3 Biology -- Human Biology: Digestive System

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Anatomy and histology 

Alimentary canal 

It is a long partially coiled tubular structure extending from mouth till anus  where food we intake pass all along to digest and absorb  and remaining forms feces to be egested.

 It is 8 to 10 M long  having various girths in various regions.

 The whole canal can be studied under following headings : 

  1. Mouth and buccal cavity .

  2. Pharynx 

  3. Oesophagus / esophagus 

  4. Stomach 

  5. Small intestine 

  6. Large intestine 

  7. Anal canal 

Mouth and buccal cavity 

Mouth : 

  • Anterior most opening of alimentary canal is surrounded by lips dorsally and ventrally  and cheeks laterally.

  • Orbicularis oris is muscle surrounding  mouth.

  • Median region depression is called philitrum.

  • Lips are with mucosal lining  and separates it from skin by  vermillion border.

  •  Going a little inside ,vestibule  are narrow vertical recesses or space in between outer lips  and inner gums /gingiva .

Buccal cavity : 

  • Cavity  with teeth in upper and lower zones called jaws . The roof of this cavity is called a palate   that separates buccal with nasal cavity .

  • Palate possess two regions:

  1. Hard palate : anterior zone supported by palatine and maxillary bone.

  2. Soft palate :  not supported by bone and made up of cles and connective tissues. This terminates  into an lymphoid structure called uvula / velum palati .

  •  Two arches are in the cavity called palatoglossal and palatopharyngeal.

  Palatoglossal : lateral to tongue .

  Palatopharyngeal : behind palate till pharynx; possess  palatine tonsils.

  • Tongue : located on the floor of buccal cavity  which is a muscular, glandular and sensory organ .

 Three parts of the tongue : 

  • Root : posterior most region  that is attached to unsocial hyoid bone  through many muscles.

  • Body :  middle  larger part .

  • Tip  :  anterior ,freely  movable region .

 Tongue is attached to the cavity by lingual frenulum. 

 Ankyloglossia :  when lingual frenulum is short and tied making speech impossible in just born babies.

 They are removed by surgical cuts.

Tongue possess papillae which indeed possess  nerve endings or taste buds.

         Papillaes in tongue :

  •   Vallate /circumvallate :  formed as an inverted V shape, 10 - 12 in number in  posterior zones of the tongue which is mostly 1/3rd back.

  • Fungiform   :  mushroom-like  scattered in 2/3rd of body.

  • Filiform : smallest papillae scattered in 2/3rd zone  with fungiform.

  • Foliate : absent  or vestigial in the human body . but is seen in rabbits.

 Vallate, foliate  and fungiform are for tastes as they possess buds while filiform are just with nerve endings so more than for taste they are for sensation.

 Tastes in tongue : 

 Sour :  by  the   sides of tongue .

 Sweet, salty  and umami (MSG :Monosodiumglucamate’s taste ): by tip of tongue

Bitter  : by  posterior tongue.

Glands in tongue : 

  1. Anterior lingual / glands of Blandis and Nuhn : It is located near the apex of the tongue . Ducts open on the ventral surface of the tongue near  frenulum . approx. 5 small ducts are there .

  2. Posterior  lingual glands : 

  • Webner glands : pure mucous , located lateral and posterior  to vallate papillae .

 Their ducts open into the dorsal surface of the tongue .

  •        Ebner gland : pure serous gland  ,located  in between muscles of tongue and foliate papillae or below circumvallate  and open into the trough  of the vallate papilla.

   Functions of tongue : 

  • It helps in locating food position during mastication.

  •  For Speech  ability.      

  • Glandular secretion  , taste perception  and sensation role.    


  •  Teeth :   singular tooth  is a calcified conical structure found in jaws of  mainly gnathostomes. 

 Origin : ecto-mesodermal  (ecto: enamel , meso : rest of dentine pulp…)

 Note : Hardest tissue in human: dentine 

              Hardest substance in human : Enamel 

Characters of mammalian teeth : 

  •  Heterodont : more than one type of teeth morphologically.

