DIGESTIVE GLANDS AND THEIR FUNCTIONS

DIGESTIVE GLANDS

To bring about the simplification of complex food molecules chemically, secretion of digestive juices take place by different glands. These are as follows:

1- SALIVARY OR MOUTH WATERING GLAND

These are exocrine glands which discharge their secretion into oral cavity. In man, there are three pairs of salivary glands; parotid, sublingual and submandibular glands. The fluids secreted by the salivary glands constitute saliva, which is a slightly acidic fluid with pH of 6.8.

Saliva is mainly a mixture of water, electrolytes (Na⁺, K⁺, Cl^–, HCO₃^–) derived from blood plasma, mucous, serum fluids and enzyme, i.e., salivary amylase or ptyalin and lysozyme (antibacterial agent).

2- GASTRIC GLANDS

Glands of stomach are called gastric glands. These are numerous microscopic, tubular glands formed by the epithelium of the stomach. Gastric glands have three major types of cells:

1 – Chief cells or peptic cells which secrete inactive precursors of gastric enzymes.

2 – Oxyntic cells secrete HCl.

3 – Mucous cells or Goblet cells secrete alkaline null cells.

The secretions from these cells form gastric juice  with pH 1.5 – 2.5 (very acidic). The gastric juice contains two proenzymes, i.e., Pepsinogen (propepsin) and prorennin and the enzymes, gastric lipase, gastric amylase, mucous and hydrochloric acid.

In human body, about 2000 – 3000 mL of gastric juice is secreted per day.

3- LIVER

It is the largest gland of the body, that lies in the upper right side of the abdominal cavity just below the diaphragm. It is heavier in males (i.e., 1.4 -1.8 kg) as compared to females (about 1.2 – 1.5 kg).

Internally, the structural and functional units of liver are the hepatic lobules (containing hepatic cells arranged in the form of cords). Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule. Fat storage cells are also present in liver.

FUNCTIONS OF LIVER

Functions performed by liver are as follows:

• Production of bile The liver secretes bile juice (hepatic bile pH 8.6). The bile is stored in the gall bladder (gall bladder bile pH 7.6). About 500-1000 mL of bile is secreted by liver in a day.
• Glycogenesis It is the conversion of the excess of glucose into glycogen by liver cells with the help of insulin secreted by the pancreas.
• Deamination It is the process by which the amino group (-NH₂) is removed from the amino acids resulting in the production of ammonia, which is converted into urea.
• Excretion Waste products and bile pigments reach the duodenum through bile and pass out with faeces.
• Glycogenolysis It is the conversion of glycogen into glucose by the liver cells with the help of insulin secreted by the pancreas.
• Haemopoiesis The process of formation of blood corpuscles is called haemopoiesis.
• Secretion of Heparin Liver also secretes heparin (an anticoagulant of blood).
• Secretion of Enzymes Liver secretes certain enzymes which play an important role in the metabolism of fats, proteins and carbohydrates in the body.
• Synthesis of Vitamin Liver synthesises vitamin-A from Beta-carotene.
• Storage Liver stores glycogen, fats, vitamin C & D, bile, blood, water, iron, copper and potassium.

4- PANCREAS

It is a soft, lobuled, greyish-pink gland which weighs about 60 gm. It is about 2-5 cm wide and 12-15 cm long, located posterior to the stomach in the abdominal cavity i.e., between stomach and duodenum.

As it is a mixed gland, the exocrine portion of the pancreas secretes an alkaline pancreatic juice (containinng enzymes and hormones) while, the endocrine part of the pancreas consists of group of cells known as Islets of Langerhans, which secrete hormones to be passed into the circulating blood, i.e., insulin and glycogen.

5- INTESTINAL GLANDS

Intestinal glands are present in the walls of small intestine and it secretes intestinal juices (containing lipolytic, proteolytic and amylolytic enzymes) commonly called as succus entericus.

MECHANISM OF DIGESTION OF FOOD

Utilization of food in humans involves five processes namely ingestion, digestion, absorption, assimilation and egestion.

Ingestion is the process of taking food in solid or liquid form into the mouth. Digestion involves the conversion of complex food components  into the simplier one by the action of various enzymes. The process of digestion begins in the mouth and gets completed in the small intestine. Absorption helps to take up the digested food by the cellular components of the body. All the digested food is taken up by the walls of intestinal villi present in the small intestine and lacteals (small lymph capillary found in villi in the small intestine). They increases the surface area and helps in the absorption of digested food.

Assimilation is the process of distribution of digested food throughout the body and Egestion refers to the elimination of undigested food formed in the lumen of large intestine (colon and rectum) through anus. The unabsorbed food is sent into large intestine where more villi absorb water.

Peristalsis gradually pushes the undigested food from small intestine to large intestine. The remaining material after reabsorption of water and ions is removed from the body through the anus.

SUMMARY OF ABSORPTION IN DIFFERENT PARTS OF DIGESTIVE SYSTEM

MOUTH

Certain drugs coming in contact with the mucosa of mouth and lower side of the tongue are absorbed into the blood capillaries lining them.

STOMACH

Absorption of water, simple sugars and alcohol, etc takes place.

SMALL INTESTINE

Principal organ for absorption of nutrients.  The digestion is completed here and the final product of digestion such as glucose, fructose, fatty acids, glycerol and amino acids are absorbed through the mucosa, into the blood stream and lymph.

LARGE INTESTINE

Absorption of water, some minerals and drugs takes place.

IMPORTANT NOTE:

PERISTALSIS – It is the necessary event of the digestive process because it is essential to move the semi-digested food in a regulated manner along the whole digestive tube. The lining of the alimentary canal has muscles which contract rhythmically all along the gut, it occurs in oesophagus when food moves into stomach, when from stomach food moves into small intestine, which in turn pushes food into large intestine and then to colon and finally into the rectum and anus.

DISORDERS OF DIGESTIVE SYSTEM

1- DIARRHOEA– Cramps, headache, loss of appetite, nausea and vomiting, slightly watery stools, etc.

2- VOMITING– Sweating, nausea, etc.

3- JAUNDICE– Skin and white portion of eye becomes yellow.

4- CONSTIPATION–  Difficulty having bowel movement.

POINTS TO REMEMBER

• We sometimes hear the noises in the stomach during digestion of food because of contraction of stomach muscles.
• Eating heavy food at night before sleeping is not advisable because by doing this, the digestion slows down and blood is directed from brain to digestion of food, which leads to poor sleep quality and many other diseases.
• Our salivary gland produces about 1.5 L saliva each day.
• Our mouth becomes watery, when we smell food or something we like the most, due to the secretion by salivary glands.