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Digestive System and How It WorksThe
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. 
Why
is digestion important?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. How
is food digested?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. Movement
of Food Through the System
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. Production
of Digestive JuicesThe
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. Absorption
and Transport of NutrientsDigested
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. How
is the digestive process controlled?Hormone
RegulatorsA
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):
- Gastrin causes the
stomach to produce an acid for dissolving and digesting some foods. It is also
necessary for the normal growth of the lining of the stomach, small intestine,
and colon.
- Secretin
causes the pancreas to send out a digestive juice that is rich in bicarbonate.
It stimulates the stomach to produce pepsin, an enzyme that digests protein, and
it also stimulates the liver to produce bile.
- CCK
causes the pancreas to grow and to produce the enzymes of pancreatic juice, and
it causes the gallbladder to empty.
Additional
hormones in the digestive system regulate appetite: - Ghrelin
is produced in the stomach and upper intestine in the absence of food in the digestive
system and stimulates appetite.
- Peptide
YY is produced in the GI tract in response to a meal in the system and inhibits
appetite.
Both
of these hormones work on the brain to help regulate the intake of food for energy. Nerve
RegulatorsTwo
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. See also:
About
Diarrhea
National Digestive Diseases
Information Clearinghouse 2
Information Way
Bethesda, MD 20892–3570
Email: http://digestive.niddk.nih.gov/about/contact.htm 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 of 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. The 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. | 