Digestive Glands and Their Secretions

Salivary glands

They are glands found associated with the mouth and their secretion is called saliva. Saliva is a slightly alkaline secretion which contains 99% water. It consists of proteins like mucin which serves to lubricate the food and also helps in its passage along the alimentary canal. It also contains the enzymes salivary amylase (or ptyalin) and lysozyme. Amylase partially digests starch, a tasteless polysaccharide, into maltose, a sweet tasting disaccharide. (The complete digestion into glucose, a monosaccharide takes place later). Thus amylase is present in the saliva of animals that have starch-rich diet. For e.g., pigs as they feed on tubers and roots (Food rich in starch). Amylase is absent in the saliva of domestic herbivores such as cows and buffaloes and in carnivores like tiger, lion, etc. Lysozyme helps to kill the harmful bacteria.

There are several small salivary glands present in different areas of the mouth like the inner lining of the lips, cheeks, palate and pharynx and on the tongue. The main salivary glands are present outside the mouth. They are parotid, the largest (found beneath the earlobe), submandibular (below each side of the lower jaw) and submaxillary(below the tongue). All of them pour their secretions into the mouth through ducts. Human beings, on an average produce about 2 to 3 litres of saliva everyday.

Liver

It is the largest gland and is found in the upper part of the abdomen on the right side just below the diaphragm. Its secretion is called bile juice. It is alkaline and rich in organic (steroid) salts called the bile salts. The alkaline nature serves to neutralize the acidic pH of the gastric juice and creates the right environment for the intestinal enzymes to function.

Diagram showing the association of Liver, Pancreas and the Duodenum

The bile salts help in the emulsification of fats. The bile salts reduce the surface tension of fat droplets breaking them into small globules which increases their surface area. The small globules form an emulsion of fats in the intestine. The bile juice is greenish yellow in colour due to the pigments called the bilirubin and biliverdin. These pigments are formed from the worn out and dead red blood cells. Thus, bile juice has an excretory function as it serves to remove the wastes from the blood stream.

The bile juice passes out of the liver through hepatic ducts which then continue as a common bile duct. It opens into the duodenum of the small intestine. But if there is no food in the small intestine, it is passed along the cystic duct into the gall bladder. The latter is the storage organ for excess bile. About 1litre of bile is produced by the liver daily.

The liver is an important organ in the body, in addition to being a digestive gland, its importance can be understood by listing its functions which are as follows:

1) It produces bile which helps in digestion of fats and lipids.

2) It converts glucose to glycogen and helps to control the level of sugar in blood.

3) It carries out deamination of excess amino acids. The resultant ammonia is converted to harmless urea and transported to the kidneys. The carboxylic of the amino acids are converted to glucose.

4) It acts as a storehouse for fats, glucose, vitamins A,D,E & K, iron and copper.

5) It stores water and thus regulates the fluid balance in the blood.

6) It produces red blood cells in the embryos. In adults, it destroys old red blood cells. The new ones are synthesized in the bone marrow.

7) It produces the clotting factor fibrinogen.

8) It produces an anti-coagulant called heparin.

9) It metabolises the toxic chemicals and renders them harmless after which they are excreted.

10) It is a site of many metabolic reactions and this generates heat to maintain the body temperature.