The Digestive System.

The digestive system in mammals is the only system which has two openings.
Enterology is the study of the Digestive system.

Parts & Origin of the Digestive System:
The digestive system is developmentally divided into: a) The G.I Tract. b) Associated Organs. ]

The G.I. Tract: begins from the oral cavity and ends at the anus. It is further divided into foregut, midgut and hindgut.
The foregut consists of the oesophagus, stomach and the part of duodenum upto the opening of the common bile duct (c.b.d.). The parts of the foregut are supplied by the coeliac artery. The oral cavity and the pharynx are dealt separately as pharyngeal gut.
The midgut is the part of the g.i.t. from the opening of the c.b.d. in the duodenum up till the right two-thirds of the transverse colon through the jejunum, ileum, caecum, appendix & ascending colon. The midgut is supplied by the superior mesenteric artery.
The hindgut extends from the left one-third of the colon up to the anal opening. This is supplied by the inferior mesenteric artery.

The Associated Organs of the digestive system are mainly the liver and the pancreas.

The G.I. Tract is ectodermal in origin exept the epithelial lining which is endodermal.
The parenchyma of the liver and pancreas are endodermal.

Mouth & Buccal Cavity:
The buccal cavity is lined by stratified squamous epithelium (non - keratinised).
The roof of the buccal cavity is formed by the palate. The palate is divided into two parts: the hard palate & the soft palate.
The hard palate is furthe divided into maxillary, pre maxillary & the palatine part. The hard palate is anterior to the soft palate.

The teeth are found in bothe jaws i.e. the maxilla & the mandible. It consists of the i) enamel ii) dentine iii) cement iv) Pulp cavity & v) periodontal membrane: crown, neck & root.




The dental fromula is designed to numerically denote the number & type of teeth in each half of each jaw. It is expressed as a fraction; the numerator denotes the maxilla & the denominator the mandible. The thousands' digit is the incisior, the hundreds' is the canine & so on. The dental formula in man is different for milk teeth and permanent teeth.
Milk / lacteal / deciduous teeth : 2102 / 2102; means each half of both the jaws contain 2 incisors, 1 canine, 0 premolars & 1 molar. The total milk teeth is 20.
Permanent teeth: 2123 / 2123. The total no. is 32.
In rabbits, the formula is 2033 / 1023.
An artificial set of teeth has 28 teeth.

The tongue is a muscular and movable organ. [ The diaphragm and the tongue are muscles with no skeletal attachment.]
It functions in: i) mastication ii) taste iii) phonation - i.e. speech.
The taste buds are the organ for gustation. They are principally of 4 types:
i) Circumvallate: found in the posterior aspect of tongue, arranged in a 'V' shape
ii) Filiform: no taste function, thread like.
iii) Folate: absent in man, present in rabbits.
iv) fungiform: found on the tipd and margins of the tongue.
The motor nerve supply is by the hypoglossal nerve - cranial nerve no xii.
The sensory (taste) is by the facial (vii) & the glosso-pharyngeal (ix) nerves.

The Pharynx provides the connection between the oral cavity and the oesophagus. In man it extends from the base of the skull to C-6 vertebra, about 12 cms. it is shorter in rabbits. It is a common pathway for food and for air.
It is divided into:
i) Nasopharynx : It is lined by ciliated columnar epithelium. It has the openings of the internal nares & the Eustachian tube.
ii) Oropharynx: It is lined by stratified squamous epithelium. The Waldeyer's ring i.e. formed by the palatine & pharyngeal tonsils
is present here.
iii) Laryngopharynx: It is also lined by stratifies squamous epithelium. It is continuous with the epiglottis.

Histology of the gut:

Microscopically the gut is generally divided into four distinct layers.
i) Mucosal lamina propria is the innermost layer. It consists of the columnar epithelium that lines the inside of the gut.
ii) Submucosa is the connective tissue layer next to the mucosa. It has blood vessels & nerves, the Meisseners' plexus; & the brunners glands.
iii) The muscularis layer next to the submucosa has smooth muscles arranged in two common patterns i.e. longitudinal & circular. The stomach has an exception with an innermost oblique layer also. The Myenteric & the Auerbach's plexus of nerves are located between the muscle layers.
iv) The serosa is continuous with the visceral peritoneum. The squamous epithelium is absent in esophagus & lower distal rectum & incomplete in the duodenum.

