Gastric juice

Gastric juice

Secretion  of gastric juice  occurs  in  three  phases.  The  first  is  the  nervous  phase.  The presence  of food  in  the  buccal  cavity and its  swallowing  trigger  reflex  nerve  impulses which  pass  along  the  vagus nerve  from  the  brain  to the  stomach.  The  sight, smell, taste  and  even  the  thought  of  food  can trigger the  same  reflex.  The gastric  glands  of the stomach  are stimulated  to secrete  gastric  juice.  This  takes place  before  the food has  reached  the stomach  and therefbre  prepares it  to  receive  food.  The nervous phase of  gastrio  secretion  lasts  for approximately  one hour. The second  phase  is  the  gastric  phase which takes  place in  the  stomach.  It involves  both nervous  and  hormonal  control. stretching of the stomach  by  the  food it  contains stimulates  stretch  receptors  in  the wall of the  stomach.  These send nerve impulses  to Meissner's  plexus in  the  submucosa,  which in  turn  sends nerve  impulses  to  the gastric,glands, stimulating  the  flow  of gastric juice.  Stretching  of the stomach  and  the presence  of food also stimulates  special  endocrine  cells  in  the  mucosa  to  secrete  thehormone  gastrin. This  reaches  the gastric  glands  by way of the  bloodstream  and stimulates  them  to  produce  gastric  juice  rich in hydrochloric  acid  for about four  hours. The third  phase  is  the  intestinal  phase  which takes  place  in  the small  intestine. When acidified  chyme  enteres  and  makes contact  with  the walls of  the  duodenum,  it triggers both nervous  and  hormonal  responses.  Receptors  in  the small  intestine  are stimulated by  the presence of food,  but the  reflexes,  which  pass  through  the  brain,  inhibit secretion  of gastric  juice and slow  the  release of chyme from  the  stomach.  This prevents  too much  food being  released  into  the small intestine  at once.  In  addition,  the mucosa of the  duodenum produces two  hormones, cholecystokinin (CCK) and secretin.  (CCK may also  be known  as pancreozymin,  but  use only one  of these  names! CCK is  easier  and  more widely  used.)  The  two  hormones  are taken  in  the  bloodstream

to  the  stomach,  pancreas  and the liver.  In  the stomach  secretin inhibits  secretion of gastric  juice and CCK inhibits  stomach emptying.

Human Bile.

Human Bile

Bile is  a viscous,  greenish  yellow fluid,  sour in  taste.  It is  secreted  by hepatocytes  (liver cells)  of the liver.  Around  0.8  to  1.0 litre  of bile are produced each day  and stored  and concentrated  in  the  gall bladder. Bile  is secreted  into  the  duodenum  through  the common  bile  duct.  The hormone cholecystokinin  controlls  the  flow  of bile. Composition  of bile:  Bile is  composed  of  about  98%  water, 0.8%  bile  salts,  A.2%o bile pigments,  0.7% inorganic  salts  and 0.6% cholesterol.  Sodium  taurocholate and sodium glycocholate  are the bile  salts.  Among the inorganic  salts,  sodium  bicarbonate  is important. No  enzymes are  present in  the  bile.  Biliverdin  and  bilirubin are  main  bile pigments. The bile  pigments are  produced  in  the  liver  from  the haem  of haemoglobin  in the  process  of  the  destruction of  red blood  cells. Bile pigments have  no function and  are excretory  products. Functions of bile:  The functions of the bile  are:

i.  Emulsifies  fats  (breaks  large  fat  or  oil droplets into  an emulsion  of microscopic

droplets),  a process  that  massively  increases  the  surface  area available for fatdigesting  enzymes  to attack.

ii.  Neutralises  the  acidic  chyme from  the  stomach  and  create the  ideal  pH for  intestinal

enzymes.

iii.  Stimulates  peristalsis  (rhythmic  involuntary  muscular contractions)  in  the duodenum

and  ileum.

iv.  Allows  the  excretion  of cholesterol,  fats  and bile  pigment.

