Saliva may be considered either as gland-specific or as whole saliva. Gland-specific saliva is secreted by the parotid, submandibular, sublingual, and minor salivary glands. Whole saliva is a mixture of secretions from the salivary glands and constituents of non-salivary origin such as gingival crevicular fluid, blood and blood derivatives, desquamated epithelial cells and other cellular components, nasal and bronchial secretions, microorganisms, microbial enzymes and other microbial products, and many extrinsic substances (e.g., food debris, toothpaste and mouthwash components).1
Salivary glands are composed of specialized epithelial cells and can be divided into acinar and ductal regions. The water-permeable acinar region is where the fluid portion of gland-specific saliva is generated as a filtrate of plasma and its production is linked to body-fluid balance and blood flow (primarily branches of the maxillary artery) through salivary gland tissue.2 Amino acids enter acinar cells by active transport and the various proteins synthesized are stored in storage granules.
In response to various secretory stimuli afferent fibers of the salivary reflex arch are activated in salivary nuclei located in the medulla oblongata. The efferent part of the secretory reflex arch is comprised of sympathetic and parasympathetic fibers. Acetylcholine and norepinephrine interacting with their plasma membrane-bound receptors, signal transduction via guanine nucleotide-binding regulatory proteins (G-proteins), and activate intracellular calcium signaling mechanisms.2
Parasympathetic activity in acinar cells promotes the active transport of anions into the ductal lumen and water follows along the osmotic gradient. Sympathetic activity is responsible for the release of various proteins synthesized by the acinar cells. The initial fluid secreted by acinar cells is isotonic. The hydrophobic ductal cells absorb Na+ and Cl- ions from the isotonic primary saliva, secrete K+ and HCO3-, contribute various proteins, and render the final saliva entering the oral cavity hypotonic.2
A number of hormones also influence salivary gland function by acting directly on the acinar and ductal components of salivary glands. Antidiuretic hormone increases water resorption from the ductal system, which manifests as reduced resting salivary flow and increased viscosity of saliva. The effects of female reproductive hormones are reflected in increased resting salivary flow during pregnancy and reduced resting salivation during menopause. Testosterone is known to increase resting salivary flow.
Saliva is composed mostly of water (≈99% of the volume of whole saliva), which serves as a solvent for its many organic and inorganic components.3-7 Two defining constituents of saliva are amylase and mucin.8-10 The parotids secrete predominantly amylase-rich serous saliva; the sublingual and minor salivary glands secrete predominantly mucin-rich mucous saliva; and the submandibular glands secrete mixed seromucous saliva. Minor salivary glands produce up to 70% of the mucin.
Healthy people secrete about 1.5L of saliva per day.3 The volume of resting or unstimulated saliva in the oral cavity at any given time is about 1 mL; submandibular glands contribute most (60%), followed by the parotids (20%), minor salivary glands (15%), and sublingual glands (5%).11 Stimulated saliva is produced primarily by the parotids in response to smell, taste, tactile (tongue, oral mucosa) and proprioceptive activity (via masticatory muscles and periodontal ligaments).11-14
A person is considered to have reduced salivary flow (hyposalivation) if the unstimulated salivary flow rate measured for 5 to 15 minutes is ≤0.1 mL/min; or if the stimulated salivary flow rate measured for 5 minutes is ≤0.5 mL/min.3 If the unstimulated salivary flow rate measured for 5 to 15 minutes is ≥1 mL/min or if the stimulated salivary flow rate measured for 5 minutes is ≥3.5 mL/min, the person is considered to have increased salivary flow (hypersalivation).3