Human Digestive SystemDIGESTION
“It is the process by which large complex insoluble organic food substances are broken down into smaller simpler soluble molecules by the help of enzymes”.
Digestion in man is mechanical (break down) as well as chemical (enzymatic hydrolysis)
HETEROTROPHIC, i.e. man is dependent upon ready made food.
TYPE OF DIGESTION
EXTRACELLULAR, i.e. digestion takes place outside the cells but within GIT.
TYPE OF DIGESTIVE SYSTEM
TUBE LIKE DIGESTIVE SYSTEM, i.e,
Digestive cavity is separated from body cavity.
It has both openings, mouth and anus.
“Complete” digestive sytem
This one way tube is known as GASTRO-INTESTINAL TRACT (GIT)
ORGANS OF GASTRO-INTESTINAL SYSTEM
The adult digestive system is a tube approximately 4.5m (15ft) long and comprises of
(A) G I T
2. ORAL CAVITY -> TEETH, TONGUE
6. SMALL INTESTINE -> DUODENUM, JEJUNUM, ILEUM
7. LARGE INTESTINE -> CAECUM, RECTUM, COLON
8. ANUS -> PAROTID
(B) ASSOCIATED GLANDS
1. SALIVARY GLANDS -> SUBLINGUAL, SUBMANDIBULAR
The anterior or proximal opening of gut, which is bounded anteriorly by lips. It opens into oral cavity.
1. Lips close the mouth.
2. Lips also help in ingestion.
(2) ORAL CAVITY
It is a wide cavity supported by bones of skull
- Cheeks form side walls.
- Tongue forms floor
- Palate forms roof
- Jaws form roof boundary of mouth.
Upper jaw is fixed while lower jaw is moveable. Both jaws bear teeth.
CONTENT OF CAVITY
Teeth and Tongue
“The hard calcified structures, meant for mastication (chewing)”
NUMBER OF SETS
Humans have 2 sets of teeth ® DIPHYODONT
The 20 teeth of first dentition, which are shed and replaced by permanent teeth.
The 32 teeth of second dentition, which begin to appear in human at about 6 year of age. It consisting of 8 incisors, 4 canines, 8 premolars and 12 molars.
+ Molars are absent in deciduous set.
HETERDONT They are embedded in gums -> THECODONT
STRUCTURE OF A TOOTH
Each tooth consist of 3 parts
1. Incisors are cutting and biting teeth. Their flat sharp edges cut food into smaller pieces.
2. Canines are pointed teeth and poorly developed in humans. They are used in tearing, killing and piercing the prey.
3. Premolars and Molars are grinders and used for crushing the food.
4. Mastication increases surface are of food for action of enzymes.
5. If one attempt to swallow a food particle too large to enter ocsophagus, it may block the trachea and may stop ventilation.
“A mixture of bacteria and salivary materials”
“A soft thin film of food debris, mucin and dead epithelial cells deposited on teeth, providing medium for growth of bacterias”
Plague plays an important role in development of dental caries, periodontal and gingival disease. Calcified plaque forms dental calculus.
Accumulation of plaque causes inflammation of gums. Continuous inflammation may spread to the root of tooth and destroy peridental layer. Eventually tooth becomes loose and falls off or may have to be extracted.
Plaque combine with certain chemicals in saliva which become harden and calcified forming deposits of calculus which cannot be removed by brushing.
When bacteria of plaque converts sugar of food into acid, the enamel (hardest substance of body, covers dentin of crown of teeth) is dissolved slowly. When dentine and pulp are attached, produce toothache and loss of teeth.
FACTOR CAUSING DENTAL CARIES
- Prolonged exposure to sugary food stuff.
- Disturbance of saliva composition
- Lack of oral hygiene
- Low levels of fluoride in drinking H2O
- Add ‘flouride’ in drinking H2O or milk
- Take ‘flouride’ tablet
- Use ‘flouride’ tooth paste.
