The digestive system of man is composed of long coiled tube. This tube extends from mouth to the anus. The main parts of the digestive system are oral or buccal cavity, esophagus, stomach, small intestine (duodenum, jejunum and ileum), large intestine (ascending colon, transverse colon, descending colon, caecum and rectum).
There are various glands associated with these regions. These are salivary gland, liver and pancreas. There are three sites of digestion in the digestive system of man. These are oral cavity, stomach and small intestine.
Digestion in Oral Cavity
- Selection of food
- Grinding or mastication
1. Selection of Food
The oral cavity is bounded by palate, tongue, teeth and cheeks. The food is tasted, smelled and felt. If the taste or smell is unpleasant, the food is rejected. Similarly, if hard objects like bone or dirt are present in the food, this food is also rejected. The senses of smell, taste and sight help in the selection of food. The tongue is sensory and muscular organ. It has taste buds. So ¡t plays an important role in the selection of food.
2. Grinding and Mastication
The food is ground by molar teeth into small pieces. This is useful because the esophagus allows relatively small pieces to pass. Secondly, the small pieces have much more surface for the action of enzymes.
3. Lubrication and Digestion
These are the main functions of the oral cavity. The Saliva performs these functions. Saliva is secreted by three pairs of salivary glands. These are:
- Sublingual glands: These are present below the tongue.
- Submaxillary glands: These glands are present behind the jaw.
- Parotid glands: These are present in front of the ears.
Composition of Saliva
The saliva has three important ingredients:
(i) Water and Mucous
Water and mucous form a slimy liquid. This liquid is used to make the food moist and lubricate the food. So this food can be chewed efficiently. Thus food can pass through the esophagus smoothly.
(ii) Sodium Bicarbonate
Sodium bicarbonate and some other salts are slightly antiseptic. Their main function is to stabilize the pH of food. Fresh saliva is alkaline with a pH nearly 8. This alkaline saliva quickly loses carbon dioxide and its pH becomes 6.
(iii) Amylase or ptyalin
It is a carbohydrate digesting enzyme. It digests starch and glycogen into maltose.
The mastication of food make food soft partly digested slimy mass. This slimy mass of food is rolled into small oval lump called bolus. The bolus is then pushed into the back of mouth by the action of tongue and muscles of pharynx. The bolus is prevented from entering into the windpipe. Following steps take place during act of swallowing:
- The tongue moves upward and backward against the roof of the mouth. This forces the bolus into the back of the mouth cavity.
- The backward movement of the tongue pushes the soft palate up. It closes the nasal opening at the back. At the same time, the tongue forces the epiglottis into a horizontal position. The epiglottis is a flap of cartilage. Thus epiglottis closes the opening of the windpipe called glottis.
- The larynx moves upward under the back of the tongue. Larynx is a cartilage around the top of the windpipe.
- The contraction of a ring of muscle partly closes the alottis.
- The epiglottis diverts the food mass to one side of the opening. So the food can enter into the partly opened glottis and safely move down to the esophagus. The beginning of the swallowing action is voluntary. But the swallowing becomes automatic when food reaches the back of the mouth. The food is then forced down into the esophagus by peristalsis.
These are the characteristic movements of the digestive tract. The food moves along the cavity of the digestive canal by peristalsis.
“Peristalsis is wave of contraction of the circular and longitudinal muscle which is preceded by wave of relaxation to squeeze the food down into the digestive canal.” Peristalsis starts just behind the mass of food in the oral cavity. It passes form the esophagus to the stomach and then passes through whole of the alimentary canal.
Sometimes, the movement is reversed. So, food may pass from the intestine back into the stomach and even into the mouth. This reverse peristalsis is called antiperistalsis. It leads to vomiting. The huger contractions are peristaltic contraction. These huger contractions are caused by low blood glucose levels. These contractions can cause uncomfortable sensation called “huger pang”. Hunger pang usually begins 12 to 24 hours after the previous meal. There may be less time in some people.
The force of gravity helps in the movement of material through the esophagus, especially when liquids are swallowed. However, the peristaltic contractions are much forceful. Even a person can swallow while doing handstand by peristalsis.
