esophagus, stomach, small intestine (duodenum, jejunum, and ileum), large
intestine (cecum, colon, rectum), and the anus.
In addition to these major structures, the GI system has several accessory
organs and glands that include the salivary glands, pancreas, gallbladder, and liver.
Food that enters the oral cavity is broken down into small pieces in the mouth.
Starches are then digested by amylase found in saliva. Small pieces of food are
voluntarily moved to the back of the mouth and moved down to the esophagus
in a process commonly referred to as swallowing. When the food reaches the
esophagus, it is moved to the stomach and intestines with an involuntary move-
ment called peristalsis.
The esophagus is a tube connecting the oral cavity to the stomach and is lined
with mucous membranes that secrete mucus. The esophagus has two sphincters.
These are the superior (hyperpharyngeal) sphincter and the lower sphincter that
prevents gastric juices from entering the esophagus (gastric reflux).
The stomach is a hollow organ that holds between 1000 to 2000 mL of con-
tents that takes about 2–3 hours to empty. It, too, has two sphincters. These are
the cardiac sphincter (located at the opening of the esophagus), and the pyloric
sphincter (that connects the stomach to the head of the duodenum).
The stomach has mucosal folds containing glands that secrete gastric juices
used to break down food (digest) into its chemical elements. Lipid-soluble drugs
and alcohol are absorbed in the stomach. There are four types of cells in the
stomach. These are:Chief Cells
Chief cells secrete proenzyme pepsinogen (pepsin).Parietal Cells
Parietal cells secrete hydrochloric acid (HCl)Gastrin-Producing Cells
Gastrin-producing cells secrete gastrin, which is a hormone that regulates the
release of enzymes during digestion.Mucus-Producing Cells
Mucus-producing cells release mucus that protect the stomach lining from the
gastric juices.
The small intestine extends from the ileocecal valve at the stomach to the duo-
denum. The cecum is attached to the duodenum, which is the site where most
medication is absorbed. Most foods are also absorbed in the small intestine.(^330) CHAPTER 18 Gastrointestinal System