Physiology of Digestion: Role of Salivary glands

 

Introduction

Salivary glands are found in the mouth. The secretion of salivary glands is called saliva. Saliva is slightly alkaline in nature containing about 99% water. It consists of proteins like mucin which helps in lubricating and passage of the food along the alimentary canal. It also contains the enzymes like salivary amylase and lysozyme. Amylase enzyme incompletely digests starch (tasteless polysaccharide) into maltose (sweet-tasting disaccharide). Thus amylase is very helpful for animals with a starch-rich diet.

For example, pigs feed on tubers and roots. It is important to note that amylase is absent in the saliva of domestic herbivore animals like cows and buffaloes and also in carnivore animals like tigers, lions, etc. The other enzyme present in the saliva is lysozyme. It helps to kill harmful bacteria.

Several small salivary glands are distributed inside the mouth on the inner lining of the lips, cheeks, palate, pharynx, and tongue. The important salivary glands which are present outside the mouth are the parotid, submandibular and submaxillary glands. All of these glands discharge their secretions into the mouth through ducts.

The food enters the digestive tract through the mouth or oral cavity. The food is chewed or masticated with the help of teeth. The masticated food is mixed up with the salivary juice secreted by the salivary glands. In man, three types of salivary glands are present in pairs. They are Parotid glands in the upper jaw, sublingual, and submaxillary in the lower jaw. The saliva contains 99.4% of water and the remaining 0.5% of proteins like mucin which is a glycoprotein. The saliva is slightly is acidic in nature having a pH of about 6.8. A number of buccal glands present in the mouth secrete mucous into saliva. The secretion and flow of saliva is stimulated by

1. The food in the mouth stimulates the salivary secretion
2. Dry food secretes more saliva than the moist food
3. The thought, sight, and smell of the food stimulates saliva
4. Acids, salts, and many other chemical agents stimulate salivary secretion.

The digestive action of the saliva depends upon the salivary amylase or ptyalin. This salivary amylase is the mixture of two enzymes α and β amylase. Salivary amylase acts on starch converting it into maltose.

Insoluble starch→ soluble starch→ Erythro dextrin→ Achrodextrin→ maltose

Maltose is the end product of starch in salivary digestion while insoluble starch is the starting component of digestion.

 

Salivary amylase activity

• It acts rapidly at the normal body temperature
• It acts more rapidly at 50 to 55o C
• At 75 o C it gets destroyed
• Amylase activity in the stomach is inhibited when the acidity of the stomach reaches a pH of less than two.
• The chloride ion is necessary for amylase activity. When the food in the mouth is mixed well with the saliva, the pepsin activity in the stomach is rapid.

When the food is masticated in the mouth in the presence of saliva and mucous, it becomes sticky. When swallowed, this sticky food reaches the stomach through the esophagus. This phenomenon is called deglutition. This process is assisted by the peristaltic action of the muscles of the esophagus. Because of the absence of the digestive glands, no digestion occurs in the esophagus.