Digestion in stomach
A special ring of muscle is present at the junction between esophagus and stomach. It is called cardiac sphincter. The cardiac sphincter contracts and closes the stomach. So it prevents the food content from moving back into the esophagus. It opens when a wave of peristalsis comes down form the esophagus.
The stomach is present below the diaphragm. It is present on the left side of the abdominal cavity. Stomach is elastic muscular bag. It stores food for some time. So it makes the feeding discontinue. It also partly digests the food.
Structure of Stomach
The stomach wall is composed of three principal layers.
- An outer layer of connective tissue.
- The middle layer is composed of smooth muscles. There are two layers of the smooth muscles. The outer is longitudinal muscles and inner is circular muscles. These muscles layers help in the churning (grinding) and the mixing of food with the stomach secretions.
- The inner layer is composed of connective tissue with many glands. This layer is called mucosa. The mucosa of the stomach possesses numerous tubular gastric glands.
The gastric gland is composed of three kinds of cells:
- Mucous cells: These cells secrete mucous.
- Parietal or oxyntic cells: They secrete hydrochloric acid (HCI).
- Zymogen cells: These cells secret pepsinogen.
The secretion of all three types of cells is collectively called gastric juice. The secretion of gastric juice is regulated by smell, sight and quality of food. Gastric juice has following three parts:
- Mucous: Mucous is a thick secretion. It covers the inner side of the stomach. So that the underlying wall of the stomach could not be digested.
- Hydrochloric acid (HCI): The HCI is secreted in concentrated form. It adjusts the pH of the stomach and makes its pH 2 — 3 for the action of pepsin. HCL also makes the food soft. It kills the microorganisms coming with food.
- Pepsin: It is an enzyme. It is secreted in an inactive form called pepsinogen. This pepsinogen becomes active pepsin in presence of acidic medium or already activated pepsin. The Pepsin hydrolyses the protein and yields peptones and polypeptides.
Formation of Chyme
The muscles of the stomach thoroughly mix up the food with the gastric juice. They at last convert this food into a semi-solid mass called chyme. Gradually, the stomach transfers this chyme into duodenum by the relaxation of the pyloric sphincter.
Roll of Small Intestine in Digestive System
Small intestine is composed of three parts called, duodenum, jejunum and ileum.
It is a large gland. Its exocrine tissues secrete a juice called pancreatic juice. The pancreatic juice is transferred from the pancreas to the duodenum by pancreatic duct. The pancreatic juice contains sodium bicarbonate (NaHCO3). It partly neutralizes the chyme coming from the stomach. The pancreatic juice has many enzymes. These enzymes digest the different components of food like carbohydrates, fats and proteins.
These enzymes are:
- Amylase: It is also called amylopsin. It digests starch into maltose.
- Lipase: It is a fat digesting enzyme. It hydrolyses fats into fatty acids and glycerol.
- Trypsin: It is secreted in inactive form called trypsinogen. The trypsinogen is activated by enterokinase. The enterokinase is an enzyme. It ¡s secreted by the lining of the duodenum. Trypsin splits proteins into peptones and polypeptides.
Liver secretes bile. The bile may be temporarily stored in the gall bladder. It is release into the duodenum through the bile duct. The bile is green watery fluid. It contains no enzyme. Its green colour is due to the bile pigments. These bile pigments are formed by the breakdown of haemoglobin in the Iiver. The bile also contains bile salts. The bile salts act on fats and emulsify them. Emulsification means breakdown of fats into small globules. So fats are easily digested by water- soluble lipase.
Sometimes, bile pigments are prevented from leaving the digestive tract. So they may accumulate in blood and cause a condition called jaundice.
The liver secretes cholesterol. Sometimes, this cholesterol precipitates in the gall bladder to produce gall stones. This may block the release of bile.
Liver converts the toxic substances ammonia into less toxic compound urea. Ammonia is produced as waste product during amino acid metabolism. The urea is transported to kidney and the kidney excretes it outside the body.
