CorpusHuman Anatomy
DigestiveFluid

Saliva

Saliva

Saliva is a hypotonic, slightly acidic to neutral fluid secreted at about 1–1.5 L a day. It lubricates food, starts starch digestion and protects the mouth and teeth.

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Overview

Volume≈1–1.5 L/day
pH≈6.0–7.4
TonicityHypotonic to plasma

Saliva is about 99.5% water. Compared with plasma it has high K⁺ and HCO₃⁻ and low Na⁺ and Cl⁻, because of ductal modification, and it is hypotonic. Its pH is about 6–7.4 and rises with flow rate as more bicarbonate is secreted. Resting flow is low; it rises more than tenfold when stimulated by taste and chewing.

Salivary α-amylase (ptyalin) breaks α-1,4 glycosidic bonds in starch. It is inactivated by gastric acid, but goes on working inside the swallowed bolus until acid penetrates, so a substantial fraction of dietary starch (up to 30–40% by Guyton's estimate) may be hydrolysed before the bolus is fully acidified. Lingual lipase, from von Ebner glands, is acid-stable and works mainly in the stomach. It is especially important in neonates, whose pancreatic lipase is immature.

Haptocorrin (R-binder) binds dietary vitamin B12 and protects it from acid in the stomach.

Parts & subdivisions

  1. Enzymes
    Salivary α-amylase (starch); lingual lipase (triglyceride); kallikrein (local vasodilatation via bradykinin).
  2. Mucins
    Glycoproteins (MUC5B, MUC7) that lubricate, coat the bolus and protect the mucosa.
  3. Antimicrobial factors
    Lysozyme, lactoferrin, histatins, salivary peroxidase/thiocyanate and secretory IgA.
  4. Mineral-protective proteins
    Statherin and proline-rich proteins keep saliva supersaturated with calcium phosphate, which allows enamel remineralisation.
  5. Haptocorrin (R-binder)
    Binds vitamin B12 and shields it from acid until pancreatic proteases release it in the duodenum.
  6. Electrolytes
    K⁺ and HCO₃⁻ higher than plasma; Na⁺ and Cl⁻ lower. HCO₃⁻ buffers plaque acid.

Function

  • Lubricates food and forms the bolus.
  • Starts digestion of starch and fat.
  • Dissolves tastants.
  • Cleanses the mouth and protects against microbes.
  • Buffers acids and remineralises enamel.
  • Moistens the mouth for speech.
Secretions

Produced by the parotid, submandibular, sublingual and minor glands. The mix shifts towards serous, parotid saliva on stimulation.

Regulation

Neural only, through the salivary nuclei in the medulla. Parasympathetic (ACh, muscarinic M3) activity gives copious watery flow. Sympathetic (noradrenaline, β and α₁) activity gives a scant, protein-rich flow. Stimuli are taste, chewing, smell, sight or thought of food, and nausea. Sleep, fear, dehydration and anticholinergic drugs suppress flow.

Clinical significance

Dry mouth (xerostomia)

Reduced saliva causes rampant cervical caries, oral candidiasis, difficulty swallowing dry food and altered taste. Causes include anticholinergics, tricyclics, antihistamines, radiotherapy and Sjögren syndrome.

Test yourself

Which salivary gland is the most common site of salivary calculi?
  1. Submandibular
  2. Parotid
  3. Sublingual
  4. Minor labial glands

Answer: Submandibular

About 80–90% of stones form in the submandibular gland. Its duct is long and runs uphill, and its saliva is more alkaline, more viscous and richer in calcium and mucin.

More digestive system questions

Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.