What are the functions of saliva?

A volume of 500–1000 mL of saliva is secreted by the parotid, submandibular and sublingual glands per day in response to the thought, smell, taste and presence of food in the mouth or stomach. Saliva is 98% water, with the remaining 2% made up of:

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  Electrolytes. Saliva is hypotonic – it has a lower Na⁺ concentration but a higher K⁺ concentration than plasma. Resting salivary pH is 7.0, but when HCO₃‾ secretion is increased, the pH rises to 8.0.

  
  
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  Proteins and enzymes, including mucin, haptocorrin, α-amylase and lingual lipase.

  
  
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  Bactericidal substances, including thiocyanate, lysozyme, lactoferrin and immunoglobulin A.

Unsurprisingly, the functions of saliva are reflected by its constituents:

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  Lubrication of food: saliva protects the pharyngeal and oesophageal mucosa from damage during swallowing. Mucin is primarily responsible for the lubrication properties of saliva.

  
  
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  Digestion:

  
  
  
  
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     – α-amylase is an enzyme identical to pancreatic amylase, which catalyses the breakdown of carbohydrate polymers. It works optimally at pH 7 – the pH of saliva – and manages to cleave up to 75% of starch before becoming denatured by the acidic environment of the stomach.

     
     
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     – Lingual lipase commences the digestion of dietary triglyceride. It is particularly important in neonates, whose pancreatic lipase is immature and not particularly effective at digesting milk fats.

     
     
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     – Haptocorrin is a protein that binds to vitamin B₁₂, protecting it from the low-pH environment of the stomach.

  
  
  
  
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  Neutralisation of acid: the HCO₃‾-containing saliva dilutes and neutralises gastric acid when the contents of the stomach either:

  
  
  
  
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     – Reflux into the oesophagus.

     
     
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     – Enter the oral cavity during vomiting. There is a large increase in salivation immediately before vomiting, which protects tooth enamel against acid erosion.

  
  
  
  
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  Antibacterial effects: despite the many bactericidal constituents of saliva, there is little evidence of any significant bacteriostatic action in humans.

How is saliva produced?

Salivary glands are composed of acinar cells and ducts surrounded by contractile myoepithelial cells. Production of saliva occurs in two phases:

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  The acinar cells produce the primary secretion by the active transport of electrolytes, followed by the passive movement of H₂O. The primary secretion is approximately isotonic: Na⁺, Cl‾ and HCO₃‾ concentrations approximately resemble those of plasma.

  
  
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  The duct cells modify the primary secretion to give a secondary secretion. Na⁺ and Cl‾ are reabsorbed, whereas K⁺ and HCO₃‾ are secreted. Reabsorption takes place at a greater rate than secretion – saliva therefore becomes more hypotonic as it progresses through the duct.

The rate of saliva production affects its composition. At higher rates saliva is rich in Na⁺ and HCO₃‾, whilst at lower rates it has a greater proportion of K⁺ and Cl‾. Aldosterone increases Na⁺ reabsorption and K⁺ secretion, similar to its effect in the kidney.

How are the salivary glands innervated?

The basic secretory unit of the salivary gland is the acinus. The acini of the parotid, submandibular and sublingual glands have both:

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  Parasympathetic innervation – stimulation produces vasodilatation of blood vessels supplying the acini and myoepithelial cell contraction, resulting in the secretion of a mainly serous, electrolyte-rich saliva.

  
  
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  Sympathetic innervation – stimulation produces vasoconstriction of blood vessels supplying the acini and myoepithelial cell contraction. This results in a brief increase in the secretion of mainly mucous saliva that is rich in amylase, followed by a period of decreased saliva production.

The origin of the parasympathetic fibres differs between the salivary glands:

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  The parotid gland is supplied by the glossopharyngeal nerve (cranial nerve IX); pre-ganglionic fibres synapse at the otic ganglion.

  
  
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  The submandibular and sublingual glands are supplied by the facial nerve (cranial nerve VII); pre-ganglionic fibres synapse at the submandibular ganglion, whilst post-ganglionic fibres travel in the lingual nerve.