Porth's Essentials of Pathophysiology, 4e

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Gastrointestinal and Hepatobiliary Function

U N I T 8

Upper Gastrointestinal Tract The mouth forms the entryway into the GI tract for food; it contains the teeth, used in the mastication of food, and the tongue and other structures needed to direct food toward the pharynx and the esophagus. The mouth also serves as a receptacle for saliva produced by the salivary glands. Saliva moistens and lubricates food, so it is easier to swallow, and it contains enzymes (amylase and lipase) involved in the initial digestion of starches and lipids. The esophagus is a straight, collapsible tube, about 25 cm (10 inches) in length, that lies behind the trachea and connects the pharynx with the stomach. The esoph- agus functions primarily as a conduit for the passage of food from the pharynx to the stomach. Its structure is uniquely designed for this purpose: the smooth muscle layers provide the peristaltic movements needed to move food along its length, and the mucosal and submucosal glands secrete mucus, which protects its surface and aids in lubricating food.

and lymph. In the large intestine, unabsorbed wastes are collected for later elimination. Structurally, the GI tract is a long, hollow tube, the lumen of which is an extension of the external environ- ment (Fig. 28-1). Nutrients do not become part of the internal environment until they have passed through the intestinal wall and have entered the blood or lymph channels. For simplicity and understanding, the GI tract can be divided into three parts. The upper part—the mouth and pharynx, esophagus, and stomach—acts as an intake source and receptacle through which food passes and in which initial digestive processes take place. The middle portion—the duodenum, jejunum, and ileum of the small intestine—is where most digestive and absorptive processes occur. The lower segment—the cecum, colon, and rectum of the large intestine—serves as a storage channel for the efficient elimination of waste. The accessory organs, which include the salivary glands, liver, and pancreas, produce secretions that aid in digestion.

Oral cavity

Parotid gland and duct Pharynx Esophagus

Sublingual and submandibular glands and ducts

Liver

Hepatic duct

Stomach

Gallbladder

Splenic flexure

Duodenum

Hepatic flexure

Transverse colon

Jejunum

Pancreas

Descending colon

Ascending colon Ileocecal junction Cecum

Ileum

Sigmoid colon

Appendix

Rectum

FIGURE 28-1. The gastrointestinal system.