stomach, small intestine, large intestine, rectum) and accessory organs (salivary
glands, liver, pancreas, gallbladder) and ducts. The alimentary canal is a hollow
tube lined with mucous membrane. The gastrointestinal tract functions to digest
food, absorb nutrients, propel the contents through the lumen, and eliminate the
waste products.
Digestion of food has both mechanical and chemical components. Both pro-
cesses begin in the mouth. Chewing, movement through the gastrointestinal tract,
and churning within the stomach are parts of the mechanical process. Saliva,
hydrochloric acid, bile, and other digestive enzymes all contribute to the chemical
process of digestion.
The esophagus extends from the oropharynx to the stomach. At the top of the
esophagus is the upper esophageal sphincter (UES) to prevent the influx of air into
the esophagus during respiration. At the bottom of the esophagus is the lower
esophageal sphincter (LES) to prevent the reflux of acid from the stomach into the
esophagus.
The contents of the esophagus empty into the stomach through the cardiac
sphincter. The stomach secretes gastrin, which promotes secretion of pepsinogen
and hydrochloric acid, pepsin, and lipase, all of which aid digestion and mucous
which helps protect the stomach lining.
The liver is a very vascular organ located in the right upper quadrant of the
abdomen under the diaphragm. It has two main lobes that are comprised of smaller
lobules. The liver stores a variety of vitamins and minerals. It metabolizes pro-
teins; synthesizes plasma proteins, fatty acids, and triglycerides; and stores and
releases glycogen. The liver detoxifies foreign substances such as alcohol, drugs,
or chemicals. The liver forms and secretes bile to aid in digestion of fat. Bile will
release into the gall bladder for storage or into the duodenum if needed for diges-
tion if the sphincter of Oddi is open due to secretion of the digestive enzymes
secretin, cholecystokinin, and gastrin. The gall bladder is a small receptacle that
holds bile until it is needed. It is located on the inferior aspect of the liver.
The pancreas is located retroperitoneally in the upper abdomen near the stom-
ach and extends from just right of midline to the left toward the spleen. The pan-
creas has both endocrine and exocrine functions. The endocrine functions include
secretion of insulin in response to elevations in blood glucose from the beta cells
of the islets of Langerhans and glucagon in response to decrease in blood glucose
from the alpha cells. The exocrine function includes secretion of trypsin, lipase,
amylase, and chymotrypsin to aid in digestion.
The small intestine is comprised of the duodenum, jejunum, and the ileum. The
duodenum attaches to the stomach, is about one foot long and C-shaped, and
curves to the left around the pancreas. The common bile duct and pancreatic duct
enter here. The jejunum is between the duodenum and ileum and is about eight
feet long. The last portion of the small intestine is the ileum which is up to twelve
feet long, depending on the size of the patient. The ileocecal valve separates the
ileum from the large intestine. The appendix is found at this juncture. The large
intestine can be broken down into the ascending colon, transverse colon, descend-
ing colon, and sigmoid colon. The sigmoid colon joins the rectum and ultimately
the anal canal.
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