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The Digestive System Detail & Brief
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
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 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
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
mixture is referred to as chyme.)
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
The Small Intestine/Bowel
The mixture of food, liquid, and digestive juice (chyme) that passes out of the stomach,
in a regulated controlled manner, enters into the small intestine/bowel. The average
total length of the normal small bowel in adults is about 7 meters/22 feet. The small
intestine has 3 segments:
● the duodenum,
● the jejunum, and
● the ileum.
● Each part or section performs an important role in nutrient absorption.
Duodenum – The chyme first enters into the duodenum where it is exposed to
secretions that aid digestion. The secretions include bile salts, enzymes, and
bicarbonate. The bile salts from the liver help digest fats and fat-soluble vitamins
(Vitamin A, D, E, and K). Pancreatic enzymes help digest carbohydrates and fats.
Bicarbonate from the pancreas neutralizes the acid from the stomach.
Jejunum – The chyme is then further transited down into the second or middle part of
the small intestine, the jejunum. Mainly in the first half of the jejunum, the majority
(about 90%) of nutrient absorption occurs involving proteins, carbohydrates, vitamins,
Ileum – The ileum is the last section of the small intestine and leads to the large
intestine or colon. The ileum mainly absorbs water, bile salts, and vitamin B12.
The ileocecal valve is a one-way valve located between the ileum and the cecum, which is
the first portion of the colon. This valve helps control the passage of contents into the
colon and increases the contact time of nutrients and electrolytes (essential minerals)
with the small intestine. It also prevents back-flow (reflux) from the colon up into the
ileum, and helps minimize the movement of bacteria from the large intestine up into the
The Large Intestine/Bowel, or Colon:The primary function of the large intestine or colon is to absorb fluids and electrolytes,
particularly sodium and potassium, and to convert remaining luminal contents into
more solid stool.
The colon absorbs on average 1–1.5 liters (about 1–1.5 quarts) of fluid every day and has
a capacity to adapt its fluid absorption to as much as 5 liters/quarts per day if needed.
Another function of the colon is to break down (ferment) dietary fiber to produce short
chain fatty acids – substances that can be absorbed and provide added nutrition.
The first portion of the colon, the cecum, is shaped like a pouch, and is the area of
storage for the contents arriving from the ileum. The second portion is the ascending
colon, where fluids are absorbed and where some stool formation begins.
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
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
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. 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.
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
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.
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.
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.
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, E, 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.
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 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.
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 GI tract in response to a meal in the system and
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 and
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.