Saliva

• Lubrication by mucins
  • Food easier to chew/swallow
  • Aids speech
  • Protects oesophagus
• Aids taste - solubilises food for easy permeation into taste buds
• Digestion using enzymes such as α-amylase and lingual lipase
• Anti-bacterial - contains lysosomes, antibodies and lactoferrin (which can bind to the iron in your food, preventing bacteria from "using" it instead of you)
• Buffer
  • Saliva is an alkaline buffer - contains bicarbonate (HCO3-)
  • Neutralises acid in food and vomit
  • HCO3- + H+ ==> H2CO3, H2CO3 ==> H2O + CO2
• Minerals
  • Saliva contains high levels of minerals like calcium and phosphate which mineralise teeth
  • This prevents enamel demineralisation by acid

The salivary glands are clusters of cells called acini

Sublingual glands (smallest)

• Located directly under the mucous membrane covering the floor of the mouth beneath the tongue.
• Secretes saliva, predominantly mucous, into the oral cavity

Submandibular glands

• Located immediately below the lower jaw (under the mandible)
• Secrete saliva that is a mixed serous and mucous secretion

Parotid glands (largest)

• Located in the cheek, between the ear and ascending branch of the lower jaw
• Secretes a serous, watery saliva

·         Plasma (excl. blood cells and proteins - too large) from surrounding capillaries moves into the acinus cells then into the lumen 

·         Water follows by osmosis

·         This makes up the primary saliva secretion - it is isotonic as it contains the same components and concentrations of those components as plasma

·         As the primary saliva secretion moves down the duct, large amounts of Na+ and Cl- are removed by ion channels in the duct cells whilst small amounts of K+ and HCO3- are added

·         This creates hypotonic saliva - it has a lower salt concentration than plasma

·         These individual ducts eventually make up one large duct that opens into the oral cavity

Low flow rate:

• Na+ and Cl- reabsorbed from salivary duct into the blood
• K+ and HCO3- added or secreted from blood to salivary duct
• Hypotonic saliva relative to plasma because

Na+/Cl- reabsorption > K+/HCO3- secretion

High flow rate:

• Salivary ducts not as efficient at reabsorbing Na+ and Cl-
• Increased concentrations of Na+ and Cl- in ducts
• Becomes rich in K+ and HCO3-  - more alkaline as more added
• Almost isotonic relative to plasma but still hypotonic
• Increased concentrations of amylase and mucus

There are pressure and chemoreceptors in the mouth which become activated through chewing/taste/tactile stimulation which increases saliva secretion. There is also input from the cerebral cortex, e.g. secretion at the thought/sight/smell of food.

Control is predominantly through the parasympathetic nervous system (though can be sympathetic depending on the situation)

Parasympathetic nerves:

• Vasodilation - there are lots of capillaries within the salivary glands which can dilate to increase blood flow
• Increased plasma transport into acinar cells
• Increased transport of HCO3- from duct cells
• Acinar cells produce more amylase
• There is high extrusion of saliva from ducts

Salivary hypofunction: reduced or totally absent saliva

• Main causes:
  • Medication, e.g. anti-cholinergics, anti-histamines and even bronchodilaters
  • Head and neck cancer, and radiotherapy
  • Autoimmune diseases
• Dry mouth (xerostomia)
• Burning mouth which is a result of having acidic food with a dry mouth
• Constant thirst
• Dental problems, may be due to:
  • the absence of calcium and phosphate which are found in the saliva; their function is to mineralise the teeth
  • overgrowth of oral bacteria/yeasts
  • pH drops due to lack of HCO3- and bacteria producing acid
  • this acid can also break down the protective enamel coating of the teeth
• Difficulty swallowing dry foods
• Loss of taste