Your Digestive System and How It Works
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 digest food.
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.
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 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 too
start swallowing by choice, once the swallow begins, it becomes involuntary and proceeds under thee
control of the nerves..
The esophagus is the organ into which the swallowed food is pushed. It connects the throat abovee
with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valvee
closing the passage between the two organs. However, as the food approaches the closed ring, thee
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 mustt
store the swallowed food and liquid. This requires the muscle of the upper part of the stomach too
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 materialss
by its muscle action. The third task of the stomach is to empty its contents slowly into the smalll
intestine..
Several factors affect emptying of the stomach, including the nature of the food (mainly its fat andd
protein content) and the degree of muscle action of the emptying stomach and the next organ too
receive the stomach contents (the small intestine). As the food is digested in the small intestine andd
dissolved into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixedd
and pushed forward to allow further digestion..
Finally, all of the digested nutrients are absorbed through the intestinal walls. The waste products off
this process include undigested parts of the food, known as fiber, and older cells that have beenn
shed from the mucosa. These materials are propelled into the colon, where they remain, usually for aa
day or two, until the feces are expelled by a bowel movement..
Production of Digestive Juices
Glands of the digestive system are crucial to the process of digestion. They produce both the juices
that break down the food and the hormones that help to control the process.
The glands that act first are in the mouth--the salivary glands. Saliva produced by these glandss
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 enzymee
that digests protein. One of the unsolved puzzles of the digestive system is why the acid juice of thee
stomach does not dissolve the tissue of the stomach itself. In most people, the stomach mucosa iss
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 otherr
digestive organs mix with the food to continue the process of digestion. One of these organs is thee
pancreas. It produces a juice that contains a wide array of enzymes to break down thee
carbohydrates, fat, and protein in our food. Other enzymes that are active in the process come fromm
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 thee
gallbladder. At mealtime, it is squeezed out of the gallbladder into the bile ducts to reach the intestinee
and mix with the fat in our food. The bile acids dissolve the fat into the watery contents of thee
intestine, much like detergents that dissolve grease from a frying pan. After the fat is dissolved, it iss
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 juicee
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 moleculess
that can be absorbed into the blood. Glucose is carried through the bloodstream to the liver, where itt
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 thee
small intestine digests table sugar into glucose and fructose, each of which can be absorbed from thee
intestinal cavity into the blood. Milk contains yet another type of sugar, lactose, which is changedd
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, 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. In a healthy adult, more than a gallon of water containing over
an ounce of salt is absorbed from the intestine every 24 hours.
How Is the Digestive Process Controlled?
Hormone Regulators
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.
Nerve Regulators
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 networkk
embedded in the walls of the esophagus, stomach, small intestine, and colon. The intrinsic nerves aree
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.
Links to other sites on the Web
Back to Other Gastroenterological Disorders
Information provided by Tomás Ordóñez M.D. Paseo Bolívar 12-101 Colonia Centro ZIP Code 31000 Chihuahua, Chihuahua, México. Tel. and Fax (14) 10 50 91.
© 1997 tomaso@mail.interred.net.mx
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