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The digestive system is a series of
hollow organs joined in a long, twisting tube from the mouth to the
anus (see figure). Inside this tube is a lining called the mucosa.
In the mouth, stomach, and small intestine, the mucosa contains tiny
glands that produce juices to help digest food.
Two solid organs, the liver and the
pancreas, produce digestive juices that reach the intestine through
small tubes. In addition, parts of other organ systems (for
instance, nerves and blood) play a major role in the digestive
system.
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When we eat such things as bread,
meat, and vegetables, they are not in a form that the body can use
as nourishment. Our food and drink must be changed into smaller
molecules of nutrients before they can be absorbed into the blood
and carried to cells throughout the body. Digestion is the process
by which food and drink are broken down into their smallest parts so
that the body can use them to build and nourish cells and to provide
energy.
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Digestion involves the mixing of food,
its movement through the digestive tract, and the chemical breakdown
of the large molecules of food into smaller molecules. Digestion
begins in the mouth, when we chew and swallow, and is completed in
the small intestine. The chemical process varies somewhat for
different kinds of food.
The large, hollow organs of the
digestive system contain muscle that enables their walls to move.
The movement of organ walls can propel food and liquid and also can
mix the contents within each organ. Typical movement of the
esophagus, stomach, and intestine is called peristalsis. The action
of peristalsis looks like an ocean wave moving through the muscle.
The muscle of the organ produces a narrowing and then propels the
narrowed portion slowly down the length of the organ. These waves of
narrowing push the food and fluid in front of them through each
hollow organ.
The first major muscle movement occurs
when food or liquid is swallowed. Although we are able to start
swallowing by choice, once the swallow begins, it becomes
involuntary and proceeds under the control of the nerves.
The esophagus is the organ into which
the swallowed food is pushed. It connects the throat above with the
stomach below. At the junction of the esophagus and stomach, there
is a ringlike valve closing the passage between the two organs.
However, as the food approaches the closed ring, the surrounding
muscles relax and allow the food to pass.
The food then enters the stomach,
which has three mechanical tasks to do. First, the stomach must
store the swallowed food and liquid. This requires the muscle of the
upper part of the stomach to relax and accept large volumes of
swallowed material. The second job is to mix up the food, liquid,
and digestive juice produced by the stomach. The lower part of the
stomach mixes these materials by its muscle action. The third task
of the stomach is to empty its contents slowly into the small
intestine.
Several factors affect emptying of the
stomach, including the nature of the food (mainly its fat and
protein content) and the degree of muscle action of the emptying
stomach and the next organ to receive the contents (the small
intestine). As the food is digested in the small intestine and
dissolved into the juices from the pancreas, liver, and intestine,
the contents of the intestine are mixed and pushed forward to allow
further digestion.
Finally, all of the digested nutrients
are absorbed through the intestinal walls. The waste products of
this process include undigested parts of the food, known as fiber,
and older cells that have been shed from the mucosa. These materials
are propelled into the colon, where they remain, usually for a day
or two, until the feces are expelled by a bowel movement.
The glands that act first are in the
mouth-the salivary glands. Saliva produced by these glands contains
an enzyme that begins to digest the starch from food into smaller
molecules.
The next set of digestive glands is in
the stomach lining. They produce stomach acid and an enzyme that
digests protein. One of the unsolved puzzles of the digestive system
is why the acid juice of the stomach does not dissolve the tissue of
the stomach itself. In most people, the stomach mucosa is able to
resist the juice, although food and other tissues of the body
cannot.
After the stomach empties the food and
juice mixture into the small intestine, the juices of two other
digestive organs mix with the food to continue the process of
digestion. One of these organs is the pancreas. It produces a juice
that contains a wide array of enzymes to break down the
carbohydrate, fat, and protein in food. Other enzymes that are
active in the process come from glands in the wall of the intestine
or even a part of that wall.
The liver produces yet another
digestive juice-bile. The bile is stored between meals in the
gallbladder. At mealtime, it is squeezed out of the gallbladder into
the bile ducts to reach the intestine and mix with the fat in our
food. The bile acids dissolve the fat into the watery contents of
the intestine, much like detergents that dissolve grease from a
frying pan. After the fat is dissolved, it is digested by enzymes
from the pancreas and the lining of the intestine.
Digested molecules of food, as well as
water and minerals from the diet, are absorbed from the cavity of
the upper small intestine. Most absorbed materials cross the mucosa
into the blood and are carried off in the bloodstream to other parts
of the body for storage or further chemical change. As already
noted, this part of the process varies with different types of
nutrients.
Carbohydrates. It is
recommended that about 55 to 60 percent of total daily calories be
from carbohydrates. Some of our most common foods contain mostly
carbohydrates. Examples are bread, potatoes, legumes, rice,
spaghetti, fruits, and vegetables. Many of these foods contain both
starch and fiber.