Tongue is a muscular fleshy structure forming floor of oral cavity. Tongue has
- a root
- a tip and
- a body
- Taste buds respond to sweet, salt, acid and bitter taste, only when these substances are dissolved in H2O of saliva.
- Taste buds are most numerous on sides of vallate papillae. They are absent on mid dorsal region of oral part of tongue.
Papillae are projections of mucous membrane which gives characteristic roughness to the tongue. These are of 3 types
- VALLATE PAPILLAE
- FUNGIFORM PAPILLAE
- FILLIFORM PAPILLAE
1. Its function is ‘Spoon-like’.
2. It mixes the masticated food with saliva
3. It helps in swalloing
4. It helps in sucking and testing food.
3 pairs of salivary glands.
Lies at base of pinnae.
It is supplied by IX cranial nerve.
(2) SUB LINGUAL
Lies at base of tongue.
Supplied by VII cranial nerve.
(3) SUB MANDIBULAR
Lies at base of lower jaw.
Supplied by VII cranial nerve
These three pairs produce about 1.5dm3 of saliva each day.
These glands are supplied by Parasympathetic Nervous System. Fibers of parasympathic N.S lie in Cranial nerves. These nerves increase their secretion.
It is a watery secretion containing 95% H2O, some mucous, amylase and Lysozyme enzyme.
- Salivation is brought about by “Parasympathetic Nervous System.”
- Saliva is secreted in response to the sight, thought, taste or smell of food.
1. Mucous of Saliva moistens and lubricates the food particles prior to swallowing.
2. Salivary Amylase or Ptylin begins digestion of starch, first to dextrins and then to maltose (dissacharide).
3. Lysozyme destroys the oral cavity pathogen bacteria. It has a cleansing action.
4. Water in Saliva, dissolve some of the molecules in food particle then they react with chemo receptors in taste buds, giving sensation of taste, hence, the H2O enables taste buds to respond.
5. Saliva is fully saturated with calcium and this prevents decalcification of teeth.
6. Saliva makes speech possible by moistening the mouth; it is not possible to talk if the mouth is dry.
7. It acts as a lubricant and enables a bolus (a rounded mass of semi-solid, partially digested food particles stick together by mucus) to be formed. The tongue pushes bolus into pharynx.
The musculo-membranous passage between mouth and posterior nares and the larynx and oesophagus.
It contains opening of oesophagus, glottis, Eustachian tube and internal nostrils.
PARTS OF PHARYNX
The part above the level of soft palate is NASOPHARYNX, which communicates with auditory tube.
It lies between soft palate and upper edge of the epiglottis.
It lies below the upper edge of epiglottis and opens into larynx and oesophagus.
FUNCTION -> SWALLOWING
Swallowing in its initial stages is voluntary but involuntary afterwards.
1. As the bolus of food moves into the pharynx, the soft palate is elevated and lodges against the back wall of pharynx sealing the nasal cavity and preventing food from entering it.
2. The swallowing center inhibit respiration, raises the larynx and closes the glottis (opening between vocal cords), keeping food from getting into trachea.
3. As the tongue forces the food further back into the pharynx, the bolus tilts the epiglottis backward to cover the closed glottis.
4. This pharyngeal act of swallowing lasts about 1 second.
This is a narrow muscular tube of about 25cm long. It connects pharynx to stomach. It passes through the thoracic cavity and penetrates the diaphragm, then it joins the stomach a few cms below the diaphragm.
MUSCLES OF OESOPHAGUS
Upper-one third is surrounded by skeletal muscles.
Lower two-third is surrounded by smooth muscles.
SPHINCTERS (MUSCULAR VALVES)
1. Skeletal muscles, just below pharynx surrounding oesophagus form Upper Oesophageal Sphincter.
2. Smooth muscles in last 4 cm of oesophagus forms Lower Oesophageal Sphincter. It seals the exit of food.
It conveys the food or fluid by Peristalsis.
