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Physiology

Gastrointestinal

Question 99 of 180

Which of the following enzymes is NOT found in saliva:

Answer:

There are three major digestive enzymes found in saliva, alpha-amylase, lingual lipase and kallikrein. Saliva also contains anti-microbial enzymes such as lysozyme. Pepsin is not produced in saliva - protein digestion first begins in the stomach.

Sites of Production

Three main pairs of glands: the parotid, submandibular and sublingual glands secrete saliva. About 1 - 2 L of saliva is produced per day, and almost all is swallowed and absorbed.

Salivary gland Contribution at rest Contribution when stimulated
Parotid gland 20% 50%
Submandibular gland 65% 30%
Sublingual gland 7-8% 10%
Minor glands 7-8% 10%

Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (Own work) [CC BY 3.0 , via Wikimedia Commons

Salivary Glands. (Image by Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (Own work) [CC BY 3.0 , via Wikimedia Commons)

Composition

Saliva is hypotonic and alkaline, and contains a mixture of both inorganic and organic constituents. Saliva composition varies according to the rate and site of production, but the main components are:

  • Water (99%)
  • Mucus
  • Digestive enzymes e.g. amylase, lingual lipase
  • Antibacterial enzymes e.g. lysozyme
  • Secretory immunoglobulins e.g. IgA
  • Electrolytes (high K+ and HCO3- concentration)

Control of Secretion

Secretion of saliva is under the control of the autonomic nervous system. There is a baseline level of secretion (about 0.5 mL/min) due to ongoing low-level parasympathetic stimulation which prevents the mouth and pharynx from drying out. On top of the baseline, increases in salivary secretion can occur in response to food activating gustatory receptors or mastication activating mucosal mechanoreceptors or in response the sight, smell and anticipation of food. However salivation is initiated, the impulses to the salivary glands travel via the autonomic nerves.

The sympathetic control of salivary production is via the superior cervical ganglion. Sympathetic innervation causes increased protein secretion resulting in increased production of a thick mucoid saliva. There is variable sympathetic innervation between the salivary glands and generally this system is far less important than the parasympathetic innervation in terms of regulating production of saliva.

The parasympathetic outflow is coordinated via centres in the medulla, and innervation occurs via the facial and glossopharyngeal nerves. This results in the following effects: increased production of saliva by acinar cells, increased secretion of bicarbonate by ductal cells, increased blood flow to the salivary glands and contraction of myoepithelium to increase the rate of expulsion of saliva. Overall, increased parasympathetic stimulation results in an increased flow of saliva that is more watery in composition. Saliva production is decreased by inhibition of the parasympathetic nervous system e.g. by sleep, dehydration, anticholinergic drugs and fear.

Function

Functions of saliva in health include:

  • General cleansing and protection of the buccal cavity by washing away food particles
  • Moistening of the buccal cavity for speech
  • Lubrication of ingested food for bolus formation and swallowing
  • Dissolving of chemicals in food allowing them to interact more efficiently with taste buds
  • Dilution and neutralisation of acid in food (and produced by bacteria)
  • Secretion of digestive enzymes (particularly amylase for early starch digestion)
  • Secretion of the antibacterial enzyme lysozyme
  • Secretion of IgA to protect against invasion of microorganisms

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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