PROPERTIES AND COMPOSITION OF SALIVA

PROPERTIES AND COMPOSITION OF SALIVA

PROPERTIES OF SALIVA

1. Volume: 1000 mL to 1500 mL of saliva is secreted per day and it is approximately about 1 mL/minute. Contribution by each major salivary gland is:

i. Parotid glands : 25%

ii. Submaxillary glands : 70%

iii. Sublingual glands : 5%. 2. Reaction: Mixed saliva from all the glands is slightly acidic with pH of 6.35 to 6.85

3. Specific gravity: It ranges between 1.002 and 1.012

4. Tonicity: Saliva is hypotonic to plasma.

COMPOSITION OF SALIVA

Mixed saliva contains 99.5% water and 0.5% solids. Composition of saliva is given in Figure 37.3.

FUNCTIONS OF SALIVA

Saliva is a very essential digestive juice. Since it has many functions, its absence leads to many inconveniences.

1. PREPARATION OF FOOD FOR SWALLOWING

When food is taken into the mouth, it is moistened and dissolved by saliva. The mucus membrane of mouth is also moistened by saliva. It facilitates chewing. By the movement of tongue, the moistened and masticated food is rolled into a bolus. Mucin of saliva lubricates the bolus and facilitates swallowing.

2. APPRECIATION OF TASTE

Taste is a chemical sensation. By its solvent action, saliva dissolves the solid food substances, so that the dissolved substances can stimulate the taste buds. The stimulated taste buds recognize the taste.

3. DIGESTIVE FUNCTION

Saliva has three digestive enzymes, namely salivary amylase, maltase and lingual lipase.

Salivary Amylase

Salivary amylase is a carbohydrate-digesting (amylolytic) enzyme. It acts on cooked or boiled starch and converts it into dextrin and maltose. Though starch digestion starts in the mouth, major part of it occurs in stomach because, food stays only for a short time in the mouth. Optimum pH necessary for the activation of salivary amylase is 6. Salivary amylase cannot act on cellulose.

Maltase

Maltase is present only in traces in human saliva and it converts maltose into glucose.

Lingual Lipase

Lingual lipase is a lipid-digesting (lipolytic) enzyme. It is secreted from serous glands situated on the posterior aspect of tongue. It digests milk fats (pre-emulsified fats). It hydrolyzes triglycerides into fatty acids and diacylglycerol.

4. CLEANSING AND PROTECTIVE FUNCTIONS

i. Due to the constant secretion of saliva, the mouth and teeth are rinsed and kept free off food debris, shed epithelial cells and foreign particles. In this way, saliva prevents bacterial growth by removing materials, which may serve as culture media for the bacterial growth.

ii. Enzyme lysozyme of saliva kills some bacteria such as staphylococcus, streptococcus and

brucella.

iii. Proline-rich proteins present in saliva posses antimicrobial property and neutralize the toxic

substances such as tannins. Tannins are present in many food substances including fruits.

iv. Lactoferrin of saliva also has antimicrobial property.

v. Proline-rich proteins and lactoferrin protect the teeth by stimulating enamel formation.

vi. Immunoglobulin IgA in saliva also has antibacterial and antiviral actions.

vii. Mucin present in the saliva protects the mouth by lubricating the mucus membrane of mouth.

ROLE IN SPEECH

By moistening and lubricating soft parts of mouth and lips, saliva helps in speech. If the mouth becomes dry, articulation and pronunciation becomes difficult.

EXCRETORY FUNCTION

Many substances, both organic and inorganic, are excreted in saliva. It excretes substances like mercury,

potassium iodide, lead, and thiocyanate. Saliva also excretes some viruses such as those causing rabies

and mumps. In some pathological conditions, saliva excretes certain substances, which are not found in saliva under normal conditions. Example is glucose in diabetes mellitus. In certain conditions, some of the normal constituents of saliva are excreted in large quantities. For example, excess urea is excreted in saliva during nephritis and excess calcium is excreted during hyperparathyroidism.

REGULATION OF BODY TEMPERATURE

In dogs and cattle, excessive dripping of saliva during panting helps in the loss of heat and regulation of body temperature. However, in human beings, sweat glands play a major role in temperature regulation and saliva does not play any role in this function.

REGULATION OF WATER BALANCE

When the body water content decreases, salivary secretion also decreases. This causes dryness of the mouth and induces thirst. When water is taken, it quenches the thirst and restores the body water content.

 

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