The Digestive System (cont.)
Stop 3: The Stomach and Small Intestine
The stomach is a sac-like organ with strong muscular walls. In addition to
holding food, it serves as the mixer and grinder of food. The stomach secretes
acid and powerful enzymes that continue the process of breaking the food down
and changing it to a consistency of liquid or paste. From there, food moves to
the small intestine. Between meals the non-liquefiable remnants are released
from the stomach and ushered through the rest of the intestines to be
eliminated.
Made up of three segments -- the duodenum, jejunum and ileum -- the small
intestine also breaks down food using enzymes released by the pancreas and bile
from the liver. Peristalsis is also at work in this organ, moving food through
and mixing it up with the digestive secretions from the pancreas and liver,
including bile. The duodenum is largely responsible for the continuing breakdown
process, with the jejunum and ileum being mainly responsible for absorption of
nutrients into the bloodstream.
A more technical name for this part of the process is "motility" since it
involves moving or emptying food particles from one part to the next. This
process is highly dependant on the activity of a large network of nerves,
hormones and muscles. Problems with any of these components can cause a variety
of conditions.
While in the small intestine nutrients from food are absorbed through the
walls of the intestine and into the bloodstream. What's leftover (the waste)
moves into the large intestine (large bowel or colon).
Everything above the large intestine is called the upper GI tract. Everything
below is the lower GI tract.
Stop 4: The Colon, Rectum and Anus
The colon (large intestine) is a five- to seven -foot -long muscular tube
that connects the small intestine to the rectum. It is made up of the ascending
(right) colon, the transverse (across) colon, the descending (left) colon and
the sigmoid colon, which connects to the rectum. The appendix is a small tube
attached to the ascending colon. The large intestine is a highly specialized
organ that is responsible for processing waste so that defecation (excretion of
waste) is easy and convenient.
Stool, or waste left over from the digestive process, passes through the
colon by means of peristalsis, first in a liquid state and ultimately in solid
form. As stool passes through the colon, any remaining water is absorbed. Stool
is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it
into the rectum, usually once or twice a day.
It normally takes about 36 hours for stool to get through the colon. The
stool itself is mostly food debris and bacteria. These bacteria perform several
useful functions, such as synthesizing various vitamins, processing waste
products and food particles, and protecting against harmful bacteria. When the
descending colon becomes full of stool it empties its contents into the rectum
to begin the process of elimination.
The rectum is an eight-inch chamber that connects the colon to the anus. The
rectum:
- Receives stool from the colon
- Lets the person know there is stool to be evacuated
- Holds the stool until evacuation happens
When anything (gas or stool) comes into the rectum, sensors send a message to
the brain. The brain then decides if the rectal contents can be released or not.
If they can, the sphincters relax and the rectum contracts, expelling its
contents. If the contents cannot be expelled, the sphincters contract and the
rectum accommodates so that the sensation temporarily goes away.
The anus is the last part of the digestive tract. It consists of the muscles
that line the pelvis (pelvic floor muscles) and two other muscles called anal
sphincters (internal and external).
The pelvic floor muscle creates an angle between the rectum and the anus that
stops stool from coming out when it is not supposed to. The anal sphincters
provide fine control of stool. The internal sphincter is always tight, except
when stool enters the rectum. It keeps us continent (not releasing stool) when
we are asleep or otherwise unaware of the presence of stool. When we get an urge
to defecate (go to the bathroom), we rely on our external sphincter to keep the
stool in until we can get to the toilet.
Next: Accessory organs in digestion »
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