Alternate rhythmic contraction and relaxation waves in the muscle layers surrounding a tube are called Peristaltic Waves.
It is the basic propulsive movement of GIT.
Distention of oesophagus.
An oesophageal peristaltic wave takes about ‘9 sec’ to reach stomach. Bolus is moved toward stomach by progressive peristaltic wave which compresses the lumen and forces the bolus ahead of it.
Peristalsis in opposite direction, i.e. from stomach towards pharynx.
- Early stages of GIT irritation.
- Over distention.
Anti peristalsis begins to occur, some minute before vomiting appears. The initial events of anti peristalsis may occur repeatedly without vomiting, called RETCHING. 1. Vomiting begins with a deep inspiration, closure of glottis and elevation of soft palate.
2. Abdominal and thoracic muscles contract, raising intradominal pressure.
3. Stomach is squeezed, lower oesophageal sphincter relaxes allowing expulsion of stomach content into oesophagus in form of VOMITUS.
Stomach is a hollow, muscular, distensible bag like organ.
Lying below the diaphragm on the left side of abdominal cavity.
It has 3 regions.
1 CARDIAC REGION
This is the anterior region which joins the oesophagus through a cardiac sphincter. It has muscous glands which helps in lubrication of food.
The middle portion is body of stomach. The part to the left and above the entrance of oesophagus is called FUNDUS of stomach. Body of stomach contain gastric glands. Gastric glands contain 3 types of cells.
- These cells are present at opening of gastric glands and secrete mucous.
- It lubricates the food and passage.
- It also protects the epithelium from self digestion by pepsin.
- They lie deeper within the glands and secrete dilute HCl having a pH of 1.5 – 2.5.
- Kills microbes
- Solublization of food particles.
- Activate the inactive enzyme pepsinogen into Pepsin.
- Deeper in the glands and secrete enzyme precursor Pepsinogen.
- After converting into Pepsin, it acts upon proteins and convert them into short chain polypeptides, Peptones.
The posterior region is the terminal narrow pyloric region or Antrum. It opens into duodenum through pyloric sphincter / pylorus.
ITS SECRETION -> GASTRIN
This region does not secrete acid. It secretes mucous, pepsinogen and a hormone GASTRIN. Endocrine cells which secrete GASTRIN are scattered throughout epithelium of antrum.
Partially digested proteins.
Activate gastric glands to produce gastric juices.
“RENIN”-ADDITIONAL ENZYME IN INFANT
In infants, RENIN is secreted which curdles the milk.
FUNCTION OF STOMACH
(1) STORAGE OF FOOD
Pylorus acts as a valve and retain food in the stomach for about 4 hours. Periodic relaxation of pylorus releases small quantities of chyme into duodenum.
(2) MECHANICAL DIGESTION
The weak peristaltic waves also called mixing waves move along the stomach wall once every 20 seconds. These waves not only mix the food with secretions but also move mixed contents forward.
(3) CHEMICAL DIGESTION
Gastric juice converts food to a creamy paste called CHYME.
6. SMALL INTESTINE
The small intestine is a coiled tube approximately 6 meters long and 2.5 cm wide, leading from stomach to large intestine. It fills most of the abdominal cavity.
There are 3 divisions.
It begins after pyloric stomach and ends at jejunum. Its length is about 30cm.
Pancreatic juice from pancreas by pancreatic duet and bile from gall bladder by common bile duct act on chyme from stomach. Both ducts open via a common opening in duodenum.
SYNTHESIS, STORAGE AND SECRETION
Bile is made in liver and enters the duodenum via the bile duct. It stores in gall bladder.
Bile is yellow in colour but changes to green due to exposure to air.
- Bile Salts
These are sodium salts of compounds of cholestrol. NaHCO3 is also present which neutralizes the acidity of gastric juice and make the chyme alkaline.
The main bile salts are for emulsification of fats.