Human Digestion in stomach:

Human Digestion  in  stomach:

Mechanical digestion:  Food  from  pharynx enters the  stomach  through  the oesophagus.  Stomach  is  a bug like  organ.  Mechanical digestion  takes  place  by the churning  action  of the  stomach.  This  is  made more efficient by the fact  that  unlike  other regions  of the gut it  posseses  three  layers  of smooth  rnuscles instead  of two.  namely  the outer  longitudinal,  middle circular  and iner  oblique  layers.  Stomach  wall contains  gastric glands  which produces a iuice  collectively  known  as gastric juice.  As a result  of

mechanical  digestion  ttre  gastric  tuice  mixes  with the food  rnass.

Chemical digestion:

Approximately 2 liters of gastric  juice  are secreted  each  day. The components  of gastric iuice 

perform  the  chemical  digestion.Role  of gastric  juice  in  digestion: The  gastric juice  contains hydrochloric acid  (HCl). mucus and pepsinogen.

i. Hydrochloric  acid (HCl):  A. Kills bacteria. B. Converts  inactive  pepsinogen  into active  pepsin. C.  Makes the  stomach  contents  pH I-2.5,  ideal  for  the optimum activities  of the  stomach enzymes.  D.  Denatures  many proteins; their tertiary structure  is  altered, making them  unfold and  so easier to  digest.

ii. Mucus:  mucous  cells  protects  the stomach  lining  from  the  digestive action of  its  own secretion  and  HCl.

iii. Pepsin: Pepsin (protein breaking  enzymes  are  called  proteasei  so,  pepsin  is  a protease)  is  a powerful  endopeptidase (the  protease  that breaks bonds  between  amino acids within  proteins  thus producing  smaller  peptides  is  called endopeptidase and  that splits  off  amino  acids from proteins  i.e. polypeptides  is  called exopeptidase) enzyme:  it  breaks  specific  peptide bonds in  the  middle of  the protein chain,  turning protein molecules  into  polypeptides. The soluble protein  of  milk is caseinogen. Human  infants  digest this milk protein  with acidic  pepsin,  just  as  adults  do  (the  stomach  of ruminant mammal  secretes  a milk curdling  errzyme  called  rennin  which  clots  and  precipitates  the  soluble milk protein casein/caseinogen  into  insoluble  calcium  salt of casein  and  slows the movement of milk through the  stomach  which  is  then  digested  by pepsin;  Rennin should not  be confused  with  renin  which  also is an enzyme  produced  by  kidney).

Digestion in Mouth

Human Digestion  in  Mouth

Mechanical digestion: Mouth cavity  is  the  chamber  just  inside  the  mouth  in  which  food  is  chewed. During cher,ving  the  muscular tongue  moves food  around  the  mouth  and  mixes  and  moistens it with saliva. The  tongue  posseses  taste  buds  that contain  receptors  sensitive  to  sweet, salty,  sour  and bitter sunstances.  A simple  (inborn)  or conditioned  (learned)  reflex results in  stimulation  of  the  salivary glands to  secrete  saliva.  The eye  and  the  olfactory  (smell) receptors  in  the  nose  are  also  important receptors  in  triggering  reflexes that  bring  about salivation.

Chemical digestion: Digestion of food  begins  immediately  after the  ingestion.  But little  is  digested  in  the
mouth, because  food  remains  in  the  mouth for  a short  time.  In  the  mouth, the  food  is  also mixed with saliva  (  about  1.5 dm3  of saliva produced daily)  produced  by three  pairs  of salivary glands whose ducts
lead  into  the  mouth.

Role of saliva  in  digestion:
i. Water  and  mucin of saliva  lubricates  and  softens  the  food.
ii.  Lysozyme  helps to kill  bacteria.
iii.  The salivary  amylase  (:ptyaline)  acts  on  cooked  starch (starches  are  long  polymers of  glucose) and begins to  break  it  first  down into  shorter polysaccharides,  and  then  to the  disaccharide  maltose.  The action  of  amylase  continues in  the  stomach  until it is destroyed by the  hydrochloric  acid  of  stomach.
iv.  Among  the  mineral  salts  the  chloride  ion  speedup  the  activity of the  enzyme. Eventually  the semisolid,  partially  digested food  particles are stuck  together  and  moulded into  a bolus (or pellet) by  the tongue,  which  then pushes  it towards  the  pharynx.