The esophagus is a 25 cm x 2 cm long muscular tube that connects the pharynx & the stomach. It has a thickest submucosa.
The upper one third is lined by stratified squamous epithelium & has voluntary muscles.
The middle one third shows transition between squamous & columnar epi.; & voluntary & involuntary muscles.
The lower one third is lined by columnar epi and has involuntary muscles.
The esophagus communicates with the stomach through the cardiac sphincter. The food bolus is moved forward by peristalsis. The peristaltic wave passes at the rate of 0.2cm - 0.5cm/sec.
The digestive glands are absent in the esophagus. The esophagus is under frequent attak from the acidic reflux from the stomach and is an important cause of esophagitis & Barett's Esophagus.

The stomach is J shaped, 25 cm long & wide bag like structure. The interior of the stomach is thrown into numerous folds of the mucosa. This is called rugae. It helps in expansion of the stomach after meals. The muscle layer is most developed in the stomach.
The mucosa contains numerous gastric glands, about 35 million in number.
The stomach is divided into 4 parts: cardiac, fundus, body & pyloric antrum. Peristalsis occurs at 2-3 min intervals.

The small intestine is divided into duodenum, jejunum & ileum. The total length in humans is around 5 metres. The S.I. has villi which are finger shaped projections of the mucosa which form the unit of absorption. Plica circularies are numerous circular infoldings of the gut throughout the S.I. The villi & microvilli with plicae increase absorption by around 600%.

The duodenum is 25 cm long. It is C or U shaped. At around the mid length, the duodenum has an opening for the common bile duct & the pancreatic duct at the Ampulla of Vater, which is guarded by the sphincter of Oddi. Villi are fewer and leaf like. Submucosal Brunner's glands which are simple branched tubular glands are present. They have goblet cells and secrete mucous. They open in the crypts of Leiberkuhn. Crypts of Leiberkuhn have 3 distinct secretions. i) glands for succus entericus / intestinal juice.
ii) Paneth cells for lysozyme.
iii) Argentaffin cells for serotonin (peristaltic stimulator), secretin (pancreas), cholecystokinin (gall bladder).

The jejunum is 2 cm long, rich in digestive glands.In comparision with ileum, it has a thicker wall, wider lumen, more vascular, and more abundant villi. Payers patches are absent. Villi have microvilli that shows brush border appearance. They show swaying type movement (segmental)
The jejunum leads to ileum. The distinguishing feature is the presence of Payers patches, i.e. lymphatic tissue found in the submucosa. might be a solitary nodule or in masses. These get infected during typhoid fever.

Motility of the gut:

The gut wall has circular and longitudinal muscles along with nerve plexus and hormones like gastrin & motilin. These help in moving the gut, essential for propelling, churning, mixing , absroption & egestion of food. The gut shows 3 types of movements.
i) Peristalsis: Wave of contractions directed proximo-distally. Occurs in proximal esophagus, stomach & intestines both L & S. Propels food forward at the rate of 0.5 - 2 cm /s. In stomach peristalsis wave occurs every 2-3 min.
ii) Segmentation movement: Stretches of the alimentary canal swell up & undergo concenteric contractions. Mixes food with digestive enzymes.
iii) Pendular: It is to and fro circular contractions, resulting in churning and mixing of food.

In rabbits the distal end of the ileum expands to form the sacculus rotundus. Fron the junction of the sacculus rotundus & large intestine arises 40 - 50 cm long tube called caecum. The largest part of the caecum is about 15 cm long called vermiform appendix. In humans the vermiform appendix is vestigeal and has lymphoid tissue. The bacretia in sacculus rotundus & caecum help in digestion of cellulose.

