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Review of the human Digestive System.

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									                          THE DIGESTIVE SYSTEM
Digestive system of (alimentary canal) of man is a long (6-9 m) tube of variable diameter. It has the following parts.


MOUTH
      It is a transverse slit present below the nose and is bound by two movable lips covered externally by skin &
      internally by mucous membrane.
      Upper lip has a tubercle in the middle a groove or philtrum above & two ridges on the sides.


VESTIBULE
      It is a narrow space between gums & lips in front & checks on the sides.
      Lips are connected with gums by superior & inferior labial frenula.


BUCCAL CAVITY
      It is large space in between the upper immovable jaw and lower movable jaw.
      The roof is formed by palate while the floor consists of throat with tongue.
      Hard palate (in front) bears transverse ridges called rugae.
      Soft palate (behind) ends in a flap or Uvula for closing internal nares.
      Tongue is a muscular, protrusible, mobile flattened soft structure which is free in front and attached basally by
      Frenulum that help in the chewing, tasting and swallowing of food.
      Taste areas of tongue: Sweet (tip) Salt(tip and sides) Sour(sides) Bitter(base).
      Mucous glands occur over the tongue are called weber’s glands.
      The upper surface bears four types of papillae– vallate, fungiform, foliate and filiform.
      Teeth : In man there are 16 teeth each Jaw. There are four different types of teeth.
      A tooth has three parts. (i) Crown (exposed part) (ii) Neck(surrounded by gums) (iii) Root(embeded in socket).
      Tooth is mainly made up of ivory like substance called dentine.
      In crown part of dentine covered by enamel (hardest substance of body).
      Cement and peri dontal ligament fixed the tooth in bony socket.
      Pulp cavity : Cavity of tooth with connective tissue, nerve fibers, and blood vessels.
In man two sets of teeth appears during life. In humans, the first set or milk teeth consists of 20 teeth. These are
called deciduous or temporary teeth. They are completely replaced by the permanent teeth by about 12 years of
age.
   DENTAL FORMULA OF HUAMN ADULT — DENTAL FORMULA OF HUMAN CHILD
           I C Pm M                   I C Pm M
           2, 1, 2 , 3                2, 1, 0 , 2
                       ,
           2, 1, 2, 3                        ,
                                      2, 1, 0 2
   (Dental formula is the representation of number of teeth in each upper and lower half jaw)
   Teeth are heterodont (different types), diphyodont (temporary and permanent) bunodont (low cusps) & thecodont
   (fixed is sockets).
   Teeth help in cutting, tearing and crushing of food.
   Salivary glands are 3pairs – parotid (below ears), submaxillary (submandibular at angles of lower jaw) and
   sublingual (below tongue).
   They secrete 1-1.5 litres of saliva per day which is poured into buccal cavity for lubrication, mastication and
   partial carbohydrate digestion.

PHARYNX
   The oral cavity passes into a pharynx. Which has three parts nasopharynx, oropharynx and laryngopharynx.
   The masticated food, which has been partially digested with the help of ptyalin (an enzyme contained in saliva),
   passes through the pharynx, and then goes down into the stomach through a tube called oesophagus.
   Some tonsils of lymphoid tissues also present at the junction of buccal cavity and pharynx.

OESOPHAGUS
   It is a muscular tube of 25-40 cm length that connects pharynx with stomach. It corresponds to the neck region of
   man.
   It has mucous glands for lubrication.
   The muscular walls of the oesophagus move in a rhythmic wavelike manner, which conveys the food
   downwards. This muscular movement is called peristalsis.
   A ring of muscle, the oesophageal sphincter, controls the opening of oesophagus into the stomach.
   Its thoracic part is without serosa.