Liver

Human Liver:

Apart  from  the  skin, the liver  is  the largest  visceral organ  of the  body. It  is  also  the  largest gland  of the  body.  It  is  a deep  brown  colored  triangular  organ  situated  just  below the diaphragm  in  the  right side  of  the  stomach.  The average  weight  of the liver of  adult  man is  1500 grams. The liver consists of four  lobes.  The  right  lobe  is  the  largest.  Beneath  this lobe  the  sac  like  gall  bladder is  attached.  The bile  secreted  by the  liver is  deposited  in the gall bladder.  The bile comes  to  duodenum  through  the common  bite duct.The functional  unit hepatocyte.  Each lobule  has  a diameter of about 1 mm. Between the of the  liver  is  known  as lobule.  The cells of the  liver  is  called.lobules branches are of hepatic  artery,

hepatic  portal vein  and  bile duct.  At  the center  of  each lobule  is a branch of  the  hepatic vein which is

connected to the hepatic  artery  and hepatic  portal vein by  blood.

Functions of Liver:

Liver's  function  is  extensive.  Different  types  of biochemical  reactions take  place liver,  so  it  is  alscl called  the  organic laboratory  or  chemical  workshop.  Some functions  of  liver  are as follows:

Storage  functions:

ii.  Storage  of glycogen: Glucose is  taken  to  the  liver  in  the  hepatic  portal vein.  If the concentration  of glucose  in  the blood  rises  above  0.1%  by weight, the  excess  is converted  to  glycogen  under  the control  of insulin,  a honnone  produced by  the pancreas.  The conversion  of slucose  to glycogen  is 

called  glycogenesis.

ii. The production  and  storage  of  bile: The liver produces  bile  salts and  adds  to them the  bile  pigment bilirubin  from  the  breakdown  of red  bloocl  cells. With  sodiumchloride  and sodium  hydrogen carbonate, cholesterol  and  water,  this  forms  the  greenyellow  fluid  known  as bile. Upto  1 dm3 of bile may  be produced  daily.  It  is temporarily  stored in  the  gall  bladder  before  being discharged  in  the gall hladder. It  is used  to emulsify  fats  before  their  digestion  by  lipase  in  the  duodenum.

iii. Storage  of vitamins:  The  liver  r,vill  store  a flumber  of  vitarnins  which  can later  be released if deficient  in  the  diet.  It  stores  mainly the  tat-soluble  vitamins  A, D, E and K, although  the  water 

soluble  vitamin B and  C can  also stored.

iv.  Storage  of  minerals: The  liver stores  minerals, e.g. iron,,  potassium,  copper  and zrflc. It  is  the liver's stores of these minerals,  along  with  vitamins,  w'hich makes  it  such  l nutritious  food.

v. Storage of  blood:  The liver,  with its  vast  complex  of blood  vessels,  forms  a large store  of blood with a capacity of up to 1500  cm3. In  the  event of haemorrhage, constriction  of these  vessels  forces blood  into the  general circulation to  raplace  that lost  and so helps  to  maintain  blood pressure.  In stressful  situations,  adrenaline also causes  constriction  of these vessels,  creating  a rise  in  blood pressure.

Metabolic  functions:

Carbohydrate  metabolism:  Liver performs  the  following  metabolic  functions related  to  carbohydrate 

metabolism:

A. When the level  of blood  sugar (glucose)  falls  below  its  normal  level,  the glycogen isreconverted  into glucose,  a process called  glycogenolysis, under the  control  of hormone  adrenaline  (produced  by the adrenal glands)  and  hormone glucagon (produced  by the  pancreas).

B. Muscles  also  break  down  glycogen  if  it needs  glucose  for  its  own respiration,  in  which case  the glycogen is  converted  to  glucose phosphate,,  but is  then  converted  to pyruvate (glycolysis)  which is used  to  produce ATP  during  aerobic  or  anaerobic  respiration. Lactic  acid (lactate)  produced  by anaerobic  respiration  in  skeletal  muscle can be converted  later  into  glucose  and  hence  glycogen  in  the liver.

C. When the  demand  for  glucose  has exhausted the  glycogen  store  in  the  liver,  glucose can  be synthesised  from non-carbohydrate sources  by the  process  called gluconeogenesis.