The large intestine: It is about 1.5 metres in humans, and around 1 m in rabbits. It begins with the caecum where the ileum ends. The patterned representation is caecum -->ascending colon-->transverse colon-->descending colon-->sigmoid / pelvic colon-->rectum-->anus.
The longitudinal muscles are not continuous in the L.I, instead they are srranged in 3 distinct bands called taenia coli. These bands are somewhat shorter than that of the intestine and thus account for the sacculated appearance called haustrations. Presence of packets of peritoneum filled with fat called appendices epiploicae. L.I. lacks villi and plicae. The sigmoid colon stores faeces.

The rectum is 75 cm long in rabbits and 13-15 cm in man. In rabbits it opens directly outside through anus which has one anal sphincter. In man there is a 4 cm long anal canal with two sphincters, the internal ( involuntary, autonomic control) & external (voluntary) anal sphincter.

The liver:

The liver is the largest organ of the human body. It is a multi-lobulated structure, in man 4 lobes :- right, left, caudate & quadrate. The right and left lobes are separated by the falciform ligament. Microscopically it is composed of hexagonal lobules, surrounded by a connective tissue sheath known as Glisson's capsule.






Functions of the liver:
i)Sectrtion of bile: non - enzymatic digestive juice.
ii) Regulates the blood sugar level. Moderator organ for homeostatic control of blood sugar by the process of glycogenesis {excess insulin - glucose --->glycogen}
Glycogenolysis {glucagon - glycogen ---> glucose }

Gluconeogenesis.
iii)Also controls lipogenesis. Excess glucose and amino acids are transformed into fats and stored in the liver cell.
iv) Excess and harmful amino acids of blood is metabolised into toxic NH3, which is again metabolised into useful ketoacids by the process of deamination in the presence of oxidase enzyme.
v) Ornithine cycle - Toxic NH3 + CO2 ------------>Urea {detoxified - excreted in urine}
vi) Haemoglobin of dead R.B.C.:-
Haemoglobin---------->Globinm + Haeme ---------> i. iron
ii. non iron---------->biliverdin-------->bilirubin -------> i.urine
ii. stool
iii. Plasma.
vii) Produces an anti coagulant heteropoly saccharide called Heparin.
viii) Prothrombin and fibrinogen are pro coagulatory proteins synth. by liver.
ix) Erythropoesis ( formation of R.B.C) in embryo.
x) As hemolytic organ
xi) Kupffer cells are liver macrophages.
xii) Site of synthesis of vit A from carotenes of carrot in the presence of enzyme carotenase.
xiii) Stores Fe & Cu as ferritin & ceruloplasmin.
xiv) Site of heat production - metabolic center
xv) Emergency storage of water.
xvi) 2nd reservoir of blood.

Physiology of Digestion
Digestion in mammals begins from the mouth. Saliva produced from the salivary glands aids this.
The saliva contains 99% water. Salivary amylase & lingual lipase are the enzymes in saliva.
Almost 1 lit. of saliva is produced each day in humans. Its pH ranges from 6.5-7.5.
Starch, sugar, glycogen ---------------------------------> Maltose, Isomaltose & limit dextrins.
(Enzyme:)Ptyalin
--> 3-4% carbohydrates
--> 30-40% starch
--> acts best in slightly
acidic pH
Stomach has specialised cells producing various enzymes and acid that helps digestion. These are:
i) Oxyntic cells / Parietal cells: These produce HCl which converts pepsinogen into pepsin. It is also antibacterial in nature. Its pH is 1.5 - 3.5. This makes the content of the stomach acidic in nature, and in reflux diseases where the gastric contents are ejected into the esophagus it is this acidity that insults the esophagus the maximum because the protective mucus is not produced.
ii) Chief / Peptic or zygomatic cells : These produce pepsinogen, which is the precursor of pepsin, the chief enzyme for protein digestion.
iii) Mucous neck cells: These produce the gastro-protective mucous.
iv) Pyloric G cells / Argentaffin cells: These produce gastrin, which incereases HCl & gastric juice secretion.

Protein Digestion:
Pepsinogen -------------------> Pepsin
(HCl )

Proteins -----------------------> Proteoses & Peptones
(Pepsin)

Milk protein in children:
Prorennin ---------------------> Rennin
(Pepsin )

Caesin ------------------------> Paracaesin
(Rennin)

Ca++ + Paracaesin -------------------------> Calcium Paracaesinate.
(Rennin )

Cal. Paracaesinate -----------------------> Proteoses & Peptones
(Pepsin)
# A small amount of gastric lipase is produced in children.