EMULSIFICATION Break down of large fat particles into small droplets so that they can mix well with H2O to form emulsions.
+ BILE PIGMENTS
BILIRUBIN and BILIVERDIN are excretory products formed by breakdown of haemaglobin of worn out RBCs in the liver.
ACTION OF ‘CHOLECYSTOKININ (CCK)’
CCK is a hormone and produced by cells of small intestine.
STIMULI FOR HORMONE RELEASE
Fatty food in duodenum.
CCK is released in blood and reaches to gall bladder and causes it to contract. Due to contraction of gall bladder, bile enters the duodenum.
Pancreatic juice is produced in pancreas by its exocrine function and secreted via pancreatic duct. It is a colourless fluid.
ACTION OF SECRETIN
Secretion is also a hormone and produced by cells of small intestine.
Acid (HCl) carried with chyme in small intestine.
It increases the secretion of pancreatic juice and also increases bicarbonate secretion in bile.
(1) TRYPSIN (PROTEASE)
It is secreted in an inactive form called Trypsinogen which is activated by action of an enzyme produced by duodenum called enterokinase.
Break proteins and long chain polypeptides into small peptide fragments.
(2) CHYMOTRYPSIN (PROTEASE)
It is also secreted in inactive form, Chymotrypsinogen which is converted into chymotrypsin by action of Trypsin.
Converts casein (milk proteins) into short chain peptide.
It is similar to salivary amylase. It acts on polysaccharides (Glycogen and Starch) and convert them into maltose (a disaccharide).
It acts on emulsified fat droplets. It splits off lipid into fatty acid and glycerol, hance the digestion of fat is completed in duodenum.
It extends from duodenum to illeum. It is 2.4 meters long. Here the digestion of food is completed.
COLLECTION OF PEPTIDASES, EREPSIN
Peptidases complete the breakdown of polypeptide into amino acids.
It converts nucleotides into nucleoside. End products of digestion, i.e, monosaccharide and A.As are liberated in lumen of small intestine for absorption in ileum.
It is the last and longest part of small intestine. Its length is about 3.6 meters long. It contains digested food in true solution form.
The inner wall (Mucosa and Submucosa) of small intestine is thrown into various folds. These folds have finger-like microscopic projections called villi.
Each villus is lined with epithelial cells having microvilli on their free surfaces.
Their walls are richly supplied with blood vessels and lymph vessels called Lacteals. Some smooth muscles are also present in villi.
MECHANISM OF ABSORPTION
Major function of ileum is absorption of digested food, which is facilitated by highly folded inner wall of intestine with villi on their surfaces.
This increases the absorptive area. Villi are able to move back and forth due to muscle fibers in them.
- The monosaccharide and A.As are absorbed into blood capillaries by Diffusion or Active Transport.
- Fatty acid and glycerol enter epithelial cells of villi, covert into triglycerols and enters Lacteals and pass into blood stream.
All capillaries converge to form hepatic portal vein, which delivers absorbed nutrients to liver.
7. LARGE INTESTINE
Small intestine opens into large intestine, which is a large diameter tube about 6.5 cm. It is not coiled by relatively has 3 straight segments.
Caecum is a blind ended pouch placed in the lower right side of abdominal cavity. It gives a 10cm long finger like projection, Appendix. Appendix is a vestigial organ, i.e. an organ present in rudimentary form and has no function but has well developed function in ancestors.
Symbiotic bacteria, present in caecum, help in digestion of cellulose, which is not digested by man, as enzyme for digestion is absent.
Colon is longest part and has 3 regions :
+ Ascending colon
+ Transverse Colon
+ Descending Colon
-> SIGMOID COLON is terminal part of Descending Colon.
Inorganic salts, water and mineral absorbed in colon. Some metabolic waste products and excess calcium of body as salts are excreted into large intestine. Each day 500 ml of intestinal content enter the colon and during its passage the amount reduced to 150 ml due to absorption of H2O.
