Secretion of Saliva

Function of Saliva

Saliva performs multiple essential roles within the gastrointestinal systems, which can be broadly summarised as contributing to digestion, protection of the oral cavity, and maintenance of oral health. These functions include:

• Lubrication and digestion – moistens food to aid mastication and swallowing and initiates carbohydrate digestion via amylase
• Antimicrobial defence – flushes away microorganisms and debris with the help of antimicrobial proteins and peptides (lysozyme, lactoferrin, peroxidases and defensins).
• Mucosal protection – salivary mucins form a lubricating barrier which protect the oral mucosa from mechanical trauma and toxins.
• Buffering and pH regulation – bicarbonate and phosphate ions buffer acids which help to maintain an alkaline oral environment.
• Tooth integrity – high concentrations of calcium and phosphate promote enamel remineralisation and protect the tooth surface.
• Taste – solubilises food molecules, allowing interaction with taste receptors and facilitating taste perception.

Constituents of Saliva

Saliva is a complex fluid composed of several components present in varying concentrations:

• Water – primary constituent making up about 99% of saliva.
• Electrolytes – sodium and chloride ions are typically present at lower concentrations than in plasma (hypotonic), whilst potassium, calcium and iodide ions are present at higher concentrations (hypertonic).
• Bicarbonate – present at a higher concentration than in plasma, contributing to buffering capacity and maintenance of an alkaline oral environment.
• Bacteriostats – substances that inhibit the multiplication of bacteria, including lysozyme, lactoferrin, immunoglobulin A (IgA) and salivary lactoperoxidase.
• Mucus (mucins) – facilitates lubrication of food and aids swallowing by forming a protective mucous layer.
• Enzymes – includes salivary amylase (initiates carbohydrate digestion), lingual lipase (fat digestion), and kallikrein (contributes to bradykinin production, a potent vasodilator).

Salivary Glands

Saliva is produced by three pairs of major salivary glands: the parotid, submandibular, and sublingual glands. These are ducted exocrine glands composed of secretory acini and a branching ductal system.

The parotid glands produce a purely serous secretion, which is watery and rich in enzymes, particularly amylase. The submandibular glands produce a mixed serous and mucous secretion and are responsible for the majority of resting salivary flow. In contrast, the sublingual glands produce a predominantly mucous secretion, which is viscous and mucus-rich.

These major salivary glands have unequal contributions to total saliva production.

Regulation of Secretion

Salivary secretion is primarily regulated by the autonomic nervous system. Parasympathetic stimulation is the dominant influence and produces a high-volume, enzyme-rich secretion, while sympathetic stimulation has a smaller effect on flow but increases protein output.

Further detail on the neural control of salivation can be found here.

Mechanism of Secretion

Saliva is produced in two main stages: acinar secretion and ductal modification.

Stage 1 – Acinar Secretion

Acinar cells secrete a primary saliva that is isotonic with extracellular fluid and contains enzymes, mucus and electrolytes. Sodium and chloride ions are actively secreted into the ducts, with water following osmotically.

The primary acinar fluid contains:

• Similar sodium and potassium concentrations to plasma
• Reduced chloride concentration
• Increased iodide concentration
• Similar bicarbonate concentration to plasma

Stage 2 – Ductal Modification

As saliva passes through the ducts, sodium and chloride are reabsorbed, potassium and bicarbonate are secreted, and little water is reabsorbed due to low ductal permeability. The final saliva has a similar volume but is hypotonic.

Ductal modification:

• Decreases sodium and chloride concentrations
• Increases potassium concentration
• Decreases bicarbonate concentration at rest and increases when stimulated.

At higher salivary flow rates, there is less time for ductal modification, so saliva more closely resembles the acinar secretion (except for bicarbonate, which rises with stimulation).

By Boumphreyfr (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Fig 1
Diagram showing the modification of saliva.

Resting vs Stimulated Saliva

At low stimulation levels, ductal modification of this low volume is extensive and the acinar secretion is highly modified.

With increased stimulation, ductal modification is reduced as the flow exceeds the ductal cells’ maximum rate of modification. This high volume acinar secretion is therefore modified less.