The intestine:
The pancreatic juice & the intestinal juices aid digestion in the intestine.
The pancreatic juice helps in the digestion of proteins, carbohydrates, fats & nucleic acids. 2-3 lit. of it is produced each day. It has a pH of 7.5 - 8.5.

# Protein Digestion: Trypsin in an endopeptide that is found in all animals. It breaks protein into small peptides but cannot digest keratin.
Enterokinase
Trypsinogen --------------------------> Trypsin
(Endopeptidase )

Trypsinogen --------------------------> Trypsin [autocatalysis]
(Trypsin )

Proteins, peptides, proteoses & ---------------------------------> smaller peptides
(Peptones Trypsin )

Fibrinogen ---------------------------------> Fibrin {blood clot}
(Trypsin )

#Chymotrypsin is a milk coagulating enzyme.
(Trypsin )
Chymotrypsinogen --------------------> Chymotrypsin

(Trypsin )
Proelastase ----------------------> Elastase

Procarboxypeptidase ----------------------> Carboxypeptidase.

(Carboxypeptidases )
Peptides ----------------------------------------------> smaller peptide chain + amino acids.

Fat Digestion: Steapsin / Pancreatic lipase.
Emulsified fats ----------------------------------> Tri, di & monoglycerides
Tri, Di & monoglycerides------------------------------------> fatty acids + glycerol

Carbohydrate digestion.
(sucrase )
Sucrose---------------------------> glucose + fructose

(Maltase )
Maltose --------------------------> glucose + glucose

(Lactase )
Lactose --------------------------> glucose + galactose


Absorption:
Majority of the absorption process takes place in the small intestine. Only water, alchohol & some drugs like aspirin are absorbed from stomach. Two processes are recognised:
i) Passive absorption : A slow physical process in which glucose and amino acids are absorbed by simple diffusion along the concenteration gradient. Water absorption follows by osmosis. No energy is required and it is a bidirectional process.
Fructose and mannose are absorbed with a carrier protein molecule (permease / translocase). This is also called facilitated diffusion.
ii) Active absorption: Na+, glucose, galactose and amino acids absorbed with the help pf energy provided by ATP, hydrolysis by Na+-K+ pump. Unidirectional, requires carrier molecule.
Absorption of fats: The end products of fat digestion are monoglycerides, fatty acids and glycerol. Glycerol is water soluble, so it is directly absorbed by the mucosal cells of the intestine.
In the intestinal lumen, fatty acids, monoglycerides & bile salts aggregate to form water soluble molecules called micelles. (0.03-4nm diam.) Each micelle has bile salts, in its outer part & the hydrophobic fatty acids and cholesterol form the core. this action of bile salts is called HYDROTROPHIC ACTION. Micelles are taken up by pinocytosis.
In the mucosal cell, fatty acids & monoglycerides are again formed into triglycerides.
Neutral fat so formed are surrounded by a coat of B-lipoprotein to form water soluble globules called chylomicrons.

Short chain fatty acids ( less than 14 carbon atoms) are directly absorbed into portal circulation. Bile salts are also extracted by liver through portal circulation. This is the entero-hepatic circulation of bile salts.

Past questions in I.O.M entrance from digestive system:
1) A dental plaque tends to form rapidly: (2004)
a.while eating b.just before brushing teeth c.during sleep d. at a constant rate.
Ans: b.

