Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Gastrointestinal System

C h a p t e r 2 8

There are sphincters at either end of the esophagus. The upper sphincter, the pharyngoesophageal sphincter, consists of a circular layer of striated muscle. It keeps air from entering the esophagus and stomach during breath- ing. The lower esophageal sphincter, also called the gas- troesophageal sphincter , lies just above the area where the esophagus joins the stomach. The circular muscle in this area normally remains tonically contracted, creating a zone of high pressure that serves to prevent reflux of gas- tric contents into the esophagus. During swallowing, there is “receptive relaxation” of the lower esophageal sphincter that allows for easy propulsion of the esophageal contents into the stomach. The lower esophageal sphincter passes through an opening, or hiatus , in the diaphragm as it joins with the stomach, which is located in the abdomen. The portion of the diaphragm that surrounds the lower esoph- ageal sphincter helps to maintain the zone of high pressure to support the function of the lower esophageal sphincter. The stomach is a pouchlike structure that lies in the left side of the abdomen and serves as a food reservoir during the early stages of digestion. The esophagus opens into the stomach through an opening called the cardiac orifice , so named because of its proximity to the heart. The small part of the stomach that surrounds the cardiac orifice is called the cardiac region ; the dome- shaped region that bulges above the cardiac region is called the fundus; the middle portion is called the body; and the funnel-shaped portion that connects with the small intestine is called the pyloric region (Fig. 28-2). The wider and more superior part of the pyloric region, the antrum , narrows to form the pyloric canal as it approaches the small intestine. At the end of the pyloric canal, the circular smooth muscle layer thickens to form the pyloric sphincter. This muscle serves as a valve that controls the rate of stomach emptying and prevents the regurgitation of intestinal contents back into the stomach.

The inner surface of the empty stomach reveals a number of longitudinal folds or ridges called rugae. The rugae, which serve to accommodate expansion and fill- ing of the stomach, almost disappear when the stomach is distended with food. Small Intestine The small intestine, which forms the middle portion of the GI tract, consists of three subdivisions: the duode- num, jejunum, and ileum (see Fig. 28-1). The duodenum, which is approximately 22 cm (10 inches) long, connects the stomach to the jejunum and contains the opening for the common bile duct and the main pancreatic duct. Bile, a fluid synthesized by the liver that breaks down lipids, and pancreatic juices, which facilitate digestion of lipids, carbohydrates, and proteins, enter the intes- tine through these ducts. It is in the jejunum and ileum, which together are approximately 7 m (23 ft) long and must be folded onto themselves to fit into the abdominal cavity, that food is digested and absorbed. Lower Gastrointestinal Tract The large intestine, which forms the lower GI tract, is approximately 1.5 m (5 ft) long. It is divided into the cecum, colon, rectum, and anal canal (see Fig. 28-1). The cecum is a blind pouch that projects down at the junc- tion of the ileum and the colon. The ileocecal valve lies at the upper border of the cecum and prevents the return of feces from the cecum into the small intestine. The appen- dix arises from the cecum approximately 2.5 cm (1 inches) from the ileocecal valve. The colon is further divided into ascending, transverse, descending, and sigmoid portions. The ascending colon extends from the cecum to the under- surface of the liver, where it turns abruptly to form the right colic (hepatic) flexure. The transverse colon crosses the upper half of the abdominal cavity from right to left and then curves sharply downward beneath the lower end of the spleen, forming the left colic (splenic) flexure. The descending colon extends from the colic flexure to the rectum. The rectum extends from the sigmoid colon to the anus. The anal canal passes between the two medial borders of the levator ani muscles. Powerful sphincter muscles guard against fecal incontinence. Gastrointestinal Wall Structure Below the upper third of the esophagus, the GI tract is essentially a four-layered hollow tube of varying diam- eter, but similar structural organization. It consists of four distinct layers: the inner mucosal layer ; the under- lying submucosal layer , the muscularis externa , and the outer serosal layer (Fig. 28-3). Mucosa The mucosa , or inner mucosal layer, is made up of an epithelium lining; an underlying loose connective tis- sue, called the lamina propriae ; and the muscularis

Lower esophageal sphincter

Fundus

Esophagus

Cardiac region of stomach

Pyloric sphincter

Duodenum

Body of stomach

Pyloric canal

Rugae

Antrum

FIGURE 28-2. Structures of the stomach.