The digestible carbohydrates are
broken into simpler molecules by enzymes in the saliva, in juice
produced by the pancreas, and in the lining of the small intestine.
Starch is digested in two steps: First, an enzyme in the saliva and
pancreatic juice breaks the starch into molecules called maltose;
then an enzyme in the lining of the small intestine (maltase) splits
the maltose into glucose molecules that can be absorbed into the
blood. Glucose is carried through the bloodstream to the liver,
where it is stored or used to provide energy for the work of the
body.
Table sugar is another carbohydrate
that must be digested to be useful. An enzyme in the lining of the
small intestine digests table sugar into glucose and fructose, each
of which can be absorbed from the intestinal cavity into the blood.
Milk contains yet another type of sugar, lactose, which is changed
into absorbable molecules by an enzyme called lactase, also found in
the intestinal lining.
Protein. Foods such as meat,
eggs, and beans consist of giant molecules of protein that must be
digested by enzymes before they can be used to build and repair body
tissues. An enzyme in the juice of the stomach starts the digestion
of swallowed protein. Further digestion of the protein is completed
in the small intestine. Here, several enzymes from the pancreatic
juice and the lining of the intestine carry out the breakdown of
huge protein molecules into small molecules called amino acids.
These small molecules can be absorbed from the hollow of the small
intestine into the blood and then be carried to all parts of the
body to build the walls and other parts of cells.
Fats. Fat molecules are a rich
source of energy for the body. The first step in digestion of a fat
such as butter is to dissolve it into the watery content of the
intestinal cavity. The bile acids produced by the liver act as
natural detergents to dissolve fat in water and allow the enzymes to
break the large fat molecules into smaller molecules, some of which
are fatty acids and cholesterol. The bile acids combine with the
fatty acids and cholesterol and help these molecules to move into
the cells of the mucosa. In these cells the small molecules are
formed back into large molecules, most of which pass into vessels
(called lymphatics) near the intestine. These small vessels carry
the reformed fat to the veins of the chest, and the blood carries
the fat to storage depots in different parts of the body.
Vitamins. Another vital part of
our food that is absorbed from the small intestine is the class of
chemicals we call vitamins. The two different types of vitamins are
classified by the fluid in which they can be dissolved:
water-soluble vitamins (all the B vitamins and vitamin C) and
fat-soluble vitamins (vitamins A, D, and K).
Water and salt. Most of the
material absorbed from the cavity of the small intestine is water in
which salt is dissolved. The salt and water come from the food and
liquid we swallow and the juices secreted by the many digestive
glands.
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A fascinating feature of the digestive
system is that it contains its own regulators. The major hormones
that control the functions of the digestive system are produced and
released by cells in the mucosa of the stomach and small intestine.
These hormones are released into the blood of the digestive tract,
travel back to the heart and through the arteries, and return to the
digestive system, where they stimulate digestive juices and cause
organ movement.
The hormones that control digestion are
gastrin, secretin, and cholecystokinin (CCK):
Additional hormones in the digestive
system regulate appetite:
Both of these hormones work on the
brain to help regulate the intake of food for energy.
Two types of nerves help to control
the action of the digestive system. Extrinsic (outside) nerves come
to the digestive organs from the unconscious part of the brain or
from the spinal cord. They release a chemical called acetylcholine
and another called adrenaline. Acetylcholine causes the muscle of
the digestive organs to squeeze with more force and increase the
"push" of food and juice through the digestive tract. Acetylcholine
also causes the stomach and pancreas to produce more digestive
juice. Adrenaline relaxes the muscle of the stomach and intestine
and decreases the flow of blood to these organs.
Even more important, though, are the
intrinsic (inside) nerves, which make up a very dense network
embedded in the walls of the esophagus, stomach, small intestine,
and colon. The intrinsic nerves are triggered to act when the walls
of the hollow organs are stretched by food. They release many
different substances that speed up or delay the movement of food and
the production of juices by the digestive organs.
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2 Information Way
Bethesda, MD
20892-3570
Email: nddic@info.niddk.nih.gov
The National Digestive Diseases
Information Clearinghouse (NDDIC) is a service of the National
Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The
NIDDK is part of the National Institutes of Health under the U.S.
Department of Health and Human Services. Established in 1980, the
clearinghouse provides information about digestive diseases to
people with digestive disorders and to their families, health care
professionals, and the public. NDDIC answers inquiries, develops and
distributes publications, and works closely with professional and
patient organizations and Government agencies to coordinate
resources about digestive diseases.
Publications produced by the
clearinghouse are carefully reviewed by both NIDDK scientists and
outside experts.
This e-text is not copyrighted. The
clearinghouse encourages users of this e-pub to duplicate and
distribute as many copies as desired.
NIH Publication No. 04-2681
May
2004
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