Saliva (Digestive Juice)

Saliva is a watery alkaline secretion delivered into the mouth from salivary glands. Parotid, mandibular and sublingual glands are the major glands whereas labial, lingual palatine and buccal gland are the minor glands.

Composition of saliva:

Water (98.5-99%)

Dry matter (1-1.5%)

Gases (O2, N2 and CO2)

Cellular constituents Inorganic substances (Na, K, Ca, Cl, HCO3)

Organic substances

(yeast cell, bacteria, protozoa)

Enzyme Other organics

Ptyaline Mucin

Carbonic anhydrase Glucose

Phosphatase Protein

Lysozymes Cholesterol

Functions of saliva:

Food can be digested in the absence of saliva, but saliva is present, it does contribute. It also has several other functions.

a. Digestion: Saliva helps the food to be digested in the mouth and then partially digested specially starch by ptyalin. As the foods do not remain in the mouth for long time, the activities of saliva may continuesin the stomach for awhile upto the pH 4.5 deactivated by HCl in the stomach.

b. Deglutition: Swallowing, termed deglutition is impossible in the absence of saliva. In order for food to be swallowed, it must be moistened by the saliva.

c. Lubrication: the mucous in saliva is an excellent lubricant. It prevents the mucous membranes from drying and cracking. The lubricating as well as the moistening, properties of saliva aid deglutition.

d. Cleansing action: constant flow of saliva keeps the mouth and teeth clean. This cleansing action is further aided by the bactericidal effect of lysozyme. In absence of saliva, there is a chance of developing dental caries or dental cavities.

e. Regulation of thirst: Saliva plays a role in the water balance of the body. During deglutition, salivary secretion is suppressed and the dryness of the oral and pharyngeal mucosa arouses the sensation of thirst.

f. Protection: Saliva protects the oral tissues. Saliva acts as buffering agent which protects the oral tissues from the corrosive acid or base, which may gain accidentally. It has bactericidal effect, although some bacteria survive in the mouth and propagate in presence of saliva which die later in the stomach.

g. Heat regulation: In animals like dog, salivation with panting is a means for the disposal of excess heat.

Regulation of salivation:

There are 3 processes of regulation of salivation.

a. Nervous regulation: The salivary glands are innervated by both sympathetic and parasympathetic nerve fibers. Parasympathetic stimulation increase watery saliva secretion (low organic component). Sympathetic stimulation causes small thick, mucous saliva.

b. Physical regulation: The physical processes are two types.

Conditional reflex (acquired reflex): Here salivation occurs in presence of food such as sight and smell of food stimulate saliva secretion reflexly although no food is actually given in the mouth. In Pavlov’s famous experiment parasympathetic stimulation of salivary gland was evoked in dogs by the sound of a ringing bell. The dog had beeb trained to anticipate eating after hearing the bell.

Unconditional or inherent or in born reflex: When food is actually given to the mouth then the secretion of salivary glands occur by unconditional reflex.

Aptyalism or Xerostomia- Is a condition of complete cessation of salivary secretion.

Hyposalivation- Reduced saliva

Hypersalivation- Excessive salivation

Sialolithiasis- Stone (calculi) in the salivary duct

c. Chemical regulation: There are some chemical substances namely acetylcholine, pilocarpine and bradykinine, atropine contribute regulation of saliva secretion.

Posted in: Physiology