  • Diphyodont : two sets of teeth in entire life. Which are : 

  • Milk teeth :   erupts at 6 months of birth, finishes to erupt in 2 years and gets replaced within 12 years old.

 Dental formula : arrangement of teeth in a half of upper or lower zone .

 For milk teeth it is : 2102  ( no premolars ).

  •   Permanent teeth :   replacement begins at 6 and gets completed  at 12   except wisdom or 3rd molar which may or may not appear and if it does it will at  17-  25 age.  

 Dental formula : 2123

 Hence , it is inferred all are diphyodont but premolars and 3rd molars. 

  •        Thecodont : embedded in sockets  or  alveoli of the jaw .

  • Bunodont : having cusps   i.e projected  parts in crown  except incisors . 

Structure of  a tooth 

      Externally it can be divided into 3 parts : 

  1. Crown : outer exposed part of teeth out of gingiva , whitish and shining in colour.

  2.  Neck : constricted part covered by gingiva.

  3. Root :  portion of teeth found generally embedded in the socket of jaw.

           Mostly a single root is present in teeth.    

                     Composition of teeth  : 

  1. Enamel :  it is the outermost exxposed shiny, whitish part covering the crown which is the hardest of all in the body.

          It  is a non living component secreted by  ameloblast cells from the pulp cavity via pulp canal. The hardness of enamel is due to the presence of fluorides.

  1. Dentine : it is the hardest tissue of the body  even more than bones .They are secreted by odontoblast cells.  Mainly it’s 70% inorganic and 30% organic substituents. Hydroxyapatite is the main constituent of this.  

  2. Pulp cavity : innermost composition with jelly like matrix supplied by blood vessels.

 Root Canal Treatment (RCT) : This in dentistry is a way to clean out this composition; PULP CAVITY.

 Cementum :  this is a specialized calcified substance that covers roots.

 In between cementum and jaw bone, there is connective tissue called periodontal ligament . This is important for movement during mastication.


Root canal’s blood vessels enter through apical foramen .

   Functions of teeth

  • Incisors : cisled, acuspid shaped so for cutting and biting 

  • Canine : pointed/dagger shaped so, for tearing flesh .

  • Premolar and molar :  plane crown so,  in grinding and chewing.

  • Aids in speech also.

Extra information: 

Tusk of an elephant : modified incisors

Tusk of  walrus :  modified canines

Fangs of teeth : modified maxillary teeth

 Pharynx : 

  • musculo - membranous tube lies behind the nasal cavity , oral cavity and larynx.

  •  From upper skull up to cricoid cartilage level  which is up to 6th cervical vertebrae.

  • It is 12.5 cm in length.

  • It  is lined by stratified squamous epithelium.

  • Overall , it is  conical  in shape.

          Parts  of conical pharynx :

  1.  Nasopharynx :  from upper  skull to uvula . The eustachian tube ends here.Cells here are pseudo stratified...Around here ,we have  Waldeyer's ring  which is ;

 Ring of tonsils  which is of adeno tonsils in pharynx wall, tubal tonsils, palatine tonsils ; (this is only in oropharynx, others all belong here) and lingual tonsils.

  1. Oropharynx : from uvula  to upper portion of epiglottis .  cells here are  non keratinised squamous epithelium.

  2. Laryngo pharynx : from upper epiglottis  to level of 6th cervical vertebrae. Here it  is also non keratinised squamous epithelium.

Posterior communicates  with 2 opening glottis and gullet .glottis is guarded by epiglottis and gullet is by sphincters.

 This is a common passage for food and air.

Oesophagus :

  • Lies back of the trachea  in the upper zone  and  back of the heart in the lower region.

  • Collapsible part lining of muscular region.

  • Lies in level of c6 to t12 level.

  • Narrowest tube of alimentary canal which is 25 cm long.

  • Possess  3 sphincters two at ends and one at middle.

  • Upper one third is skeletal muscle while the rest is smooth muscles which are involuntary.


 Stomach :     

  •  Most dilated sac  of J shaped is located covering the epigastric, left hypochondriac and  umbilical region ; which is just below the diaphragm on the left side of abdomen.

