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.

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