by particular chemical substances (taste buds). The signals transmitted by these receptors are conveyed to the autonomic nervous system (salivation centers located in the bulb), where they are reworked to stimulate glandular secretion. The same result is obtained when the organism is subjected to certain stimuli, such as certain smells, the sight of a particularly palatable food or memories that evoke food. The whole mechanism has the purpose of preparing the mouth to accept food.
The efferent nerve fibers that innervate the salivary glands belong mainly to the parasympathetic nervous system. However, a significant contribution is also provided by the orthosympathetic system. Both stimulate salivary secretion and this is one of the few, if not the only case, in which, together with the digestive tract, the two systems perform the same function (generally the orthosympathetic inhibits, while the sympathetic stimulates). However, there is a small difference between these regulatory mechanisms: while in normal conditions both stimulate the salivary glands, in particular situations (strong emotion or fear), the action of the sympathetic is reversed and salivary secretion is eliminated.
Saliva deficiency is called xerostomia and can result from injury to the salivary glands, from the use of drugs, from psychological disorders, from some diseases such as mumps (mumps) and from a state of general dehydration of the organism.
Excess saliva is instead identified by the term "ptyalism" or "sialorrhea", also due to the use of certain drugs, mental illnesses, pregnancy, the initial installation of dental prostheses, inflammatory conditions of the oral cavity, to the excess of interdental tartar and to tumors that affect the first tract of the digestive system.