D. Carbohydrate  in  the  body which  can  not be used  or stored  as  glycogen  is converted into  fats  and  stored.

ii.  Fat  metabolism:  The liver performs  the  following  functions  ralated  to fat  metabolism:

A. The  liver  can break  down fats  into  fatty acids and  glycerol  for  respiration if  glucose  is in  short supply. Fatty acids  are coverted  to acetyl groups  which  combine  with coenzyme  A to  form  acetyl coenzyme  A. This enters  Kreb's  cycle  for oxidation and produce  energy.

B. The  liver  converts  excess carbohydrate  to fat.

C. Removes  cholesterol  from  the  blood and  breaking  it  down  or, when necessary, synthesising  it.

D. Desaturation of  fats:  Fats are stored in  the  body  in  saturated  form. This means  that they  cannot
take  any  more  hydrogen into their  composition.  Before  saturated  fats  can be used  by  the tissues  ofthe body,  the  hydrogen  must  be removed,  and this  occurs  in the  liver. The resultant  unsaturated fats  can be used  to provide .

iii. Protein metabolism: The liver  performs the  following  important  functions  related to proteion metabolism:

A. Formation  of urea:  Amino  acids  are taken  to the  liver  in  the  hepatic  portal  vein. Those  that are in excess of  the  body's  needs  cannot be stored. The nitrogen-containing part of the amino  acids is removed and changed  to  urea. The  process  is  sometimes referred  to as deamination.

B. Transamination:  The  systhesis  of  amino  acids  by the transfer  of amino  group  from an amino acid to another  organic  acid is  called  transamination.  Transamination  is  the means of producing  amino  acids which  are  deficient  in  the  diet. The essential  amino acids  (8  in  number: valine,  isoleucine,  leucine, lysine,  methionine,  phenylalanine, tryptophan,  threonine)  can not  be synthesised by  transamination  in  the live  and  must be  obtained  from  the  diet.

C. Plasma protein  production:  Plasma  proteins,  albumin,  globulin,  fibrinogen and prothrombin  are vital components  of  plasma  and  the  majority of thern  are  synthesised from  amino  acids  ion  the liver.

iv.  Formation  and  breakdown  of red  blood  cells:  The  fetus relies solely on the liver  for the  production of red blood  cells.  In an  adult,  this  role  is  transferred  to  the  bone malTow.

v. Breakdown  of haemoglobin  of red  blood cells: The  adult  liver,  however,  continues to  break  down red  blood cells  at the  end of their  120-day  life  span. The phagocytic macrophage  cells  that  is,  Kupffer cells  lining  the sinusoids  carry  out this  breakdor.vn.The  haemoglobin  they  contain  is broken  clown  into haem  and  globin.  Globin  is  the protein  part of  the  molecule  and is  broken  down to  its  individual amino  acids. The iron  is  removed  from  haem and  the remaining  part of the  molecule  forms  a green pigment  called  biliverdin  which  is  converted  to  bilirubin,  yellow in  colour and  a component  of  bile. The accumulation  of bilirubin  in  the blood is  a symptom  of liver disease and produces a yellowing  of the skin,  a condition  known  as  jaudice. The iron  is  either  stored in  the liver  cells as a compound  called ferritin  or used  in  ttie formation  of  new  red  blood  cells by  the  bone  maffow. The liver  produces haematinic principle,  a substance  needed  in  the  formation  of red blood  celis.Vitamin  Bn deficiency results  in  pernicious  anaemia.

xi.Hormone  breakdown:  To  varying degrees,  the  liver  breaks  down all  hormones. Some, such  as testosterone, are rapidly  broken down  whereas otheres,  like  insulin,  are destroyed more  slowly.

xii.Detoxification:  The  liver  is  ideally  situated  to  remove,  or render  harmless, toxic material absorbed by the  intestines.  Foreign  organisms  or material are  ingested  by the Kupffer  cells  while  toxic  chemicals are made safe  by chemical  conversions within hepatocytes. Alcohol  and nicotine  are two  substances dealt with  in  this  way.

xiii.Production of heat: The liver,  with its considerabie  metabolic  activity,  can  be used to  produce  heat in  order  to  comb  at a fall  in  body temperature.  This reaction,  triggered by  the hypothalamus,  is  in  response  to  adrenaline,  thyroxine  and nervous stimulaion. Whether the  liver's  activities  produce excess  heat  under  ordinary circumstances  is a matter  of some  debate.

xiv.Cholesterol  Production:  The liver  produces  cholesterol  and  production  of cholesterol  is  the  begining  of the  sysnthesis  of other  steroid  molecules.The  major source  of  cholesterol  is the diet, and
many  dairy  products  are  rich  in  cholesterol  or fatty acids from  whichcholesterol  can be  systhesised.

