Section 21. Disorders of the Gastrointestinal System >
Part 1. Presenting Signs and Symptoms >
Chapter 381. Normal Structure and Function of the Gastrointestinal TractAlan N. Mayer
Topics Discussed: brunner glands; esophagus; gastroenterology and hepatology; gastrointestinal function; gastrointestinal muscularis; gastrointestinal submucosa; gastrointestinal tract; intestinal epithelium; intestinal lamina propria; intestinal villus; intestine, large; intestine, small; stomach.
Sections: Embryology and Developmental Biology, Digestion and Absorption, Gastrointestinal Motility, Immunologic Functions of the Gastrointestinal Tract, The Microbial Ecosystem, References.
Excerpt:"The gastrointestinal (GI) tract is a continuous tube beginning
at the mouth and ending at the anus (Fig. 381-1).
Its main function is to digest food and absorb nutrients and fluid.
It is subdivided into 4 regions: (1) esophagus, (2) stomach, (3)
small intestine, and (4) large intestine. The liver and pancreas
directly communicate with the GI tract via ducts that join with
the duodenum, the most anterior segment of small intestine. At the
cellular level, the tissue architecture of the gut tube is similar
throughout, consisting of 4 concentric layers.1 From
inner to outer they are (1) mucosa, (2) submucosa, (3) muscularis
propria (externa), and (4) adventitia or serosa. The mucosa is composed
of epithelium, lamina propria, and muscularis mucosae (Fig.
381-2).The esophagus extends from the posterior pharynx to the stomach,
serving as a conduit for the passage of food. Esophageal epithelium
is nonkeratinized, stratified, and squamous, designed to withstand
abrasion as ingested food passes through it. Submucosal glands secrete
mucins for lubrication and acid protection. The esophageal epithelium
is not well suited to resist chemical insults, making it vulnerable
to damage from caustic ingestions or chronic exposure to gastric
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