The digestive system is made up of the digestive
tract—a series of hollow organs joined in a long, twisting tube
from the mouth to the anus—and other organs that help the body
break down and absorb food (see figure).
Organs that make up the digestive tract are the mouth, esophagus,
stomach, small intestine, large intestine—also called the
colon—rectum, and anus. Inside these hollow organs 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. The digestive tract also contains a layer of smooth muscle
that helps break down food and move it along the tract.
Two “solid” digestive organs, the liver and the pancreas, produce
digestive juices that reach the intestine through small tubes
called ducts. The gallbladder stores the liver’s digestive juices
until they are needed in the intestine. Parts of the nervous and
circulatory systems also play major roles in the digestive system.
Why is digestion important?
When you eat foods—such as bread, meat, and vegetables—they are
not in a form that the body can use as nourishment. 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 the body can use them to build
and nourish cells and to provide energy.
How is food digested?
Digestion involves mixing food with digestive juices, moving it
through the digestive tract, and breaking down large molecules of
food into smaller molecules. Digestion begins in the mouth, when
you chew and swallow, and is completed in the small intestine.
How is the digestive process controlled? Hormone 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 main hormones that control digestion are gastrin, secretin,
and cholecystokinin (CCK):
Gastrin causes the stomach to produce an acid for
dissolving and digesting some foods. Gastrin is also necessary for
normal cell growth in the lining of the stomach, small intestine,
and colon.
Secretin causes the pancreas to send out a digestive juice
that is rich in bicarbonate. The bicarbonate helps neutralize the
acidic stomach contents as they enter the small intestine.
Secretin also stimulates the stomach to produce pepsin, an enzyme
that digests protein, and stimulates the liver to produce bile.
CCK causes the pancreas to produce the enzymes of
pancreatic juice, and causes the gallbladder to empty. It also
promotes normal cell growth of the pancreas.
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 digestive tract in response to a
meal in the system and inhibits appetite.
Both of these hormones work on the brain to help regulate the
intake of food for energy. Researchers are studying other hormones
that may play a part in inhibiting appetite, including glucagon-like
peptide-1 (GPL-1), oxyntomodulin (+ ), and pancreatic polypeptide.
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