DIGESTON IN MAN:

 The digestive system of man consists of a long coiled tube that extends from the mouth to the anus.

 Its main parts are :

 i . Oral or buccal cavity . 

ii . Esophagus

iii. Stomach 

iv . Small intestine ( duodenum , jejunum and ileum ) 

v. Large intestine ( ascending colon , transverse colon , descending colon , caecum and rectum ) 

Some specialized glands associated with the digestive system are :

 i . Salivary glands 

ii . Liver 

iii . Pancreas . 

There are three sites of digestion in the digestive system of man . 

i . Oral cavity

 ii . Stomach

 iii . Small intestine

Q.No.  

Describe the role of oral cavity in the digestive system of man in detail .

 Answer 

DIGESTION IN ORAL CAVITY:

 There are several functions of oral cavity .

a ) Selection of food

 b ) Grinding or mastication

 c ) Lubrication

 d ) Digestion . 

a ) Selection of food : 

when food enters the oral cavity ( the cavity bounded by palate , tongue , teeth and cheeks ) it is tasted , smelled and felt . If the taste or smell is unpleasant or if hard objects like bone or dirt are present in the food , it is rejected . Oral cavity is aided in selection by the senses of smell , taste and sight . Tongue being sensory and muscular organ plays the most important role in selection of food through its taste buds .

 b ) Grinding or mastication :

 After selection , the food is ground by means of molar teeth into smaller pieces . This is useful because : 

( a ) The esophagus allows relatively small pieces to pass through and 

( b) Food becomes easier to digest when it is in small particles , because small pieces have much more surface for the enzyme to attack and much less solid material to penetrate .


c ) Lubrication and digestion : 

These are the main functions of the oral cavity accomplished by saliva . Saliva is secreted by three pairs of salivary glands namely sublingual glands situated below the tongue ; sub maxillary glands behind the jaws and parotid glands in front of the ears . Saliva produced by these glands contains three important ingredients .

 i ) Water and mucous , 

ii ) Sodium bicarbonate and some other salts , iii ) Carbonate digesting enzymes , amylase or ptyalin . 

Water and mucus together make a slimy liquid , which serves to moisten and lubricate the food so that it can be chewed efficiently and passed through the esophagus smoothly . Sodium bicarbonate and some other salts are slightly antiseptic but their main function is to stabilize the pH of food . Fresh saliva is alkaline with a pH nearly 8 quickly losses carbon dioxide and gets to pH 6. Ptyalin is a carbohydrate digesting enzyme , which digests starch and glycogen to maltose .

Swallowing : 

As a result of mastication , the softened , partly digested , slimy food mass is rolled into small oval lump called bolus , which is then pushed to the back of the mouth by the action of tongue and muscles of pharynx which ensure that the food does not enter the windpipe . Following are the events , which occur during swallowing :

 i ) The tongue moves upwards and backwards against the roof of the mouth , forcing the bolus to the back of the mouth cavity .

 ii) The backward movement of the tongue pushes the soft palate up and closes the nasal opening at the back . At the same time the tongue forces the epiglottis ( a flap of cartilage ) into more or less horizontal position thus closing the opening of the windpipe ( the glottis ) . 

iii ) The larynx , cartilage round the top of the windpipe moves upward under the back of the tongue .

 iv ) The glottis is partly closed by he contraction of a ring of muscle .

 v ) The food does not enter the partly open glottis , because the epiglottis diverts the food mass to one side of the opening and safely down the esophagus . The beginning of the swallowing action is voluntary , but once the food reaches the the esophagus by peristalsis . back of the mouth , swallowing becomes automatic . The food is then forced down the esophagus by peristalsis.

Peristalsis :

 These are characteristics movements of the digestive tract by which food is moved along the cavity of the canal . It consists of the wave of contraction of the circular and longitudinal muscles preceded by the wave of relaxation thus squeezing the food down along the canal . Peristalsis starts just behind the mass of food from the buccal cavity along the esophagus to the stomach and then along the whole alimentary canal . Occasionally the movements are reversed ; with the result food may be passed from the intestine back into the stomach and even into the mouth . This movement is called anti peristalsis , leading to vomiting . Hunger contractions an peristaltic contractions which are increased by low blood glucose levels and are sufficiently strong to create an uncomfortable sensation often called a " hunger pang " . Hunger pangs usually begin 12 to 24 hours after the previous meat or in less time for some people . 

Gravity assists the movement of material through the esophagus , especially when liquids are swallowed . However , the peristaltic contractions that move material through the esophagus are sufficiently forceful to allow a person to swallow , even while doing a headstand .

Q No. :

 Describe the role of stomach in the digestive system of man in detail . 

Answer : 

DIGESTION IN STOMACH:

 The stomach is situated below the diaphragm on the left side of the abdominal cavity.

Structure of Stomach: 

Cardiac Sphincter : 

At the junction between esophagus and the stomach there is a special ring of muscles called cardiac sphincter . When the sphincter muscles contract , the entrance to the stomach closes and thus prevents the contents of the stomach from moving back into the esophagus . It opens when a wave of peristalsis coming down the esophagus reaches it . 

