The digestive system is made up of the digestive tract—a series of hollow
organs joined in a long, twisting tube from the mouth to the anus—and other
organs that help the body break down and absorb food (see figure).
Organs that make up the digestive tract are the mouth, esophagus, stomach,
small intestine, large intestine—also called the colon—rectum, and anus. Inside
these hollow organs 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. The digestive tract also contains a layer of smooth muscle that
helps break down food and move it along the tract.
Two “solid” digestive organs, the liver and the pancreas, produce digestive
juices that reach the intestine through small tubes called ducts. The
gallbladder stores the liver's digestive juices until they are needed in the
intestine. Parts of the nervous and circulatory systems also play major roles
in the digestive system.
Why is digestion important?
When you eat foods—such as bread, meat, and vegetables—they are not in a
form that the body can use as nourishment. 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 the body can use them to
build and nourish cells and to provide energy.
How is food digested?
Digestion involves mixing food with digestive juices, moving it through the
digestive tract, and breaking down large molecules of food into smaller
molecules. Digestion begins in the mouth, when you chew and swallow, and is
completed in the small intestine.
Movement of Food Through the System
The large, hollow organs of the digestive tract contain a layer of muscle
that enables their walls to move. The movement of organ walls can propel food
and liquid through the system and also can mix the contents within each organ.
Food moves from one organ to the next through muscle action called peristalsis.
Peristalsis looks like an ocean wave traveling through the muscle. The muscle
of the organ contracts to create 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 you are able to start swallowing by choice, once the swallow begins,
it becomes involuntary and proceeds under the control of the nerves.
Swallowed food is pushed into the esophagus, which connects the throat
above with the stomach below. At the junction of the esophagus and stomach,
there is a ringlike muscle, called the lower esophageal sphincter, closing the
passage between the two organs. As food approaches the closed sphincter, the
sphincter relaxes and allows the food to pass through to the stomach.
The stomach has three mechanical tasks. First, it stores the swallowed food
and liquid. To do this, the muscle of the upper part of the stomach relaxes to
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 kind of food
and the degree of muscle action of the emptying stomach and the small
intestine. Carbohydrates, for example, spend the least amount of time in the
stomach, while protein stays in the stomach longer, and fats the longest. As
the food dissolves into the juices from the pancreas, liver, and intestine, the
contents of the intestine are mixed and pushed forward to allow further
digestion.
Finally, the digested nutrients are absorbed through the intestinal walls
and transported throughout the body. 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 pushed into the colon, where they
remain until the feces are expelled by a bowel movement.
Production of Digestive Juices
The digestive 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. An enzyme is a substance that speeds
up chemical reactions in the body.
The next set of digestive glands is in the stomach lining. They produce
stomach acid and an enzyme that digests protein. A thick mucus layer coats the
mucosa and helps keep the acidic digestive juice from dissolving 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. One of
these organs, the pancreas, 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.
The second organ, the liver, produces yet another digestive juice—bile.
Bile is stored between meals in the gallbladder. At mealtime, it is squeezed
out of the gallbladder, through the bile ducts, and into the intestine to mix
with the fat in food. The bile acids dissolve fat into the watery contents of
the intestine, much like detergents that dissolve grease from a frying pan.
After fat is dissolved, it is digested by enzymes from the pancreas and the
lining of the intestine.
Absorption and Transport of Nutrients
Most digested molecules of food, as well as water and minerals, are absorbed
through the small intestine. The mucosa of the small intestine contains many
folds that are covered with tiny fingerlike projections called villi. In turn,
the villi are covered with microscopic projections called microvilli. These
structures create a vast surface area through which nutrients can be absorbed.
Specialized cells allow absorbed materials to cross the mucosa into the blood,
where they are carried off in the bloodstream to other parts of the body for
storage or further chemical change. This part of the process varies with
different types of nutrients.
Carbohydrates. The Dietary Guidelines for
Americans 2005 recommend that 45 to 65 percent of total daily calories be from
carbohydrates. Foods rich in carbohydrates include bread, potatoes, dried peas
and beans, rice, pasta, fruits, and vegetables. Many of these foods contain
both starch and fiber.
The digestible carbohydrates—starch and sugar—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 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.
Sugars are digested in one step. An enzyme in the lining of the small
intestine digests sucrose, also known as table sugar, into glucose and
fructose, which are absorbed through the intestine into the blood. Milk
contains another type of sugar, lactose, which is changed into absorbable
molecules by another enzyme in the intestinal lining.
Fiber is undigestible and moves through the digestive tract without being
broken down by enzymes. Many foods contain both soluble and insoluble fiber.
Soluble fiber dissolves easily in water and takes on a soft, gel-like texture
in the intestines. Insoluble fiber, on the other hand, passes essentially
unchanged through the intestines.
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. Then in the small intestine, several enzymes from the
pancreatic juice and the lining of the intestine complete the breakdown of huge
protein molecules into small molecules called amino acids. These small
molecules can be absorbed through 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 intestine. The bile
acids produced by the liver dissolve fat into tiny droplets and allow
pancreatic and intestinal enzymes to break the large fat molecules into smaller
ones. Some of these small molecules are fatty acids and cholesterol. The bile
acids combine with the fatty acids and cholesterol and help these molecules
move into the cells of the mucosa. In these cells the small molecules are
formed back into large ones, 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 food that is absorbed through the small intestine are
vitamins. The two 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, E, and K). Fat-soluble vitamins are stored
in the liver and fatty tissue of the body, whereas water-soluble vitamins are
not easily stored and excess amounts are flushed out in the urine.
Water and salt. Most of the material absorbed through the small intestine is water in which
salt is dissolved. The salt and water come from the food and liquid you swallow
and the juices secreted by the many digestive glands.
How is the digestive process controlled?
Hormone 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 main hormones that control digestion are gastrin, secretin, and
cholecystokinin (CCK):
·
Gastrin causes the stomach to produce an acid for dissolving and digesting some
foods. Gastrin is also necessary for normal cell growth in the lining of the
stomach, small intestine, and colon.
·
Secretin causes the pancreas to send out a digestive juice that is rich in
bicarbonate. The bicarbonate helps neutralize the acidic stomach contents as
they enter the small intestine. Secretin also stimulates the stomach to produce
pepsin, an enzyme that digests protein, and stimulates the liver to produce
bile.
·
CCK causes the pancreas to
produce the enzymes of pancreatic juice, and causes the gallbladder to empty.
It also promotes normal cell growth of the pancreas.
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 digestive 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. Researchers are studying other hormones that may play a part
in inhibiting appetite, including glucagon-like peptide-1 (GPL-1),
oxyntomodulin (+ ), and pancreatic polypeptide.
Nerve Regulators
Two types of nerves help control the action of the digestive system.
Extrinsic, or outside, nerves come to the digestive organs from the brain
or the spinal cord. They release two chemicals, acetylcholine and adrenaline.
Acetylcholine causes the muscle layer of the digestive organs to squeeze with
more force and increase the “push” of food and juice through the digestive
tract. It also causes the stomach and pancreas to produce more digestive juice.
Adrenaline has the opposite effect. It relaxes the muscle of the stomach and
intestine and decreases the flow of blood to these organs, slowing or stopping
digestion.
The intrinsic, or inside, nerves 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.
Together, nerves, hormones, the blood, and the organs of the digestive
system conduct the complex tasks of digesting and absorbing nutrients from the
foods and liquids you consume each day.
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