Your Digestive System and How It Works
On this page:
- Why is digestion important?
- How is food digested?
- How is the digestive process controlled?
The digestive system is a series of hollow organs joined in a
long, twisting tube from the mouth to the anus. 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 Juices
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
Absorption and Transport of Nutrients
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
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?
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
- 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,
- 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
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
- 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.
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
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