Pancreas

Human Pancreas:

Pancreas is  an irregularly  shaped, pink  colored  gland.  It  is  located  in  the bend  of  the duodenal loop.  lt  is  a compound  gland,  dual  firnction  exocrine  and endocrine.  The exocrine  part  make up  99  percent  of the  glandular tissue  of pancreas.  They  secrete digestive  juice  called  pancreatic juice.  This  fluid  contains  water,  some  salts,  sodium hydrogen carbonate and  a mixture  of about 15 enzymes. The  pancreatic juice  passes  into the  duodenum  through  pancreatic  duct  which  joins the  common  bile  duct .The  endocrine  structures  are called  islets  of  Langerhans  which produce two  hormones insulin  and  glucagon.

Function:  Pancreatic juice  plays  important  role  in  digestion  and  the  hormones  controls

the carbohydrate  metabolism.

lntestinal Gland

Human lntestinal  Gland

The  mucosa of  small intestine  possesses  numerous finger-like  projections  called villi (sing.villus). At the base of  the  villi  the epithelium  folds  in places  to form  narrow  tubes.  These tubes are called  crypts  of Lieberkuhn  (fig.  3.5). In  the crypts  of Lieberkuhn  there present  a kind  of cell called Paneth  cetl.  In  the  duodenum there  in  addition  rounded  Brunner's  gland.

Cells  of  the  intestinal  mucosa, are subjected to  considerable wear  and are constantly undergoing replacement.  Cells  deep  in  the crlpt of  Lieberkuhn  divide  rapidly and  migrate up the villus.  The cells reach  the tip  of the villus  in  about two days.  there they  shed,  along with their  membrane enzymes, into the  lumen  at the  rate  of some  17 billion  a day  along length  of the intestine. Before they  are  shed, however, these  cells  differentiate into absorptive  cells  that  transports nutrients into  the network  of  blood and  lymph  vessels, once  digestion is  completed.  The  average  life  of these  cells  is  about 5 days.

Functions:

i. The  epithelial  cells  of villi produce  membrane enzyme which  helps in  digestion.

ii. The  cells  of  the  crypts secrete  intestinal  juice,  a slightly  alkaline  fluid  which contains

water  and  mucus and  helps to  increase  the volume  of fluid  in  the  gut.

iii. Paneth  cells  secrete  lysozyme,  the  bacterial  enzyme  which  kills  bacteria.

iv. Goblet cells  secrete  mucus.  which  protects  the  intestinal  wall.

v. Bru,,ner's  glands  secrete mucus which  als, r'oiects  the  intestinal  rvall.

Gastric Gland:

Human Gastric  Gland:

The  deep tubular  glands of the stomach wall are called  gastric  gland .Collectively  the  secretion  of the  gastric glands is  called  gastric juice. Approxirnately  2 liters  of gastric  juice are secreted  each  day.  There arc  4 kinds off cells  found  in the  gland.  Each  type  of cell produces  a specific secretion  which  are as.

\

i.  Oxyntic  cell  (:Parietal  cell): Produces  Hydrochloric

acid  (HCl):

Function:

a. It  is  a strclng  bacteriocide  and kills bacteria.

b.  Converts  inactive  pepsinogen  into active  pepsin.

c.  Denatures  proteins  and  helps  to soften tough   connective  tissue  in  meat.

ii.  Mucous cells:  Secretes  mucus.

Fuction:  Mucus  protects  the stomach  lining  from  the digestive action  of its own secretion  and HCl.

iii. Argentaffine cells:  Secretes  gastric  intrinsic  factor.

Function: Helps  in  the absorption  of vitamin  B12.

iv.  Chief cells  (also  called  zymogen  cells,  peptic  cells): These  cells  secrete pepsinogen.  Hydrochloric  acid converts  pepsinogen  into  active  pepsin. Pepsin then helps in  digestion.

