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U N I T 8
Gastrointestinal and Hepatobiliary Function
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.
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.
Parotid gland
and duct
Oral cavity
Pharynx
Esophagus
Sublingual and
submandibular
glands and ducts
Liver
Stomach
Hepatic duct
Gallbladder
Duodenum
Hepatic flexure
Ascending colon
Ileocecal junction
Cecum
Appendix
Splenic flexure
Pancreas
Transverse colon
Descending colon
Jejunum
Ileum
Sigmoid colon
Rectum
FIGURE 28-1.
The gastrointestinal system.
