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Saliva, a hyposmotic liquid, is secreted by salivary glands located in the oral cavity. Comprising of 99% water, it also contains a small percentage of inorganic and organic substances. These include mineral salts, especially chlorides and bicarbonates of sodium, potassium, and calcium, enzymes like amylase, mucin, lysozyme, and immunoglobulins.
Saliva production is carried out by different glands. The submandibular glands are responsible for producing 60% of saliva, the parotid glands 30%, and the sublingual glands 5%. The remaining 5% is produced by minor salivary glands. The secretion of saliva by these glands can increase significantly under strong stimulation.
The saliva secreted by these glands varies in characteristics. The parotid glands secrete a more fluid saliva rich in ptyalin, the submandibular glands secrete mixed saliva, while the sublingual glands produce a viscous liquid due to its high mucin content.
Saliva plays several crucial roles in our bodies, with the primary ones being:
The process of digestion begins in the mouth, where a mechanical system (chewing) is aided by chemical reactions facilitated by saliva. Saliva transforms food into a bolus, an almost uniform mixture of chopped and insalivated food, protecting the pharynx and esophagus from any sharp or excessively large food fragments.
Saliva also contains enzymes, such as lipase and salivary amylase or ptyalin, which aid in digestion. Amylase, in particular, begins to digest cooked starch, breaking down the internal bonds of the starch molecule, leading to the formation of maltose, maltotriose, and dextrins.
Saliva also serves a hygienic function for the oral cavity. The water and mineral salts in saliva pass between the teeth, removing any food residues.
Saliva provides lubrication for the oral cavity, facilitating swallowing and speech. This property is attributed to its mucin content, a protein which, when mixed with the water present in saliva, takes on a viscous consistency. This protein also protects the oral cavity and larynx from abrasions from food fragments.
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.
The salivary glands work in a continuous cycle, secreting saliva continuously, albeit in varying quantities. During sleep, approximately 0.3 ml of saliva is secreted per minute, while when awake this quantity 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, and by chemoreceptors activated by particular chemical substances. These signals are conveyed to the autonomic nervous system, where they are reprocessed to stimulate glandular secretion.
The lack of saliva, known as xerostomia, can result from damage to the salivary glands, use of certain drugs, psychological disorders, certain diseases, and a general state of dehydration. On the other hand, an excess of saliva, identified as "ptyalism" or "drooling," can also be due to the use of certain drugs, mental illnesses, pregnancy, the initial installation of dental prostheses, inflammatory conditions of the oral cavity, excess interdental tartar, and tumors affecting the first part of the digestive system.
As shown, saliva plays a crucial role in maintaining oral health and facilitating digestion. It's a vital component of our bodies, working tirelessly to keep us healthy and functioning optimally.