- This is the first juice coming in contact with food. It is secreted by the salivary glands. The salivary glands are located outside the GIT. Food mixes with the saliva on chewing (or mastication).
- The main salivary glands are the parotid, submaxillary (also called submandibular) and sublingual glands. The minor salivary glands are lingual, labial, buccal and palatine glands.
- Depending on the secretion and histological appearance, there are two types of acini in the salivary glands. These are:
- Serous acini :
- Their cells have round nuclei with collection of secretory (zymogen) granules at their apexes; these secrete thin
- watery saliva rich in enzymes (ptyalin).
- Mucous acini :
- Their cells have flattened basal nuclei; these secrete thick viscous saliva rich in mucin Demilunes: sometimes, the mucous acini have caps of serous acini over them; these caps (crescentic in shape) of serous acini over the mucous acini are called demilunes.
- Table showing the histological type and the percentage contribution of the different salivary glands to total saliva.
|Gland||Histological type Secretion||% contribution of total saliva|
- Composition of saliva
This is about 1.5 litres/24 hours at a rate of about 1ml/min. The rate of secretion is maximum during meals and minimum during sleep pH
Max potassium ions present in : (AIIMS May 09).
B. Jejunal secretions
Resting salivary glands : slightly less than 7
During active secretion : approaches 8
- Hypotonic to plasma
Water : 99.5%
Solids : 0.5%
Organic : 0.3 %
Inorganic : 0.2%
- Ductal Modification
- The salivary juice formed in the acini first drains into ducts called intercalated ducts. The intercalated ducts drain the salivary juice into another type of ducts called striated ducts; these finally open into the oral cavity. As the saliva flows through these ducts, its composition gets modified.
- The saliva first formed in the acini (also called as primary secretion) is isotonic with plasma and its ionic composition is approximately same as that of plasma. However, as this saliva flows through the ducts:
- Na and Cl gets absorbed
- K and HCO3- is added
- It becomes hypotonic (this is because the ducts are relatively impermeable to water)
- Thus, the final composition of the saliva secreted in the oral cavity depends on the rate of salivary flow through the ducts. More the rate of flow, less will be the ductal modification.
- At low rates
- The saliva is hypotonic, slightly acidic, and rich in K; but has less Na and Cl
- At high rates
- The saliva is still hypotonic (but closer to isotonic), with higher concentrations of Na and Cl.
- Effect of aldosterone on ductal modification Its action on the salivary ducts is similar to its action on the collecting ducts of the kidney i.e. it increases Na absorption and increases K secretion. Thus aldosterone increases the K concentration and decreases the Na
- concentration of saliva. In Addisonâ€™s disease (where aldosterone is less), there is a high Na/K ratio in the saliva.
Enzymes in the saliva
- Salivary alpha amylase or ptyalin :
- Lingual lipase
Other constituents of the saliva
- Mucins :
These are glycoproteins produced by the mucous acini of the salivary glands. Their function is to lubricate the food and help in food bolus formation; they also bind bacteria and protect the oral mucosa.
The secretory immunoglobulin IgA is present in the saliva; it helps to fight against bacteria and viruses.
- Lysozymes :
These are groups of enzymes which attack the walls of bacteria and destroy them
- Lactoferrin :
This binds iron and is bacteriostatic
- proline-rich proteins
These protect tooth enamel and bind toxic tannins
This enzyme present in the saliva acts on alpha 2 globulin to produce bradykinin; bradykinin is a polypeptide which causes vasodilatation.
- Nerve growth factor
This is a polypeptide produced by the submaxillary salivary gland. It is useful for the growth and maintenance of the sympathetic and sensory nerves.
This is a gastrin-like substance present in the saliva
Functions of saliva:
a. Preparation of the food for swallowing (bolus formation) :
Saliva mixes well with the food and the mucus present in the saliva acts as a lubricant. Food is made into a bolus, which can be easily swallowed.
To appreciate the sensation of taste, food has to be dissolved. Saliva acts as a Solvent which dissolves the food materials and helps in stimulating the taste buds.
Saliva keeps the oral cavity moist; it helps in the movements of the lips and tongue. These factors help in speech.
d. cleansing action
Continuous secretion of saliva washes off the food residues, bacteria and desquamated epithelial cells. Lysozymes present in the saliva destroy bacteria. Thus, saliva cleans the oral cavity/teeth and helps in oral hygiene. Patients suffering from xerostomia or atyalism (deficient salivation) have more chances of dental infection than normal people.
Salivary alpha amylase helps in starch digestion
Saliva excretes mercury, lead and KI compounds.
g. Buffers in saliva
These help to maintain the oral pH at about 7.0. They also help neutralize gastric acid and relieve heartburn when gastric juice is regurgitated into the oesophagus.