Stomach :

 It is an elastic muscular bag that stores food from meals for some time , making discontinuous feeding possible .

Stomach Wall: 

The stomach wall is composed of three principal layers: 

i ) An outer layer of connective tissue

ii ) Middle layer of smooth muscles

 iii ) Inter layer (mucosa) of connective tissue with many glands.

 The middle layer 'of muscles consists of outer longitudinal and inner circular muscles. These muscles layers help in churning and mixing the food with the stomach secretions.

 Gastric Glands: 

The mucosa of the stomach possesses numerous tubular gastric glands, which are composed of three kinds of cells.

 a ) Mucous cells, that secrete mucus 

b) Parietal or oxyntic cells secrete hydrochloric acid

 c) Zymogen cells, which secrete pepsinogen. The secretion of all these cells is collectively called gastric juice. The secretion of the gastric juice is regulated by smell, sight and quality of food. 

Feedback Mechanism: 

If more protein is present in the food it stimulate the production of gastrin hormone from the gastric endocrine lining, which is carried by blood to the gastric glands and stimulate them to produce more gastric juice. Thus more proteins mean more gastrin and more gastric juice for digestion.

Prevention of stomach walls: Mucus is a thick secretion that covers the inside of the stomach. It prevents the underlying walls from being digested. 

Functions of Hydrochloric Acid:

 Hydrochloric acid is secreted in concentrated form. 

a) It adjusts the pH of stomach contents ranging from 2-3 for the pepsin to act on proteins. 

b) It softens the food and kills many microorganisms taken in along with the food c) Hydrochloric acids convert prorennin to rennin, which in turn coagulates soluble milk protein, caseinogens into soluble calcium salt of casein. This conversion takes place in the presence of calcium ions. The calcium salt of casein is then digested by pepsin. 

Functions of Pepsin: 

Pepsin is an enzyme secreted in an inactive form called pepsinogen. Pepsinogen is activated to pepsin when exposed to the acidic medium or to some already activated pepsin. Pepsin hydrolyzes protein to yield peptones and polypeptides. Hardly any amino acid is produced by its action.

 Chyme: 

The muscles of stomach wall thoroughly mix up the food with gastric juice and eventually convert it to semi-solid mass called chyme. Gradually the stomach empties into the duodenum through the relaxed pyloric sphincter. 

Heart burn.

A Heart burn, or Pyrosis, is a painful burning sensation in the chest usually associated with the back flush of acidic chyme into the esophagus. Causes of this are overeating, eating fatty food, lying down immediately after a meal, consuming too much alcohol or caffeine,smoking.

O no. Describe the role of small intestine in the digestive system of man in detail. 

DIGESTION IN SMALL INTESTINE:

 Small intestine in man consists of duodenum, jejunum and ileum.

 1.DUODENUM: 

Duodenum is about 20-25 cm long, which leads into jejunum and then ileum. When chyme passes from stomach into duodenum, its acidity stimulates the releases of secretions from pancreas, liver and duodenal cells, Hepatic and Pancreatic secretion are also stimulated by a hormone called secretin. which is produced by the intestinal mucosa on the entry of acidic food from stomach. The acidity stimulates secretin production in Duodenum and secretin is carried by blood to pancreas, which is stimulated to produce pancreatic juice. Secretin also inhibits gastric secretin.

 PANCREAS:

 Pancreas is a large gland whose exocrine tissue secretes a juice that flows through pancreatic duct into the duodenum.

 Pancreatic Julce: 

Included in this juice are enzymes that digest all principal components of food i.e.carbohydrates, fats and proteins. 

Amylase & Maltose:

 A Carbohydrate-digesting enzyme is pancreatic amylase also called amylopsin, which digests starch into maltose. 

Lipase: 

Fat digesting enzyme is lipase, that hydrolyzes a a small percentage of fats into fatty acids and glycerol. 

Trypsin: 

Trypsin is secreted as inactive trypsinogen, which is activated by enterokinase, an enzyme secreted by the lining of the duodenum. Trypsin splits proteins into peptones and polypeptides. Pancreatic juice also contains sodium bicarbonate, which partly neutralize the chyme coming from the stomach. This is necessary because enzymes of the pancreas do not work well in acid conditions.

Functions of Pancreas:

 Four important functions of pancreas are to: 1) Produce bicarbonate ions, which neutralize acids in the duodenum. 

2) Produce digestive enzymes that are released in to duodenm.

3) Produce insulin, which controls cellular uptake of glucose and its conversion into glycogen. 

4) Produce glucagon, which stimulates the conversion of glycogen into glucose:

LIVER: 

Liver secretes bile, which may be temporanly stored in the gall bladder and released into the duodenum through the bile duct. 

Bile: 

The bile is green, watery fluid. It contains no enzymes, but its green colour is due to the bile pigments, which are formed from the breakdown of hemoglobin in the liver. Composition is as follows. 