DIGESTIVE GLANDS

HUMAN DIGESTIVE GLANDS

The  glands  whose secretion  help  in  the  digestion  of food  is  called  digestive  glands.  The digestive  glands  of man are classified into  two  groups,  e.g.-

i.Glands  situated  outside  the alimentary canal but  are connected  intimately  with ducts.

ii.Glands present  in  the wall of alimentary  canal.

The  glands  outside  the  alimentary  canal  are salivary  glands,  liver,  pancreas,  and the glands in  the  wall of  alimentary  wall  are gastric  glands and  some  glands of intestine.

A. Salivary  Glands:

Around  the  mouth  there  are  three  pairs  of salivary glands which are-

i. Parotid  gland: Parotid  glands  are the largest  of salivary  glands,  The  glands are  present  below each ear  and  the ducts  from  the  glands  opens  into buccal  cavity  near  the  upper  molar.

ii. Submaxillary  or Submandibular glands:  Submandibular  glands  are present  at the  posterior part of mandible and  their  ducts open  into  the buccal  cavity.

iii. Sublingual glands:  Salivary  glands are  located  below  the  tongue anteriorly.  The ducts  also open  into the  buccal  cavity.

Saliva:

The secretion  of  salivary  glands  is  called saliva.  Abont  15 litres  of  saliva  are  secreted  by the salivary  glands  each  day.  Saliva  is  a watery  (over  99-%;2f  saliva  is  water)  secretion contaiping  the  enzymes  q4livary amylase  (or  ptyaline)  and  lysozyme.  It  also  contains mucin, sodium,  potassium,  bicarbonate  and  chloride  ions.

Functions  of  saliva:

i. The water and  mucin  in  saliva  moisten,  soften,  and  lubricate  dry  food  so that  it  is  more easily  chewed and  swallowed.

ii.  The  enzyme  salivary  amylase breaks down  starch into  the  soluble  sugar  maltose.  This reaction  is  the  tirst  stage of  carbohydrate  digestion,  Food  remains  in  the  mouth for only  a few seconds  and  little  digestion occurs  here,  but  after swallowing  the  action ofsaliva  continues  for some time  in  the  stomach.  It is  finally  stopped when  the  acid  in stomach  digestive  juice penetrates  the  food  and  destroys  the amylase.

iii.  Amylase,  and  another  enzyme in  saliva  called  lysozyme,  help to  remove carbohydrate  food and  bacteria  from  between  the  teeth  and  thus  help to  prevent  tooth decay.

iv.  Saliva moistens the mouth, tongue,  and  lips,  which  facilitates  talking.  Saliva production  stops

during nervousness  or severe  illness,  making speech difficult.

v. By  dissolving  food,  saliva makes  it  possible  for  the  chemicals  within  it  to  reach  the taste buds in  the tongue.  This is  important  because taste  buds are  not stimulated  by  dry food.

vi.  Bicarbonate  in saliva acts as a buffer, which means  that  it  keeps saliva at a more or less constant  level  of weak alkalinity.  This helps  prevent  tooth  decay by reducing the strength of mouth acids  that  dissolve  tooth  enamel.  During sleep  saliva  production slows  down

considerably.

vii.  Chloride  ions  speed  up the  activity of the  enzymes.

viii. Mucin helps to  bind  food particles  together  and  lubricae  them  to  assist  swallowing.

Teeth

HumanTeeth

The teeth  are the  hardest  part of the body.  The jaws  bear  teeth  which are used  to  chew  or masticate food  into  smaller  pieces. The arrangement  and number  of teeth  is  called  its dentition. Humans  have  two  successive  sets  of teeth.  The  deciduous  or  milk teeth  appears first,  and  are progressively  replaced by  the  permanent teeth.  The  teeth  have  different shapes  and size  and possess  uneven  biting  surface.  This  is  called  heterodont  dentition. Milk dentition  appear  at  the age of  about 6 month  and  lasts  until  the age  of  5-6. There  are 20 milk  teeth.  Human  possess up to  32 permanent  teeth.  Each half  of the  jaw  bears  2 incisors,  1 canine,  2 premolars  and  3 molars.  The  basic structure  and  function  of each type  of teeth  is  as follows  

i.  Incisor:  Incisor  situated  at the  front  of the  buccal  cavity.  They  have  flat  sharp  edge.

