The digestive system of man is composed of a long coiled tube. This tube extends from the mouth to the anus. The main parts of the digestive system are the 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 the salivary glands, liver, and pancreas. There are three sites of digestion in the digestive system of man. These are the oral cavity, stomach, and small intestine.
Digestion in the Oral Cavity
- Selection of food
- Grinding or mastication
1. Selection of Food
The oral cavity is bounded by the 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 a 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 of 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 makes the food soft partly digested slimy mass. This slimy mass of food is rolled into a small oval lump called a bolus. The bolus is then pushed into the back of the mouth by the action of the tongue and muscles of the pharynx. The bolus is prevented from entering the windpipe. The following steps take place during the 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. The larynx is cartilage around the top of the windpipe.
- The contraction of a ring of muscle partly closes the glottis.
- 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 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 a wave of contraction of the circular and longitudinal muscle which is preceded by a 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 from the esophagus to the stomach and then passes through the 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 contractions.
These huger contractions are caused by low blood glucose levels. These contractions can cause an uncomfortable sensation called “huger pang”. The hunger pang usually begins 12 to 24 hours after the previous meal. There may be less time for some people.
The force of gravity helps in the movement of material through the esophagus, especially when liquids are swallowed. However, peristaltic contractions are much more forceful. Even a person can swallow while doing a handstand by peristalsis.
Digestion in stomach
A special ring of muscle is present at the junction between the esophagus and stomach. It is called the 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 from the esophagus.
The stomach is present below the diaphragm. It is present on the left side of the abdominal cavity. The stomach is an elastic muscular bag. It stores food for some time. So it makes the feeding discontinue. It also partly digests 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 the inner is circular muscles. These muscle 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 the 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 mucously.
- 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 the 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 an 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 gastric juice. They at last convert this food into a semi-solid mass called chyme. Gradually, the stomach transfers this chyme into the duodenum by the relaxation of the pyloric sphincter.
Roll of Small Intestine in the Digestive System
The 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 the 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 an inactive form called trypsinogen. The trypsinogen is activated by enterokinase. Enterokinase is an enzyme. It ¡s secreted by the lining of the duodenum. Trypsin splits proteins into peptones and polypeptides.
The liver secretes bile. The bile may be temporarily stored in the gall bladder. It is released into the duodenum through the bile duct. The bile is a green watery fluid. It contains no enzyme. Its green color is due to the bile pigments. These bile pigments are formed by the breakdown of hemoglobin in the liver.
The bile also contains bile salts. The bile salts act on fats and emulsify them. Emulsification means the 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 the blood and cause a condition called jaundice.
The liver secretes cholesterol. Sometimes, this cholesterol precipitates in the gall bladder to produce gallstones. This may block the release of bile.
The liver converts the toxic substances ammonia into less toxic compound urea. Ammonia is produced as a waste product during amino acid metabolism. The area is transported to the kidney and the kidney excretes it outside the body.
Jejunum and ileum
The jejunum is the second portion of the small intestine. It extends from the duodenum to the ileum. It is about 2.4 meters in length. So it is about two-fifths of the small intestine. The lower three-fifths of the small intestine forms the 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 form 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 a 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 a lacteal. Lacteal is a part of the lymphatic system.
- It ¡s covered with a covering of epithelial cells.
The electron microscope shows that the epithelial cells of villi have numerous closely packed cylindrical processes called microvilli.
The total area for absorption is increased due to these enfolding villi and microvilli. The absorption of the different compounds takes place by the following methods:
- Sugars and Amino Acids: The blood in the capillaries of villi absorb sugars and amino acids by diffusion or active transport.
- Fats: Some of the fatty acids and glycerol are also absorbed by the blood. However, a large number of fatty acids and glycerol enter 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 lipoprotein droplets.
These droplets are transferred into the blood vessels through the thoracic lymph duct. Later, these lipoproteins are hydrolyzed by blood plasma enzymes. They then enter the body cells. These fats may use in respiration or stored as fat in the liver or in the muscles the under the 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 from time to time and allows a small amount of residue food to enter the large intestine.
Many humans lack the enzymes for the digestion of lactose in milk. So they develop intestinal diarrhea from consuming milk products. The epithelial cells of villi are constantly shed into the intestine. There is rapid cell division in crypts. These cells replace the cells.
Roll of Large intestine in the Digestive System
The caecum is a blind sac. It projects from the large intestine between the ileum and colon. A finger-like process arises from the blindness of the caecum. It is called the 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 cases, such an 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. The colon opens into the rectum.
It is the last part of the large intestine. Feces are temporarily stored in the rectum. Finally, these feces are removed through the anus at the internals. The anus is surrounded by two sphincters. The internal is a smooth sphincter, while the outer sphincter has stripped muscles.
As the rectum fills with feces, it gives rise to the defecation reflex. This reflex can be consciously inhibited in adults. But it cannot be controlled in infants. Gradually, the child learns to bring this reflex under control.
The function of the large intestine
The large intestine performs the 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 the 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 a condition may be fatal.
- Constipation: It is another 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 the anus. Feces contain a large number of bacteria, plant fibers, broken mucosal cells, mucous, cholesterol, bile pigments, and water.