What Does the Saliva Contain?
In a normal individual with neither systemic nor oral disease, the salivary glands in a healthy individual secrete approximately 500-1500 ml of saliva daily. The pH of saliva varies around 6-7.4. Saliva is composed of multiple components, but 99 % of this content is water, and the remaining 1% comprises organic substances, L amylase or Ptyalin, lingual lipase, small amounts of urea and uric acid, cholesterol, mucin, lysozymes, inorganic substances like Na+, cl-, K+, HCO3- etc.
In addition to the oral functions of chewing and swallowing, the important physiological role that takes place in the oral cavity is the lubrication of food. Moreover, saliva is an essential oral fluid that contains several immunoglobulins and plays a vital role in immunity. In addition, it possesses a potent antimicrobial activity and a buffering capacity. It also plays a crucial role in the cleansing and lubrication of the mouth and the food intake, along with the primary function of the chemical digestion of polysaccharides obtained through diet.
What Are the Functions of the Salivary Glands?
The saliva is secreted mainly by three pairs of salivary glands. These are also known as major salivary glands. The largest of the major salivary glands is the parotid gland which weighs approximately 20-30 g and is located at the angle of the jaw. The parotid gland is a pure serous gland meaning that its secretion of saliva is more watery and fluid-like in consistency, containing a water content of 90 % or more. This gland is primarily responsible for 25% of the total salivary secretion. The ducts of the parotid gland open into the inner side of the right and left cheek, with the salivary secretions pouring into the oral vestibule. The submandibular salivary gland is the second largest salivary gland located bilaterally, i.e., one on each side around the location of the angle of the mandible. This gland is composed of serous and mucous acini, with the serous acini that are more dominant within the gland structure. In addition, this gland possesses an s-shaped duct that opens a pathway for the secretions into the sublingual papilla and is located lateral to the frenulum lingua near the tongue. The sublingual glands are the smallest of the three major salivary glands that lie below the oral mucosa on the floor of the mouth. This gland contains serous and mucinous acini, with the mucous acini more dominant or predominating. The ducts of the sublingual glands are approximately 8-20 in the count, and they open into the oral cavity through the summit of the sublingual fold (the sublingual fold is the raised mucosal ridge in the gland that starts from the sublingual papilla and runs both laterally and backward). A few ducts of the sublingual gland may also open into the submandibular duct.
What Are the Differences in Salivary Content in Different Salivary Glands?
The important medical fact to note is that both the sublingual and submandibular glands have the salivary secretion with increased concentration of proteins and hence are more viscous compared to the watery or liquid parotid gland secretions of saliva. In addition, several more minor glands secrete saliva throughout the oral cavity.
However, minor salivary glands located in the tongue secrete lipase. The composition of saliva that varies from individual to individual always depends on the salivary flow rate. This secretion is controlled basically by the autonomous nervous system by parasympathetic stimulation. Human saliva in healthy individuals is more watery with minimal enzymatic and organic content. When the sympathetic nervous system stimulates saliva, even small amounts contain rich organic material. As we know, reflex secretions are a significant component of human life as conditioned reflexes like sight, smell, and the thought of food or hunger can stimulate or increase the salivary flow rate.
What Are the Phases of Salivary Secretions?
There are four phases of salivary secretions:
- Cephalic phase: in this first phase, even before the entry of food into the mouth, the conditioned reflexes that are sight and smell of food stimulate the salivary secretion.
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The buccal phase: It is characterized by the predominance of unconditioned reflexes like the salivary secretion naturally present to lubricate the food in the mouth. The food stimulates the buccal receptors of the oral cavity, called the buccal phase.
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Esophageal phase: Though the degree of salivary secretion is lower in this phase, the food eventually enters the esophagus.
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Gastric phase: This occurs due to the food being ingested in our stomach that stimulates salivary secretion, though to a lesser degree. Especially when there is gastric irritation or when a person feels nauseating, salivary secretion is increased, which is initiated through the gastric phase.
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The intestinal phase: It occurs only when there is irritant material or food within the upper intestine.
What Is the Primary Function of Saliva?
The functions of saliva mainly are 
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The chemical digestion of polysaccharides, and complex dietary sugars, including starch, are converted to disaccharide maltose by the amylase enzyme. This enzyme acts only at the average concentrations of salivary buffering at a ph of around 5.8-7.4, depending on the individual's salivary flow rate. This enzyme activation is terminated by the consequent acidic content of the gastric juices (1.5-1.8 ph) degrades the enzyme effect.
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Lubrication of dietary components or starch or food: the high content of the salivary secretions not only prevents the food from drying out and being moistened but is converted into the semi-solid bolus that makes an individual capable of swallowing the food.
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Nonspecific immune defense: a potent combatant of microbes and the infectious organisms in the oral cavity through lysozymes and immunoglobulins.
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Cleansing and taste sensations: An adequate salivary flow is necessary to keep the palate and the oral cavity, including the tongue, moist, soft, and smooth, free from any form of abrasiveness to prevent damage to the oral mucous membranes. Though the primary mechanism of the taste receptors is in the tongue, saliva also plays a role in the stimulation of taste receptors in the taste buds of the language.
Conclusion:
To conclude, saliva has multiple roles in the human immune system for maintaining oral and systemic health through its contribution to the digestive, lubricating and defensive functionality. It plays a major role in digestion of food also.
