Introduction:
In human physiology, we know the body's multiple lines of defense to counteract the bacterial, viral, or other pathogenic causes seeking entry into the body/host immune system. One such important defense system other than being the vital buffer for the oral cavity is our saliva, which is composed mainly of inorganic constituents like water and other electrolytes along with a small proportion of organic content.
Let us explore in this article precisely the mechanism of how your salivary glands—the major salivary glands—the parotid, submandibular, and sublingual glands—secrete saliva alongside the minor salivary glands located in different areas of your oral cavity. Glandular secretion has been a debated topic in dental research; however, with modern investigating techniques and over the last few decades, scientific research has delved deep into the near-exact mechanisms of how salivary secretions occur from the salivary glands.
When Does Salivary Flow Controlling the Saliva Composition Get Breached?
It is important to note that salivary composition can be a highly variable phenomenon that continuously varies as a quantitative phenomenon from individual to individual. These include several factors, such as the type of gland secreting saliva, the time of day, and the degree and kind of salivary response causing saliva stimulation. One of the most studied aspects of modern dental research is the effect of the salivary flow rate on salivary composition.
Would you be surprised if I told you that the salivary composition also changes with the change in flow?
In many immunocompromising diseases or underlying systemic disorders affecting salivary flow, causing
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The condition of xerostomia, or dry mouth.
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Low, moderate, or recurrent cancers of salivary glands.
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A result of secondary metastasizing tumors from other organs to the salivary glands.
Saliva, which remains a primordial inorganic buffer of the oral cavity, thus has its immune line of defense breached in these disease conditions. It is only natural in this state of disease patterns or progression that when its flow is affected, the composition of saliva also tends to vary easily, affecting the individual's oral and systemic health status.
Why Is Salivary Flow Zero During Sleep State?
One may wonder why the oral cavity is continuously permeated by this effective salivary barrier against the bacterial or viral pathogens in the mouth—would there be almost virtually zero flow in the state of sleep? This is because the human sleep cycle virtually eliminates all its protective functions. This is the reason why, before sleep, dental professionals commonly advocate for their patients to brush thoroughly or use a mouth rinse to cleanse the oral cavity.
By following proper oral hygiene every night, the rationale is that the microorganism-based fermentation cannot happen in your oral cavity, which leads to a plethora of oral diseases such as dental caries or periodontal diseases, where the lack of saliva in the sleep state can trigger the proliferation or the activity of the dental anaerobic pathogens such as the cariogenic, aciduric streptococcus or staphylococcus bacteria, for instance or the periodontal anaerobes, etc.
What Is the Circadian Variation in Salivary Flow and Heidenhain’s Law?
When researchers through different dental studies tried measuring the salivary flow over a 24-hour window or period of a routine day without stimulating activities like chewing, feeding, hunger, etc., they found a naturally physiologic circadian flow of saliva in your mouth. This means that the salivary glands, be it the major or the minor, produce saliva physiologically through 24 hours, and glandular stimulation is natural for human beings.
According to the famous Heidenhains law proposed for salivary flow, the statement is that with an increase in the saliva flow, the composition of your saliva also changes eventually. For example, the concentration of saliva at the resting state is different from that in the sleep state (where it is virtually zero) and again different during varied psychologic states—of hunger, excitement, anger, sudden appetite or food cravings, chewing, feeding, etc.
This law states that a non-linear change can be observed in the concentration of the salivary components. The law demonstrates that while specific components like sodium (Na+) increase with saliva's increasing rate or flow, ions like potassium (K+) tend to significantly dip down in their concentration as the salivary flow rate decreases.
What Are the Neurobiological Concepts of Glandular Secretion?
Generally, the Heidenhains law holds that the salivary flow in the human body is directly under the control of the autonomous nervous system. While the parasympathetic nervous system exerts the most effective power in producing a higher saliva flow, sympathetic stimulation is much less active in comparison, according to neurobiological studies. In most immunocompromising or systemic disease states or in cancers of the oral cavity or salivary glands, where the salivary flow would be significantly impacted, causing a plethora of oral issues like dental caries, periodontal disease, rampant or aggravating carious lesions in between contact areas of the tooth in affected individuals, or with xerostomia triggering oral red, white, or fibrotic lesions, etc., in such cases, your dentist or maxillofacial surgeon will prescribe drugs that will be stimulating or downplaying the salivary functions based on your disease pattern. The drugs prescribed by your dentist in disease states would influence either the parasympathetic nervous system or the parasympathetic and sympathetic receptors to stimulate more saliva in xerostomia patients (by the receptor activation on the acinar cells of major salivary glands).
Conclusion:
To conclude thus, saliva is potently not only an oral buffer but also the immune line of defense for the oral cavity, comprising several ions like sodium (Na+), potassium (K+), chlorine (Cl-), bicarbonates (Hco3-), etc. in its inorganic constituents do change according to the increase or decrease in the salivary flow. These changes are regulated by the parasympathetic nervous system, primarily in humans, and neurobiological studies show that the salivary flow always determines the composition of saliva and, in turn, the state of oral health or disease.