STOMACH
   Located below the diaphragm & is saclike, J shaped structure.
   It helps in mechanical churning and chemical digestion of food. It also acts as food reservoir.
   Stomach is differentiated into four parts- cardiac part (present near the heart), fundus (extends superiorly, from
   the cardiac part) filled with air and gases, body (middle dome shaped part), pyloric part (distal part of
   stomach),which is differentiated into three parts antrum, canal and sphincter.Pyloric canal and pyloric sphincter
   guards the opening between the stomach and duodenum.
SMALL INTESTINE
   Small intestine is about 6 to 7 metres in length & 2.5 cm in diameter, consists of three parts :
   (a)    Duodenum : It is the first part (proximal) of intestine and is about 25 cm (10 inches) long. It is
          C shaped.
   It has ampulla of vater which receives both bile duct from liver and main pancreatic duct from pancreas and
   whose opening in duodenum is guarded by sphincter of oddi.
   Brunner’s glands present mainly in duodenum secrete mucus and bicarbonate ions etc.
  (b) Jejunum : It is about 1 to 1.5 metres long and narrower than duodenum, with abundant plicae(circular
      folds), villi & digestive gland. It is the seat of major digestive activity.
 (c) Ileum : It is much longer about 3 to 3.5 metres long & 2-3 cm in diameter in adult man and more coiled. It
      is distal in position. Its distal end is dilated into a small bulb like structure sacculus rotundus. Both the
      jejunum and ileum are greatly coiled and are suspended by mesentery.




It is characterised by aggregated lymphoid follicles called Peyer’s patches and finger like villi.
Bases of villi possess crypts of Liberkuhn where intestinal glands are located, which secretes intestinal juice.
Fine lymphatic channels (Lacteals) are extended up to villi for absorption and transportation of lipids.
Two nerve plexuses of Autonomic nervous system are :
(a)   Auerbachs plexuses – controls movement of food in gut.
(b)   Meissners plexuses – control movement of villi.
HORMONAL CONTROL OF ENZYME SECRETION




LARGE INTESTINE
   It is wider than small intestine, about 1.5 to 1.8 metres long. It is divisible into 3 parts Caecum, colon & rectum.
   Caecum : It is a small blind pouch situated at the junction of ileum and colon.
   Its blind end is produced into vermiform appendix. In man it is a vestigial organ.
   Internally, there is an ileocaecal valve. Here wall of ileum is thickened & muscular to function as ileocaecal
   sphincter. It slows down the passage of ileac contents into colon. It is present in sacculus rotundus.
   Infection of the appendix is called appendicitis.
   Rupture of appendix results in peritonitis.
Colon : It is the largest part of large intestine an inverted U-shaped tube about a metre long.
In humans the large intestine ascends from the caecum as the ascending colon these crosses to left side as the
transverse colon and descends again as the descending colon.
Colon has longitudinal bands called taeniae coli and small pouches named haustra.
Colon is dividied into four parts.
Ascending colon : Extends upto liver on the right side. Absorbs fluids and salts.
Transverse colon : Crosses the abdominal cavity below the pancreas. Remove more water from the waste
materials.
Descending colon : Running downwards on the left side. Hold resulting waste.
Sigmoid or pelvic colon : S-shaped and enters the pelvis and joins the rectum. [Hence colon is concerned with
conservation of water, sodium and other minerals and formation of faeces].
Rectum : It is the last part of alimentary canal. It opens to the exterior by anus.
It bears longitudinal folds.
It is a storage chamber for faeces.
Rectum absorbs excess of water from the undigested food, secretes mucus & helps in egestion of undigested
faecal matter.
Enlargement of rectal veins causes Piles or haemorrhoids.
Anal canal : It connects rectum with anus, & is about 3cm long.
ANUS
      It is the terminal inferior opening of alimentary canal which is guarded by an internal involuntarily sphincter &
      an external voluntary sphincter.

HISTOLOGY OF HUMAN GUT
    Four major layers of different cell types form the wall of the alimentary canal.
    Starting in its cavity or lumen.
    (i) Mucosa : (the innermost layer) moist & friction resistant lining epithelium.
    Which contains the secretory and absorptive cells.
    At the base of the mucosa are some smooth cells.
    (ii) Sub mucosa : (Just outside the mucosa)
    Soft connective tissue layer with blood and lymph vessels & nerve supply.