Incisors  (total  8)  are  used  for  cutting  and biting  food.

ii.  Canine: They  are pointed  teeth.  Canines (total  4) are  used  for  grasping.

iii.  Premolar:  They  possess  one  or two  roots  and  two cusps  (projections  on the  surface  of this  tooth  is  called  cusp).  They  (total  8) are specialized for  crushing  and grinding. They  many  also  be used to tear  food.

iv.  Molar: They  possess  more  than  one  root, upper  molars  have  3 roots,  lower  molars 2. Each  has 4 or 5 cusps.  They (total  12)  are used  for  crushing  and  grinding  the  food.

Generalized  structure of a tooth:  The tooth  is  typically  consists  of 3 parts:

i.  Crown: The visible  part  above  the  gum.

ii.  Root: The  part  hidden  below the gum.

iii. Neck: The tooth  between  crown and  root.

The crown  is  covered  witir  enarnel.  Enamel is  the  hardest part  of the  body.  The neck of the tooth  is  surrounded  by gum.  Beneath  the  enamel  is  the bone  like  material,  dentine which forms the  bulk  of the  tooth.  It  is  tough  but  not  so hard  as enamel  or as resistant  to decay.  It  contains numerous  small  canals  (canaliculi)  containing  cytopiasmic  extensionsof the  odontoblast,  the dentine  producing  cell. Dentine surounds  a  pulp  cavity  with  blood vessels, odontoblasts  and nerves  running  through  it.  The  blood  supply  nourishes  the living  bone  with food  molecules and oxygen,  and  also  removes  waste  products.  The nerves  allow to sense pressure  and  touch,  and, if the  nerve  is  exposed  by damage,  we experience  intense  pains. The root  of the  tooth  is  covered with cement,  a substance  similar  to  bon:.  Numerous fibres,  connected  to the  cement  at one  end and  the  jaw  bone at  the  other,  anchor  the  tooth firmly  in  place.  However,  it  is  still able to move slightly  and  this  reduce the  chances  of it being  sheared  off during  chewing.

Human Alimentary Canals

Human  Alimentary Canals

The alimentary  canal  (Fig 3.1) is  a coiled  muscular  tube  that  leads  from  the  mouth to  the
anus. It is  about 9 meters (30  feet)  long.  The  alimentary canal enables  man to:
A.  ingest,  or take  in  food.
B. pass  food  through  its  body,  propelling  it along the  alimentary  canal  by peristalsis: rhythmic  contractions  of the  gut wall.
C. break  down food material into  smaller  pieces. This is  mechanical  breakdown.
D. digest complex  food  molecules such  as carbohydrates,  proteins  and  fats  into  simpler ones.  This is  chemical  breakdown.
E. absorb simple  food  molecules  such  as amino  acids, sugars  and  fatty  acids.
F.  absorb  water  and  some  mineral  salts  (before  they  are lost  from  the  body  as waste).
G. egest, or eliminate, undigested  food material from  the  body.

i.Mouth  opening:The  transverse  aperture  or opening below  the  nostrils  and bounded by lips  is called  mouth aperture.  The  alimentary  canal begins  from  here. 

Function:  Food materiais are  ingested  (the  act of taking  food  into  the  buccal  cavity  is

called  ingestion)  through  this  opening.

ii,Buccal  cavity: The  wide cavity beyond  the  mouth aperture  is  called  buccal  cavity or mouth. It contains  the  muscular  tongue  and  is  supported by  jaws  in  which  the teeth  are set in  sockets. Tongue  posses taste  buds.  Ducts of paired  salivary  glands  lead  into  the buccal  cavity.  The roof of the mouth  is  called  palate.

Function:  Food  is  taken  into  the buccal cavity.  The  lips,  tongue  and  teeth  work  together to capfure and received  food,  to move food  about the mouth and to  cut,  grind  and chew  food  into smaller  pieces.  Saliva  from  the  salivary  glands  mixes  with  the  food. Saliva  helps in  the digestion  and  swallowing  of the  food.

iii.Pharynx: The  funnel-shaped  structure  beyond  the mouth cavity  is  called  pharynx.  It is consists  of  nasopharynx  (connected  to  nasal  cavity),  and laryngopharynx (connected to  larynx).