Rectum is last portion, it stores faeces for some time.
When the faeces enter into rectum, it brings about a desire for defecation. The process by which faeces passes out is called Egestion.
Many symbiotic bacteria in large intestine provide the body with a source of vitamin and A.As, especially vitamin B complex and K, which are absorbed in blood stream. Administration of Broad-spectrum antibiotics destroys these bacteria and a vitamin deficiency results, which is then make up by vitamin intakes.
External opening of digestive system is ANUS.
Two sphincters surround the anus:
+ Internal Sphinter -> made up of smooth muscle and under Autonomic control (involuntary control).
+ Outer Sphincter -> made up of skeletal muscle and under Somatic Control (voluntary control).
Faecus consists of:
Dead bacteria, cellulose, Plant fibers, dead mucosal cells, mucous, cholesterol, bile pigment derivatives and H2O.
(DIAGRAM “DIGESTIVE SYSTEM” FROM BOOK XI)
Liver is the largest organ and gland of body. It weighs about 1.5 kg . It is also called ‘HEPAR’.
It is reddish brown in colour.
It lies below the diaphragm on right side.
LOBES OF LIVER
Liver has 2 lobes, i.e. Right and Left. Left is further divided into two lobes.
FUNCTIONS OF LIVER
‘AS A METABOLIC FACTORY’
It maintains the appropriate level of nutrients in blood and body. It is performed in 3 ways.
A. GLUCOSE METABOLISM
1. Additional (Surplus) Glucose is converted into Glycogen by action of INSULIN after every meal. This is called Glycogenesis.
2. Glycogen is splitted into Glucose for body needs. This is called Glycogenolysis.
3. New glucose for body requirement is formed by non-carbohydrate compounds. This is called Gluconeogenesis.
B. A.AS METABOLISM
A.As are also stored after deamination (removal of NH2 group), which forms Urea.
C. FATTY ACID METABOLISM
It also processes F.As and stores the products as Ketone Bodies, which are released as nutrients for active muscles.
‘AS A DETOXIFICATION CENTER’
Poisons and toxic substances, which can harm the body, are degraded into harmless compounds. It excrete out bile pigments and waste products.
‘AS A STORAGE ORGAN’
It stores vitamins and also produces proteins and coagulating factors of blood.
It lies on undersurface of liver, a pear shaped organ.
It concentrates and stores the bile secreted by liver.
Two hepatic ducts from liver bring bile and join the cystic duct from gall bladder. This form common bile duct, which joins Pancreatic duct coming from pancreas bringing pancreatic juice. These 2 ducts open into duodenum at same opening.
A large elongated gland situated transversely behind the stomach, between spleen and duodenum.
PARTS OF PANCREAS
It is the right extremity and directed downwards.
Left extremity is transverse and terminates close to spleen.
The main portion in middle.
Pancreatic duct opens into duodenum with common bile duct and delivers pancreatic juices.
WORKING AS A GLAND
It works both as an endocrine and exocrine gland.
Endocrine part consists of ISLETS OF LANGERHANS.
The islets contain.
α cell (ALPHA)
Produce GLUCAGON which increases blood glucose level.
β cell (BETA)
Produce INSULIN which reduces blood glucose level.
Δ cell (DELTA)
Produce Somatostatin (SS) which inhibit the release of many harmones.
P P cells
Secrete pancreatic polypeptide.
The exocrine part consists of pancreatic acini. Acini are secretory unit that produce and secrete pancreatic juice into duodenum which contain enzymes essential to digestion.
DISORDERS OF ‘GIT’
Abnormal frequency and liquidity of fecal discharges. It is the rapid movement of fecal matter through large intestine.
It may be caused by infection of intestinal wall (mucosa) by a virus or bacteria. Due to infection, mucosa becomes irritated and motility of intestinal wall increases.