Quantity = 500-1000 ml / day 

Water = 89-98 % 

Salts = Chlorides Carbonate phosphates of Na, K, Ca, and Bicarbonates of Na.

 No enzyme is present. 

FUNCTIONS OF BILE :

1. Salts present in the bile emulsify the fats. Break them up into small globules, which are then digested by water-soluble lipases (fat digesting enzymes)

2)  Bile help in the absorption products of fat digestion and vitamin A, D, E 2. and K. 

If the bile pigments are not passed through digestive system they accumulate in the blood. It causes Jaundice cholesterol when precipitate in gall bladder, they form Gall stones. Which block the release of bile. Hazards of liver: If bile pigments are prevented from leaving digestive tract, they may accumulate in blood, causing a condition known as jaundice. Cholesterol, secreted by the liver, may precipitate in the gall bladder to produce gallstones, which may block release of bile. The liver is easily ruptured because it is large, fixed in position, and fragile or it may lacerate by a broken rib. Liver rupture or laceration may result in severe internal bleeding. The liver may become enlarged as a result of heart failure, hepatic cancer or may be damaged due to hepatitis or being alcoholic. 

Functions of Liver: 

Four important functions of the liver are to: 

1) Produce bile, that emulsifies fat 

2) Converts end products of digestion into other molecules

3) Regulate components of blood

 4) Convert toxic substance, ammonia, which is a waste product of amino acid metabolism, to less toxic compound, urea, which is then excreted by kidneys.

 2. JEJUNUM:

 Jejunum is the second portion of the small intestine extending from the duodenum to the ileum. It is about 2.4 meter in length comprising about two fifth of the small intestine. 

1 ) ILEUM. 

Lower three fifth of the small intestine from jejunum is the ileum.


Function of intestinal juice: 

The food, which escapes undigested from the duodenum, is completely digested in the jejunum and ileum by a group of enzymes contained in the intestinal juice.

QNo. How  does the digested  food is absorbed in the small intestine?

 Answer :

ABSORPTION OF FOOD: 

Nearly all absorption of the products of digestion takes place in the ileum. 

Internal Structure of Ileum: 

The internal surface of ileum has many folds, which exhibit velvety appearance due to the presence of numerous finger-like outgrowths called villi. 

Villi: 

Each villus is richly supplied with blood capillaries and a vessel called lacteal of lymphatic system with a covering of epithelial cells. Electron microscope reveals that these cells have countless, closely packed cylindrical processes, microvilli. 

Functions of villi: 

The total area of absorption becomes incredibly large due to the enfolding, villi and microvilli. Absorption of carbobydrates & proteins: Simple sugars and amino acids are absorbed by diffusion or active transport into the blood capillaries through the microvilli.

Absorption of fatty acids: 

Some of the fatty acids and glycerol are also absorbed into blood stream. However, a large proportion of fatty acids and glycerol enter the epithelial cells of villi, where they recombine into fats. These fats then enter the lacteals. Proteins present in lymph vessels combine with fat molecules to form lipo-protein droplets. These pass into blood stream via thoracic lymphatic duct. The lipo- proteins are subscquently hydrolysed by blood plasma enzyme and enter body cells, where they may be used in respiration or stored as fat in the liver, muscle of under the skin.The intestinal contents are pushed along the alimentary canal by normal peristaltic activity. At the end of ileum, there is an ileocolic sphincter that opens and closes time to time to allow a small amount of residue from the ileum to enter the large intestine.

Q No. Descrilbe the role of large intestine in the digestive system of man in detail?

 Answer: 

Large intestine: 

The large intestine is composed of a caecum, colon and rectum. Caecum: Caecum is a blind sac that projects from the large intestine between ileum and colon. From the blind end of the caecum there arises a finger like process called appendix. The appendix, some times get inflamed due to entrapping and than putrification of food causing appendicitis, which has to be removed surgically in many instances.

Colon: 

The material that passes from the small intestine to the large intestine contains a large amount of water, dissolved salts and undigested material. Water and salts are absorbed into blood, while undigested material is rejected as feces. 

Fecal Contents: 

The fecal matter contains a Jarge number of bacteria, plant fibers, sloughed off mucosal cells, mucous, cholesterol, bile pigments and water. Large intestine also abrbors a large population of useful bacteria that synthesize some vitamins especially vitamin K which are absorbed in blood. If the absorption of water and salts does not take place due to infection, drug action or emotional disturbance, a condition known as diarrhoea occurs. If this condition is unchecked, dehydration develops that may prove to be fatal. The other extreme condition is constipation, which is caused by the excessive absorption of water. 

Rectum: 

Rectum is the last part of large intestine, where feces are temporarily stored and rejected through anus, at intervals. Two sphincters surround anus, the internal is of smooth and outer of striped muscles.

 Elimination of Feces: 

Under normal conditions, as the rectum is filled up with feces, it gives rise to defecation reflex. This reflex can be consciously inhibited in individuals other than infants. Gradually the child learns to bring this reflex under control.