C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
677
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
Esophagus
Cardiac region
of stomach
Fundus
Antrum
Pyloric sphincter
Lower esophageal
sphincter
Body of
stomach
Rugae
Pyloric
canal
Duodenum
FIGURE 28-2.
Structures of the stomach.
