Gastric Glands and Their Secretions

Salivary Glands

Salivary glands are in the mouth and their secretion is called saliva. Saliva is a slightly alkaline secretion which is 99% water. It consists of proteins like mucin which serves to lubricate the food and also the passage along which the food passes to facilitate easy swallowing. It also contains the enzyme called the salivary amylase or ptyalin. It partially digests starch, a polysaccharide, into maltose, a disaccharide. (The complete digestion into glucose, a monosaccharide takes place later)

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) or submaxillary
• sub lingual (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

Liver is the largest gland and is found in the upper part of the abdomen on the right side of the body just below the diaphragm. Its secretion is called bile juice. It is alkaline and rich in organic 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.

The bile salts act on the fats and emulsify them (breaking them into small globules) which increases their surface area. 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 is secreted out of the liver through hepatic ducts which then continue as common bile duct. It goes 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.

Functions of Liver

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:

• It produces bile which helps in digestion of fats and lipids
• It converts glucose to glycogen and helps to control the level of sugar in blood
• It carries out deamination of excess amino acids. The resultant ammonia is converted to harmless urea and transported to the kidneys. The carboxylic acids remaining from the amino acids are converted to glucose.
• It acts as a storehouse for fats, glucose, vitamins A,D,E and K, iron and copper
• It stores water and thus regulates the fluid balance in the blood
• It produces red blood cells in the embryos. In adults, it destroys old red blood cells. The new ones are synthesised in the bone marrow.
• It produces the clotting factor fibrinogen
• It produces an anti-coagulant called heparin
• It metabolises the toxic chemicals and renders them harmless after which they are excreted
• It is a site of many metabolic reactions that generates heat to maintain the body temperature

Pancreas

Pancreas are present at the end of the duodenum. It secretes pancreatic juice which has an almost neutral pH (6 to 7). Pancreatic juice consists of enzymes like the pancreatic amylase (to act on carbohydrates), trypsin (to act on proteins) and lipase (to act on lipids and fats). This juice is secreted into the common bile duct and then into the duodenum. Trypsin is secreted in its inactive form trypsinogen and converted to its active form in the intestine. About 700cc of pancreatic juice is secreted everyday.