GI
TRACT
Your
gastrointestinal, or "GI" tract, runs from your mouth all
the way to your anus. It is essentially a very long
and windy tube through which food is broken apart, digested,
and the nutrients absorbed into your system. To get
a good understanding of the process which turns your lunch
into a BM (Bowel Movement), lets follow the course of a
turkey sandwich with mayo and lettuce through your GI tract.
You
start off by taking a bite out of the sandwich and your
teeth chew it up. Saliva (water and enzymes) from your salivary
glands (parotid, sublingual, and submaxillary) moistens
the food and begin to digest the starch in the bread.
A chewed up ball of sandwich (bolus) in your mouth then
is swallowed and travels down your ESOPHAGUS. The ESOPHAGUS
is a muscular tube about 22-30 cm long that passes through
the middle of your chest, through your diaphragm, and attaches
to your 
STOMACH.
A SPHINCTER - a muscle that works like the drawstring of
a purse - relaxes to let the food into your stomach, and
then tightens to keep food from going back up the esophagus.
Your stomach makes hydrochloric acid and enzymes which break
down the protein - in this case, the turkey. If the sphincter
isn't working just right, one gets the acidic stomach contents
refluxing back into the esophagus. This is Gastro-Esophageal
Reflux Disease, or GERD. This is also known as heartburn.
The stomach is very muscular and also acts to grind up the
food by squeezing and relaxing. Okay, our turkey sandwich
is now essentially mincemeat. What next?
The
stomach is connected to the SMALL INTESTINE, and another
sphincter opens to let the food through. The small
intestine is another hollow tube. If fully stretched
out, it would measure between 15 and 34 feet. It's
divided into three sections. You can't tell where
one section starts and the other stops with the naked eye
- only under a microscope. The three sections, in order,
are: the DUODENUM, the JEJUNUM, and the ILEUM.
Our
chewed-up sandwich now enters the DUODENUM. The LIVER
makes bile, which is green and helps the digestion of fats.
Bile is stored in the GALL BLADDER, and conveniently squirted
into the DUODENUM when food enters. PANCREATIC juice also
enters the duodenum. The PANCREAS makes strong enzymes which
help break down the fats, carbohydrates, and proteins in
the mayonnaise, bread, and turkey, respectively. This is
where most of our sandwich is fully broken down! The pancreatic
juice also contains bicarbonate, which neutralizes the strong
hydrochloric acid the stomach has contributed to the mixture.
The
tail end of the DUODENUM, the JEJUNUM and the ILEUM absorb
the nutrients from the broken down food. They also reabsorb
water from the food mixture, and from all the saliva and
other secretions that were used to break down the food.
The small intestine also contains helpful bacteria which
aid the digestion of certain vitamins. It may take
2-4 hours for food to pass from one end of the small intestine
to the other.
After
the remaining undigested food, including the chewed-up lettuce,
passes through the last part of the small intestine (the
ILEUM), it passes through another SPHINCTER and enters the
LARGE INTESTINE, also known as the COLON.
The
colon is normally inhabited by a large number of bacteria
(see the section on gas), and is between 3 and 7 feet long
when fully stretched. The COLON's main function is to dry
out the remaining food mixture to form stool. It does
this by reabsorbing water. Stool slowly forms as it
goes through the colon, starting out as a runny mixture
in the beginning and ending as soft, but formed, stool.
The colon also makes mucus to help the stool slide along
through the colon. The turkey sandwich may take up
to 48 hours total to complete its journey through the gut
- most of that time is spent in the colon.
At
this point, what's left in the stool? Most of the
carbohydrates, fats and protein have been digested and the
nutrients extracted. What remains is undigested and indigestible
food, which includes fiber - like most of the lettuce in
the sandwich, bile, and mucus. Stool also contains a large
amount of bacteria - in fact, bacteria make up around 60%
of the final stool!
The
last parts of the colon are the SIGMOID COLON (so named
because it is slightly shaped like an "S"), the RECTUM,
and the ANUS. The RECTUM is where stool is stored before
excreted. The opening through which stool leaves your
body is called the ANUS (double-pointed arrow). The ANUS
has two muscular sphincters, the Internal (I), and the External
(E) sphincters. These strong muscles are crucial in
keeping the stool in your RECTUM until you can find a nice
toilet. You can consciously control the external sphincter,
but not the internal one. The Levator Ani (L) is part
of the pelvic floor muscles that also help keep you from
moving your bowels before you find a toilet.