Jejunum and ileum
Jejunum is the second portion of the small intestine. It is extends from the duodenum to the ileum. It is about 2.4 meter in length. So it is about two fifth of the small intestine. The lower three fifth of the small intestine forms ileum.
The digestion of food is completed in the jejunum and ileum. The wall of the jejunum and ileum secretes intestinal juice. The intestinal juice has many enzymes for digestion. These enzymes act on food and forms final products.
Absorption of Food
Nearly all the absorption of the products of digestion takes place in the ileum.
The internal surface of the ileum has many folds. These folds are numerous finger-like outgrowths called villi. These villi give velvety appearance to the inner wall of the ileum. Each villus is composed of three parts:
- It is richly supplied with blood capillaries.
- It has a vessel called lacteal. The lacteal is a part of lymphatic system.
- It ¡s covered with a covering of epithelial cells.
Electron microscope shows that the epithelial cells of villi have numerous closely packed cylindrical processes called microvilli.
The total area for the absorption is increased due to these enfolding villi and microvilli. The absorption of different compound takes place by following methods:
- Sugars and Amino Acids: The blood in capillaries of villi absorbs sugars and amino acids by diffusion or active transport.
- Fats: Some of the fatty acids and glycerol are also absorbed by blood. However, a large amount of fatty acids and glycerol enters into the epithelial cells of villi. In these cells, fatty acids and glycerol recombine to form fats. These fats then enter into lacteals. Some proteins are also present in lacteals. These proteins combine with the fats molecules to form lipo-protein droplets.
These droplets are transferred into the blood vessels through thoracic lymph duct. Later, these lipo-proteins are hydrolyzed by blood plasma enzymes. They then enter the body cells. These fats may use in the respiration or stored as fat in the liver or in the muscles of the under skin.
Transport of food through the ileum
The intestinal contents are pushed along the alimentary canal by normal peristaltic activity. There is an ileocolic sphincter at the end of the ileum. This sphincter opens form time to time and allows a small amount of residue food to enter into large intestine.
Many humans lack the enzymes for the digestion of lactose in milk. So they develop intestinal diarrhea from consuming milk products. The epithelia cells of villi are constantly shed into intestine. There is rapid cell division in crypts. These cells replace the cells.
Roll of Large intestine in Digestive System
The caecum is a blind sac. It projects from large intestine between ileum and colon. A finger like process arises from the blind of the caecum. It is called appendix. Sometimes food entraps in the appendix. The decay of this food causes inflammation of the appendix. This inflammation of the appendix is called appendicitis. In some case, such inflamed appendix has to be removed surgically.
It is the largest part of the large intestine. It has three parts: an ascending part, a transverse part and a descending part. Colon opens into rectum.
It is the last part of the large intestine. Feces are temporarily stored in the rectum. Finally these feces are removed through anus at internals. Anus is surrounded by two sphincters. The internal is smooth sphincter, while the outer sphincter has stripped muscles. As the rectum fills with feces, it gives rise to defecation reflex. This reflex can be consciously inhibited in adults. But it cannot be controlled in the infants. Gradually, the child learns to bring this reflex under control.
Function of the large intestine
The large intestine performs following functions:
1. Absorption of water and salts
The material that passes from the small intestine to the large intestine has a large amount of water, dissolved salts and undigested material. Most of the water and salts are absorbed into blood. There can be two abnormalities in the absorption of water.
- Diarrhea: If the absorption of water does not take place, it causes diarrhea. Diarrhea may be caused due to infection, drug action or emotional disturbances. If diarrhea remains unchecked it may cause dehydration. Such condition may be fatal.
- Constipation: It is other extreme condition. In this case, excessive water is absorbed.
2. Synthesis of Vitamins
A large population of bacteria lives inside the large intestine. These bacteria synthesize some vitamins especially, vitamin K. This vitamin k is absorbed by the blood and used in the body.
3. Removal of feces
The feces are also removed by the large intestine through anus. Feces contain a large number of bacteria, plants fibers, broken mucosal cells, mucous, cholesterol, bile pigments and water.