Brief Look at the Gastrointestinal System

The gastrointestinal (GI) system consists of the alimentary canal and the digestive tract that begins with the oral cavity and extends to the anus. The major structures of the GI system are the oral cavity (mouth, tongue, and pharynx)

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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 movement 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 contents 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 duodenum. 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.

The duodenum releases secretin, which is a hormone that suppresses gastric acid secretion. This results in the intestinal juices having a higher pH than the gastric juices in the stomach. The hormone cholecystokinin is also released. It simulates the flow of bile into the duodenum. Hormones, bile, and pancreatic enzymes trypsin, chymotrypsin, lipase, and amylase digest carbohydrates, protein, and fat in preparation for absorption in the small intestine.

The small intestine lead into the large intestine where undigested material from the small intestine is collected. The large intestine also absorbs water and secretes mucus while moving the undigested material—using peristaltic contractions—to the rectum where it is eliminated through defecation.

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