THE DIGESTIVE SYSTEM
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
follows the path: mouth, esophagus, stomach, small intestine,
large intestine, rectum, anus.
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.
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.
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
of Food Through the System
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.
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.
PATH OF DIGESTION
Mechanical and chemical digestion begin in the mouth where
food is chewed. The glands that act first are in the
mouth—the salivary glands. Saliva produced by these glands
contains an enzyme called ptyalin that begins to digest
the starch from food into smaller molecules (maltose).
no digestion occurs here. 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 next set of digestive glands is in the stomach lining.
This is where protein begins it digestion. The stomach lining
produce stomach acid (HCl) and an enzyme called pepsin 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.
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.
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).
Intestine: The small intestine is where most chemical
digestion occurs. 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.
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
three major classes of nutrients that undergo digestion in the
small intestine are: proteins, lipids (fats) and carbohydrates
acids and gylcerol
lipase with help from bile (not an enzyme)
food broken down in the small intestine is the size of molecules
and can now pass through the villi into the blood stream through
the process of diffusion.
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. 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.
Intestine - Colon -- These
materials are propelled into the colon, where they remain, usually
for a day or two. Its function is to absorb water from the remaining
indigestible food matter, and then to pass useless waste material
from the body. It also compacts feces, and stores fecal matter
in the rectum until it can be discharged via the anus in defecation.
Dietary fiber, or simply called fiber, refers to plant cell
wall components that are not digestible.
The large intestine houses over 700 species of bacteria that
perform a variety of functions. The large intestine absorbs
some of the products formed by the bacteria inhabiting this
region. Undigested polysaccharides (fiber) are metabolized to
short-chain fatty acids by bacteria in the large intestine and
absorbed by passive diffusion. Cellulose is not digested
at all in the human.