ABSORPTION OF DIGESTED FOOD

While within the alimentary canal  the food ,strictly speaking , is still outside the body .In  order to be use to organism ,it most be absorbed in the blood stream .The  entry of the digested food  from the lumen of the gut ,specially intestine , into the blood stream is called absorption .                                                            Very little  absorption  takes place either in the buccal cavity or in the stomach but these can be scarcely classified as food .By  far the greater  part of the absorption  takes place in the ileum .The  food is  in a very soluble and easily diffusible  form and hence  it can be easiy absorbed throug  its wall into the blood  and lymph  cappilaries . The  absorptive area of the  small intestine  is enormously increased by the development  of villi in this region .In man ther are 5,000,000  minute waving villi , providing a very large inner surface area [5 times the body surface ]  for absorption .Each  villus is wellsupplied  with the capillary blood vessels and lacteals  are concerned with the absorption of fat .The  oval peyer's  patches or lymphoid nodules  manufacture certain white  corpuscles  known as lymphocytes  which also helps in the absorption of fat .The vermiform appendix which has the structure of peyer's patches is also absrptive in a high degree. The digested carbohydrates  chiefly as glucose  and protein  must pass through where it  break up into cappilaries .Thus all the carbohydrates chiefly as glucose and proteins as amino acids are observed into the villus  cappilaries and lead  to the  factor of the hepatic portal vein .The hepatic portal vein  passes into the liver  where it break up into  capillaries .Thus all the  carbohydrates  and proteins must  pass through the liver before entering to the general circulation .In this respect the liver provides  a very important regulatory  mechanism .Any exccess of blood sugar  is always changed into glucose  and stored in liver  cells .On the other hand , if there is shortage of sugar in the blood , the glycogen stored in the liver  is turned once again  into glucose by and enzyme  known as glycogenase and released into the blood .In this way the liver   not only acts as a regulator  but also as a store house of carbohydrates .                                                                                                                           The amino acids are brought by the hepatic vein  to the liver and are allowed to go into the general circulation. Thecells of the body  absorb amino acids  are dealt with in the liver ;  by a complex actions the amino acids are split up. The carbon , hydrogen and oxygen  unite to form glycogen  or fat and the nitrogen forms ammonia .Ammonia is highly poisonous and is not released  into the blood  in any quantity in mammals .It combines  with carbon dioxide under the influence  of certain enzymes to form  urea .The breaking down of amino acids is known as deamination.                                                                                                                             The  digested fat , as fatty acids  and glycerol , passes  directly into the lacteals  where  it is at once changed  into minute droplet of fat  so that this vessels contain a milky white emulsion .The removal from the intestinal  contents of the digested carbohydrates ,proteins and fats leaves afluid residue containing  undigested and indigestable food , remais of bacteria  and largely of undigested cellulose  .This residue passes into the caecum  and colon where  further process of absorption takes place  but it is chiefly of water .A largely proportion of water is  absorbed during the passage  of this fluid residue  through the colon  so that by the time it reaches  the rectum the residue  becomes semi-solid and form the  faeces , which is thrown out at intervals.