2) Which of the following is not produced in man: (1998)
a. Somatostatin b.serotonin c. chymosin d. adrenaline.
Ans: c

3) The pH of saliva is about: (1995)
a. 1.5 b. 3.5 c.6.8 d.7.4
Ans: c

4) E.coli is the normal flora of: ( 1994/95)
a. colon b. intestine c.ileum d.stomach
Ans: a

5) Salivary gland which opens in the floor of the mouth on either side of the frenulum of tongue is: (1995)
a. Parotid b. Sub-mandibular c. sub-lingual d. none
Ans: b

6) Which of the following is a mucus generating factor: (1995)
a. excessive heat and cold b. corosion by chemical
c. irritant gas chemicals d. all of the above
Ans: d

7) Which of the following organ is non functional in humans: (1993)
a.Pinna b. vermiform appendix c. wicdom teeth d. all
Ans: d

8) Jaundice is caused by malfunction of: (1999)
a. Pancreas b. liver c. kidneys d. spleen
Ans: b

9) One feels drowsy after meal because ( 1993)
a. Increased b.p. b. decreased blood supply to the brain
c. increased body wt. d. reduced blood supply to the intestine
Ans: b

10) Iron is absorbed from: (2005)
a. ileum b. jejunum c. duodenum d. caecum
Ans c

11) The dental formula for permanent teeth is: (1993)
a) 2102 b.2123 c. 2321 d. 2033

12) Gastric acid secretion is stimulated by: (1993)
a. food in the mouth b. histamine c. gastrin d. all


13) Brunner's glands are characteristic of (1993)
a. stomach b. duodenum c. ileum d. jejunum.
Ans: b

14) The detoxification/ metabolism of drugs takes place in the: (1993 ,2003)
a. liver b. blood c. brain d. kidneys
Ans: a

15) Which of the following is not the function of liver: (2005)
a. Bile formation b. dogestion of proteins c. urea formation d. plasma protein synthesis.

16) The amount of bile secreted per day os about: (1994)
a. 500 ml b. 100 ml c. 1000 ml d. 10 ml
Ans a

17) Gall bladder and: (1997)
a. Insulin b. Gastrin c. cholecystokinin d. serotonin
Ans: c

18) Water is mainly absorbed from (1996/ 1998)
a. Duodenum b. Small intestine c. stomach d. colon
ans: b

19) The organ known as reservoir of stool is: (1997)
a. sigmoid colon b. rectum c. caecum d. vermiform appendix.
Ans: a

20)The organ rudimentary in man but functional in animals is: (1995)
a.ileum b. caecum c. appendix d. anus
ans: c

21) Vermiform appendix in humans may play a part in:
a. digestion b. hormone synthesis c. immune system d.enzyme secretion
Ans c

22) Cholecystectomy is:
a. surgical removal of gall bladder b. gall bladder stones
c. gall bladder cancer d. gall bladder inflammation
Ans: a

23) A congenital anomaly in which upper lip has a distinct cleft is called:
a. hare lip b. mouse lip c. rat lip d. rodent lip
Ans a

24) Maximum energy is obtained from the oxidation of:
a. Fat b. Carbohydrate c. Protein d. Alchohol
Ans a

25) Which of the following is non functional in man:
a. 3rd molar b. 2nd molar c. incisor d. premolar]
Ans:a

26) Alchohol absorption takes place mainly in:
a. ileum b. duodenum c. liver d. stomach
Ans d

27) The longest part of alimentary canal is:
a. esophagus b. stomach c. ileum d. colon
Ans c

28) Laxatives are used for the treatment of:
a. diarrhoea b. constipation c. piles d. vomiting
Ans b

29) Which of the following is not the part of large gut:
a. ileum b. rectum c.caecum d. vermiform appendix
Ans a

30) The first permanent tooth erupts at the age of:
a. 2 yrs b. 4 yrs c. 6 yrs d. 7 yrs
Ans c

31)The number of molars in an adult aged 40 yrs is:
a. 10 b. 12 c 14 d. none
Ans b

32)Dental carries is caused by:
a.bacteria b. viruses c. fungus d. food particles
Ans a

33)In liver energy is stored in the form of:
a. glucose b. glycogen c. sucrose d. fructose
Ans a

34) The sodium absorption in intestine is mainly by the process of :
a. active transport b. diffusion c. passive transport d. facilitated diffusion
Ans:

35) In which of these disease Vit B12 is not absorbed from the intestine:
a. Iron def. anemia b. Pernicious anemia c. thallasemia d. Spherocytosis

36) Starch is digested by:
a. Maltose b. Invertase c. Diastase d. sucrase
Ans c.