|
|
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 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 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 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, 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.
|
|
|
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 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.
|
