Why is digestion important?
How is food digested?
How is the digestive process controlled?
For More Information
The
digestive system.
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?
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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