Cholera is a bacterial disease caused by VIBRIO CHOLERA. It can cause diarrhoea. It causes extreme amount of HCO3- (bicarbonates ion) and Na and H2O to be secreted in faeces. It may causes death.
It is caused by nervous tension. In the young and elderly, diarrhoea may lead to a serious depletion of H2O and inorganic salts.
Acute inflammation of intestines especially of the colon.
Pain in abdomen, tenesmus (straining), frequent stool containing blood and mucus.
PROTOZOA. (like amoebic dysentery)
- PARASITIC WORMS.
- BACTERIA. (like bacillary dysentery)
- CHEMICAL IRRITANTS.
Infrequent or difficult evacuation of faeces. OR Slow movement of faeces through large intestine.
Faeces becomes hard due to long time available for H2O absorption.
Irregular bowel habits that have developed through a life time of inhibition of normal defection reflaxes.
- Laxatives are used
- Substance which hold H2O with them
Also called HAEMORRHOIDS Varicose dialatation of veins occurring in relation to anus, resulting from a persistence increase in pressure.
Venous dialatation covered with modified anal skin.
Dilatation of veins covered by mucous membrane.
The pressure exerted to defecate stretches skin with vein and causes dilation.
Can be avoided by regular habit of defecation and by use of fiber diet.
Impairment of the power or function of digestion, usually applied to epigastria discomfort following meals.
May be due to peptic ulcer.
- Heart burn.
- Flatulence (distended with gas)
- Anorexia, nausea, vomiting with or without abdominal pair.
Dyspepsia in which symptoms resemble those of peptic ulcer, although no ulcer is detectable. It is caused by disturbance in moter function of alimentary tract.
(6) PEPTIC ULCER
Since pepsin, is a protein digesting enzyme, it may digest the stomach wall, the first part of duodenum or rarely lower part of oesophagus where stomach juices frequently refluxes. This condition is called Peptic Ulcers.
- GASTRIC ULCERS
- DUODENAL ULCERS
- Excessive secretion of acid and pepsin.
- It may be hereditary.
- Psychogenic factors.
Complications of peptic ulcers are perforation, haemorrhage and obstruction. INVESTIGATIONS
1. Acid output of stomach is studied.
2. Ulcers cavity may be shown up on X-rays after ingestion of insoluble barium sulphate (Barium meal).
3. It may be seen using optical instrument passed down through oesophagus (endoscopy)
(7) FOOD POISONING
Also called GASTRO-ENTRITIS
By bacteria, virus, protozoa. ‘Salmonella’ species are very common.
Allergy, irritating food or drink.
Vomiting and diarrhoea within 48 hours.
(8) MAL NUTRITION
Any disorder of nutrition due to unbalanced diet or due to defective assimilation or utilization of foods.
An organism may be deficient or may receives excess of one or more nutrients for a long period of time.
Deficiency is known as under-nutrition. It is most common problem of under developed countries.
Excess is known as over-nutrition. Obesity with heart problems and reduced life expactency are its symptoms and are more common in developed countries.
(9) OBESITY AND OVER WEIGHT
Increase in body weight beyond the limitation of skeletal and physical need as the result of accumulation (excessive) of fat in the body.
It is the most common nutritional disorder. It is most prevalent in middle age. It may be hereditary or family tendency over weight results in rate of mortality.
(10) ANOREXIA NERVOSA
Loss or lack of appetite for food is called Anorexia.
An eating disorder affecting young females, characterized by refusal to maintain a normal minimal body weight, intence fear of gaining body weight, intense fear of gaining weight or becoming obese. Sometimes accompanied by spontaneous or induced vomiting.
(11) BULIMIA NERVOSA
Exclusively found in women and the age of onset is slightly older than for anorexia.
Recurrent episodes (bouts) of binge (uncontrolled) eating. Lack of self control over eating during binges.
Attacks occur twice a week and involve rich foods such as cakes and chocolates and dairy products