The Structure and Function of the Digestive System
Your digestive system is uniquely constructed to perform its specialized
function of turning food into the energy you need to survive and packaging the
residue for waste disposal. To help you understand how the many parts of the
digestive system work together, here is an overview of the structure and
function of this complex system.
The mouth is the beginning of the digestive tract; and, in fact, digestion
starts here when taking the first bite of food. Chewing breaks the food into
pieces that are more easily digested, while saliva mixes with food to begin the
process of breaking it down into a form your body can absorb and use.
Located in your throat near your trachea (windpipe), the esophagus receives
food from your mouth when you swallow. By means of a series of muscular
contractions called peristalsis, the esophagus delivers food to your stomach.
The stomach is a hollow organ, or "container," that holds food
while it is being mixed with enzymes that continue the process of breaking down
food into a usable form. Cells in the lining of the stomach secrete a strong
acid and powerful enzymes that are responsible for the breakdown process. When
the contents of the stomach are sufficiently processed, they are released into
the small intestine.
Made up of three segments — the duodenum, jejunum, and ileum — the small
intestine is a 22-foot long muscular tube that breaks down food using enzymes
released by the pancreas and bile from the liver. Peristalsis also is at work in
this organ, moving food through and mixing it with digestive secretions from the
pancreas and liver. The duodenum is largely responsible for the continuous
breaking-down process, with the jejunum and ileum mainly responsible for
absorption of nutrients into the bloodstream.
Contents of the small intestine start out semi-solid, and end in a liquid
form after passing through the organ. Water, bile, enzymes, and mucous contribute
to the change in consistency. Once the nutrients have been absorbed and the
leftover-food residue liquid has passed through the small intestine, it then
moves on to the large intestine, or colon.
The pancreas secretes digestive enzymes into the duodenum, the first segment
of the small intestine. These enzymes break down protein, fats, and
carbohydrates. The pancreas also makes insulin, secreting it directly into the
bloodstream. Insulin is the chief hormone for metabolizing sugar.
The liver has multiple functions, but its main function within the digestive
system is to process the nutrients absorbed from the small intestine. Bile from
the liver secreted into the small intestine also plays an important role in
digesting fat. In addition, the liver is the body’s chemical
"factory." It takes the raw materials absorbed by the intestine and
makes all the various chemicals the body needs to function. The liver also
potentially harmful chemicals. It breaks down and secretes many drugs.
The gallbladder stores and concentrates bile, and then releases it into the
duodenum to help absorb and digest fats.
Colon (large intestine)
The colon is a 6-foot long muscular tube that connects the small intestine
to the rectum. The large intestine is made up of the cecum, 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 cecum. The large intestine is a highly specialized organ that is
responsible for processing waste so that emptying the bowels is easy and
Stool, or waste left over from the digestive process, is passed through the
colon by means of peristalsis, first in a liquid state and ultimately in a solid
form. As stool passes through the colon, water is removed. Stool is stored in
the sigmoid (S-shaped) colon until a "mass movement" empties it into
the rectum 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, or feces, it
empties its contents into the rectum to begin the process of elimination.
The rectum (Latin for "straight") is an 8-inch chamber that
connects the colon to the anus. It is the rectum's job to receive stool from the
colon, to let the person know that there is stool to be evacuated, and to hold
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, disposing its contents. If the contents cannot be
disposed, the sphincter contracts and the rectum accommodates so that the
sensation temporarily goes away.
The anus is the last part of the digestive tract. It is a 2-inch long canal
consisting of the pelvic floor muscles and the two anal sphincters (internal and
external). The lining of the upper anus is specialized to detect rectal
contents. It lets you know whether the contents are liquid, gas, or solid. The
anus is surrounded by sphincter muscles that are important in allowing control
of stool. 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 internal
sphincter is always tight, except when stool enters the rectum. It keeps us
continent when we are asleep or otherwise unaware of the presence of stool. When
we get an urge to go to the bathroom, we rely on our external sphincter to hold
the stool until reaching a toilet, where it then relaxes to release the
© Copyright 1995-2005 The
Cleveland Clinic Foundation. All rights reserved.