By Austin Che

      The savory bacon, lettuce, and tomato sandwich lies temptingly on  the
table. Salivary glands begin to cause salivation in my mouth and gastrin  is
released in my stomach. As the sandwich enters the  oral  cavity,  sometimes
known as the mouth, the incisors, canines,  premolars,  and  molars  quickly
tear the sandwich to pieces. By now the anti-bacterial agents in the  saliva
has assured the complete deadness of the food. Salivary  amylase  hydrolyzes
the polysaccharides into maltose. After playing  around  with  the  mush  of
dead food, my tongue expertly shapes the food into a ball like bolus.  After
the common sandwich has been transformed  into  a  wonderful  bolus,  it  is
pushed into the pharynx, which doesn't seem much different from  my  throat.
While swallowing, the dutiful epiglottis  blocks  the  trachea  against  the
entry of food as the lungs are usually not too fond of bacon,  lettuce,  and
tomato sandwiches. Passing by the epiglottis sentry,  the  sandwich,  now  a
bolus, inches into the esophagus for its journey onward. Peristalsis,  waves
of contraction and relaxing of smooth muscles,  pushes  the  hesitant  bolus
through the cardiac orifice and into the stomach. Inside  the  stomach,  the
bolus is immediately  hit  by  the  gastric  juice  that  knows  no  friend.
Destroying everything in sight, the gastric juice proves to be a  formidable
enemy against the toughest bacteria and strongest sandwiches. Pepsin in  the
gastric juice hydrolyzes proteins, cleaving them into  pieces.  About  every
twenty seconds, the bacon in the sandwich is mixed with the lettuce and  the
tomato until the truly  mixed  up  sandwich  advances  to  the  next  stage,
becoming the acid chyme. The pyloric sphincter squirts  piece  by  piece  of
the chyme, once a sandwich, to proceed on the longest part of its journey  -
the dreaded small intestine. Many other cohorts  have  teamed  up  with  the
small intestine to attack what was now only the  chyme.  The  pancreas  adds
its assortment of hydrolytic enzymes as well as bicarbonate,  which  offsets
the acidity of the no longer sandwich. The  liver  produces  bile  which  is
stored in the gallbladder and released into the small intestines, which  aid
in the digestion and absorption of fats. With  everything  being  thrown  at
the poor sandwich within the  first  twenty-five  cm  or  so  of  the  small
intestine, the sandwich cannot but help feel unwanted as  everything  yearns
to break it apart. Within this section of the small  intestine,  also  known
as the duodenum,  secretin  is  used  to  signal  the  pancreas  to  release
bicarbonate to neutralize the invader. Cholecystokinin is  another  hormone,
causing bile to be released into  the  small  intestine.  Enterogastrone  is
also released, as the fatty sandwich needs more time to digest,  peristalsis
is slowed down. The commander enterokinase starts the  battle  by  signaling
the beginning of the attack. Disaccharidases attack  disaccharides  such  as
maltose,  sucrose,  and  lactose.  The  army  also  includes   trypsin   and
chymotrypsin which break polypeptides into shorter chains.  Carboxypeptidase
splits off one amino acid  at  a  time,  while  its  partner  aminopeptidase
distracts the protein from the opposite direction. Dipeptidases  also  exist
which sit in the intestinal lining digestion the easy  fragments  consisting
of only two or three amino acids. Nucleases hydrolyze DNA  and  RNA  present
in food. After bile salts have emulsified fat,  lipase  hydrolyzes  the  fat
molecules. Past the duodenum, the attack dies down, and what remains of  the
sandwich limps on to the jejunum and ileum. Many villi  and  microvilli  can
be found on the small intestines ushering out what it deems  to  be  worthy.
Some nutrients are transported out of  the  small  intestines  by  diffusion
while others are coupled to  the  active  transport  of  sodium  across  the
membranes. All the  nutrients  enter  the  capillaries  or  lacteals,  which
converge into the hepatic portal vein, leading to the liver.  In  the  liver
is where the prisoners of war are converted to become more useful  in  their
new home. The colon or large intestine takes what remains  of  the  sandwich
after it leaves the small intestine. The cecum is located  at  the  junction
of the colon and the small intestine. Travelling  through  the  colon,  much
water is absorbed,  combining  the  no  longer  recognizable  sandwich  into
feces. Within the colon are many bacteria who can not but be pleased at  the
tasty sandwich. The cellulose fibers found in parts  of  the  sandwich  help
move the food along the digestive tract. Soon, the end of the  journey  will
approach. Remaining unwanted wastes  are  thrown  into  the  rectum,  to  be
released through the anus and back out into the  world  as  something  other
than a sandwich with bacon, lettuce, and tomatoes.  It  is  truly  difficult
being a sandwich.