Digestion: How it Works
Why Is Digestion
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 (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
There are also two solid digestive organs, the liver and
the pancreas, which produce 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.
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
Digestion involves the mixing of food, its movement through the digestive
tract, and 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
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 stomach 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
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 its juice 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 carbohydrates, fat, and protein in our 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 Nutrients
Digested molecules of food, as well as water and minerals from the diet,
are absorbed from the cavity of the upper small intestine. The absorbed
materials cross the mucosa into the blood, mainly, and are carried off
in the bloodstream to other parts of the body for storage or further chemical
change. As noted above, this part of the process varies with different
types of nutrients.
Carbohydrates: An average American adult
eats about half a pound of carbohydrate each day. Some of our most common
foods contain mostly carbohydrates. Examples are bread, potatoes, pastries,
candy, rice, spaghetti, fruits, and vegetables. Many of these foods contain
both starch, which can be digested, and fiber, which the body cannot digest.
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. There are two different types of vitamins, 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,
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. In a healthy adult, more than a
gallon of water containing over an ounce of salt is absorbed from the
intestine every 24 hours.
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 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.
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.