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).
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