  • This is also 25 cm long.

  • Parts of stomach :      

  1. Cardia : small  part where bollus enter through cardiac sphincter.

  2. Fundus :Above orifice and is of dome shape and mainly contains gases this portion is basically absent in frogs.

  3. Body: it is the large portion of the stomach where food is temporarily stored and has a lot of  rugae.

  4. Pylorus :it is a funnel shaped portion connected with  duodenum via pyloric sphincter.  .

  • Curves and omentum  in stomach : 

 Lesser curvature: it is the medial small concave surface of the stomach .

Greater curvature :  this is a larger outer convex surface of the stomach.

 lesser omentum :it is mesentery  from lesser curvature connecting stomach with liver.

 Bigger omentum :it is mesentery from Greater curvature joining stomach to  coelom wall.

 This is also called policeman of the abdomen as it localises infection in abdomen .

The main purpose of these structures is insulation by storing fat and helding the structure firmly and preventing sticking of visceral and parietal peritoneum.

Cells in mucosa  of  stomach

Following are the cells in the stomach:

  • Parietal/ oxyntic cells : Creates hydrochloric acid and Castle intrinsic factor this factor protects vitamin B12 .so in efficiency of this factor eventually lead to Pernicious  anaemia.

  • Peptic/  chief/zymogen  cells : they  secrete in-active pepsin term as pepsinogen.

  •  G cells / argentaffin/enteroendocrine cell : they are named so because they take colour of Silver and produce gastrin and ghrelin secretion.

  • Mucus/ Goblet cells :  as the name suggests, they secrete mucus for Lubrication.

Functions Of stomach

  • Temporary stories of food

  •  mechanical and chemical digestion

  •  HCL plays a vital role in creating an acidic medium to activate pepsinogen Pro rennin in babies and kill microorganisms .

     Small  intestine 

  • Long coiled tube extending from pylorus to ileum of about  3 M in an alive person and double of that in cadaver.

 Can be divided into 3 parts :

  1.  Duodenum : first portion that is widest among the other two .

                        25 cm long C Shaped extending from pylorus till                   duodeno jejunal flexure.

  1. Jejunum :  empty region after death ; 2.5 m long in dead; in between duodenum and ileum

  2. Ileum :    3.5 m long; from end of jejunum to ileocecal junction.

            Mucosal cells of intestine  in crypts of lieberkuhn : 

  • Mucous cells : secretes mucous 

  • Epithelial cells : secretes enzymes like trypsinogen

  • Paneth cells: secretes lysozyme 

  • Entero endocrine / argentaffin cells:also in stomach mucosa; secretes hormones like secretin, duocrinin

  • Stem cells :  undifferentiated cells that supplements dying cells.

Large intestine : 

  • Wide  tube extending from ileocaecal junction till before anus .

  • 1.5 m long; shorter than small intestine but named large because of its  large cross section.

  Divided into following parts : 

  • Caecum: also called blind pouch with  a semi vestigial vermiform  appendix as it’s immunological aid is being discovered . But this is functional  in case of rabbits ; for cellulose digestion via bacterias.  Here in the human gut   3 to 10 times more bacteria are present than total cells in our body.  For instance : Bacteroides fragilis.

  • Colon :  question mark ?  shaped . they possess taenia coli i.e. 3 muscle bands of muscle in wall of colon. The walls also are with sacculations called haustra

Epiploic appendages are small pouches of the peritoneum filled with fat .

4 sub parts : 

  • Ascending colon : from  caecum towards liver.

  • Descending colon :  moves away fro spleen.

  • Transverse colon :  straight  joining both above colons.

  • Sigmoid colon : U shaped structure connecting to rectum.

  • Rectum:  distal portion  from recto sigmoid to anal canal. 12 cm long for temporary storage of feces, muscles here contract under reflex.  

 Anal canal : 

  •  Terminal part of GI TRACT .

  • 3.8 cm long  with  internal involuntary and external voluntary  sphincters

  • Bursting of veins due to inflammation  by bacteria  so that stool is seen in stools called Piles.

You can refer to following video too: 


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