      (iii) Muscularis externa : (external to the submucosa)
      Formed of a circular inner layer and a longitudinal outer layer of smooth muscle.
      (iv) Serosa : (outer most layer of the gut) It is a fibrous coat.
      The mucosa is produced into millions of microscopic folds or finger like projections called villi. supplied with a
      network of blood capillaries & also a network of lymph vessels, the largest of which it the central lacteal.
      Besides, the cells that line the surface of the villi, produce numerous microscopic bristle-like projections called
      the microvilli or brush border.
      On the surface of the mucosal epithelium are billions of single cell mucous glands, called mucous or goblet
      cells, secrete mucus that acts as a lubricant and protects the epithelial surface from excoriation and digestion.
      Surface areas of the gastrointestinal tract are lined by invaginations of the epithelium (pits) or in the submucosa.
      These pits of the intestine are called crypts of liberkuhn, contain specialized secretary cells.
      Besides, the stomach & upper duodenum contain many deep tubular glands.
DIGESTIVE GLANDS
   (i) Salivary glands : In man their are 3 pairs of salivary glands.
        (a) Parotid : largest below and in front of ears.
   Parotid or stenson’s ducts open in vestibule just outside the 2nd upper molars
        (b) Submaxillary or : Submandibular, medium sized at the angles of lower jaw.
   Submaxillary or wharton’s ducts do so near the lower central incisors.
        (c) Sublingual or Rivinus duct open under the tongue.
   Salivary glands secrete saliva (pH 6.8)
   Saliva contains an enzyme called salivary amylase, (Ptyalin) Which breakdown starch of the food into maltose (a
   simpler sugar).
   About 1.0 –1.5 litres of saliva is produced daily. It contains 99% water, some salts, mucin, lysozyme, thiocyanate
   ions (both are antibacterial agent), lactoferrin and ptyalin.
   Salivation increased by parasympathetic nervous system while decreased by sympathetic nervous system.




   Ptyalin converts starch and glycogen of cooked or backed food into limit dextrins, maltose, and isomaltose
   making the food sweet (on through chewing) Lysozyme is anti bacterial agent.
   Mucin and water lubricates the food for proper chewing and swallowing. Mucin also protects the soft lining of
   buccal cavity from abrasion.
   Salivary gland, absent in human beings are infraorbital glands.
   Mumps : It is painful swelling of parotid glands when they are infected with paramyxovirus.
   Accessory salivary glands : They are additional minor salivary glands which occur in i.e. buccopharyngeal
   cavity buccal, palatal, labial, lingual.
   Accessory salivary glands mainly produce mucus
   Mucus is a glycoprotein and help to neutralize the acid in stomach and protect stomach wall against the action of
   HCl and protein digestive enzyme.
Gastric Glands : It is estimated that human stomach has about 35 million gastric glands producing 2 to 3 litres of
      gastric juice per day.
      They are branched and unbranched tubular glands. Which are often named after the region of stomach as
      cardiac, principal and pyloric.
      Cardiac glands mostly secrete mucus.
      Pyloric glands also secrete mucus some of them produce hormone gastrin.
      Principal (Fundic) glands occur in body and fundus part of stomach. They posses.
      (a) Peptic cells (Chief or zymogenic cells) yield pepsinogen (A zymogen or proenzyme) and gastric lipase.
      In children, another proenzyme called prorenin is also produced.
      (b) Oxyntic cells (parietal cells). Yield HCl and intrinsic factor (castle’s Intrinsic Factor) HCl makes the gastric
           juice acidic (pH 1.5 to 2.0).
      (c) Mucous Neck (Goblet) cells : They secrete mucus.
      (d) Argentaffin cells produce gastrin, motilin, somatostatin, histamine and 5-hydroxytryptamine.
      Intestinal glands : Formed by surface epithelium of small intestine, occurs as crypts of leiberkuhn and Brunner’s
      gland.
      This juice is slightly alkaline and contains digestive enzymes, water, mineral salt (Cl¯, HCO¯3 and PO¯4 etc.)
      and mucous.




      Liver It is situated in the right upper part of the abdomen below the diaphragm. Largest digestive gland. Its
      weight is about 1.5 kg in man.
In man it has two lobes small left lobe and large right lobe.
The latter is fu
								
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