Function:  Food  passes  to oesophagus  through  the pharynx.

iv.Oesophagus:  The oesophagus  is  a straight,  narrow,  thick  walled  muscular  tube  about 25 cm  long,  leading  from  the  pharynx to the  stomach. 

Function: Food passes to stomach  through  oesophagus  by  peristaltic  movement of the muscular  wall.

v. Stomach:  The stomach  is  a J-shaped  muscular  bag  located  just  below  the  diaphragm. The aperture  in  the  junction  between  oesophagus  and  stomach  is  called  cardiac aperture  and  the sphincter  muscle  called  cardiac sphincter.  The  stomach  has four parts,  e.g.-  cardiac  end, fundus, body  and  pyloric  end.  The  exit from  the  stomach  into the  duodenum  is  controlled  by the circular sphincter  muscle called pyloric sphincter. Gastric  glands are  present  in  the  gastric mucosa.

Function:  The  main function  of the  stomach  is  to  store  the  food  from  a meal,  furn  it  into a liquid  and  release  it  in  small quantities at a time  to  the  rest  of  the alimentary  canal. It is  also  a site  of partial  digestion.

vi. Small  intestine:  The small  intestine  is  about  6.35  meters  long  in  adults  and  consists of 

three  main parts,  the  duodenum,  the  jejunum  and  ileum.  Finger-like  projections called  vills (sing.  villus)  are  present  in  the inner  wall of ileum.

Function: Digestion  of food  is  completed  in  this  region.  Also  absorbs  the  vast  majority

of small soluble  food molecules  produced  by dige

vii.Large  intestine:  The  region from  the end  of the ileum  to  the  anus  is  called  large intestine.  It  is  about 1.5 meters long,  much shorterthan  small intestine  and  consists of caecum,  colon and  rectum.

a.Caecum:  Caecum is  the  first  part of  the  large  intestine  and  is  a  blind  saclike  structure. From its  lower  end  extends  a small  finger-like  tube  called  vermiform  appendix. Appendix plays no part in  digestion.  The  inflammation  of the  appendix  is  called appendicitis.

b.Colon: It is  a tubular  structure and  consists  of  - i.  ascending  colon  ii.  transverse colon, iii.  descending colon and  iv.  sigmoid  colon.

c. Rectum: It is  a  muscular  structure  at the  end  of  the  large  intestine.

Function:  The large  intestine  absorbs  water.  Minerals also diffuse  or are actively transported  into the  bloodstream  from  the  colon.  It  forms  and  expels  undigested  food residue  in  the  process of egestion.

viii. Anus: The  aperture  through which the  rectum  communicate  with  the  outside  is called  anus.  Sphincter  muscles are  present  in  the  anus.

Function:  Det-ecation  takes  place  through  the  anus  (discharge  of faeces  or stool  from  the
body  is  referred  to  as  defecation).
Teeth

Human DIGESTION AND ABSORPTION

DIGESTION  AND ABSORPTION

The physiological process by which, in the presence of enzyme and other substances, the complex large molecules of organic food masses are broken down into simple, smaller molecules, e.9., carbohydrate into glucose, protein into amino acids and fats into fatty acids and glycerol, that can be absorbed by the body cells is called digestion. Digestion and absorption occurs in the alimentary canal, or more plainly the gut, which runs from the mouth to the anus. As the gut is continuous with the outside surface of the body, the food in the gut is considered to be 'outside' of the body. The type of digestion takes place in the alimentary canal is called extracellular digestion. Food can only be absorbed into the body after it has been ingested and broken down physically by the teeth and muscles of the gut wall (mechanical digestion). and chemically by its enzymes into molecules of a suitably small size to be absorbed through the gut wall (chemical digestion). From here the nutrients enter the blood or lymph and are delivered to the cells of the body tissues where they undergo assimilation. Undigested food is egested through the anus. The system responsible for the ingestion, digestion, absorption and removal of undigested food is called digestive system. The digestive system consists of alimentary canal and the digestive glands.