The human body produces saliva, secreted into the mouth primarily by three pairs of major salivary glands: the parotid, submandibular, and sublingual glands. The average adult generates between 0.5 and 1.5 liters of saliva every day. This substantial daily output serves multiple purposes, from initiating digestion to actively protecting the teeth and oral tissues.
The Actual Quantity Produced
The secretion rate of saliva varies significantly depending on the body’s activity and is divided into two primary categories: unstimulated and stimulated flow. Unstimulated, or resting, flow occurs constantly to maintain oral moisture at a rate of approximately 0.3 to 0.4 milliliters per minute.
The stimulated flow rate occurs during eating, chewing, or in response to sour tastes, increasing dramatically to between 4.0 and 5.0 milliliters per minute. This tenfold increase ensures enough fluid is present to process food efficiently. During sleep, production slows to a near-halt, dropping to as low as 0.1 milliliters per minute.
The nervous system tightly regulates this difference in flow, ensuring the mouth is kept moist throughout the day without excessive waste. The daily total of up to 1.5 liters is an accumulation of these varying high and low production periods.
Essential Roles in Health and Digestion
Saliva plays a direct role in the initial stages of chemical digestion through the action of specific enzymes. Salivary \(\alpha\)-amylase (ptyalin) begins the breakdown of complex carbohydrates, such as starch, into smaller sugars like maltose. This process starts before food is swallowed and depends on a near-neutral pH for optimal function.
Another digestive enzyme, lingual lipase, is secreted in the mouth but is only activated in the acidic environment of the stomach. Once activated, this enzyme breaks down triglycerides, or dietary fats, into fatty acids and diglycerides. This initial fat digestion is important in infants for processing milk fat.
Beyond digestion, saliva provides protection for the oral cavity. It acts as a buffer system, utilizing bicarbonate, phosphate, and proteins to neutralize acids produced by oral bacteria after eating. This buffering action helps maintain a stable pH, which prevents the demineralization of tooth enamel and protects against dental decay.
Saliva also contains numerous non-immune proteins with antimicrobial properties that form part of the innate defense system. Lysozyme breaks down the peptidoglycan matrix in bacterial cell walls, leading to destruction. Lactoferrin binds to iron, starving bacteria of an essential nutrient and inhibiting their growth. Histatins are peptides that show antifungal activity, particularly against Candida albicans, a common cause of oral thrush.
Finally, the mucus component of saliva aids in the formation of a smooth food mass, or bolus, necessary for comfortable swallowing. The lubrication saliva provides is also essential for speech articulation, allowing the tongue and lips to move without friction.
Variables That Change Saliva Flow
The rate of salivary flow is controlled by the autonomic nervous system, which responds rapidly to internal and external cues. The parasympathetic system is the primary driver of high-volume, watery saliva secretion, stimulating the glands via acetylcholine. Conversely, the sympathetic nervous system causes the secretion of a smaller volume of thicker, protein-rich saliva.
Hydration status directly impacts saliva volume, as the fluid is approximately 99% water. Dehydration causes the body to conserve fluid, leading to a decrease in salivary flow and the sensation of thirst. Emotional states also modulate production; anticipation of food stimulates a high flow rate, while anxiety or fear can transiently reduce flow due to sympathetic activation.
A significant factor influencing flow is the use of certain medications, which interfere with the nervous system’s control signals. Common drug classes like antihistamines, antidepressants, and some high blood pressure medications can block parasympathetic signals. This anticholinergic effect substantially reduces flow, even in healthy individuals.
When Production Goes Wrong (Xerostomia and Sialorrhea)
When the regulatory system fails to maintain normal production, two primary conditions may arise: xerostomia and sialorrhea. Xerostomia is the subjective feeling of insufficient saliva, often caused by hyposalivation. Hyposalivation is an objectively low flow rate, typically defined as an unstimulated rate below 0.1 milliliters per minute.
Chronic hyposalivation significantly increases the risk of dental decay, gum disease, and oral infections due to the loss of saliva’s protective and buffering functions. Sjögren’s Syndrome, an autoimmune disorder, is a chronic systemic cause of xerostomia that targets the salivary glands.
Sialorrhea involves the excessive accumulation of saliva in the mouth, often leading to drooling. This condition is usually not due to overproduction, but rather an impairment in the ability to swallow or clear the saliva. Neurological disorders, such as Parkinson’s disease or cerebral palsy, can disrupt the swallowing reflex, resulting in sialorrhea.