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U N I T 8
Gastrointestinal and Hepatobiliary Function
mucosa, composed of smooth muscle cells that can
contract and change the shape and surface area of the
mucosal layer. The mucosal layer performs numerous
functions in its role as an interface between the body
and the external environment, including production of
the mucus that protects and lubricates the inner lining
of the GI tract lumen, secretion of the digestive enzymes
and substances that break food down, absorption of the
breakdown products of digestion, and maintenance of a
barrier to prevent the entry of noxious substances and
pathogenic organisms. Lymphatics within the mucosa
serve as the body’s first line of immune defense. The epi-
thelial cells in the mucosal layer have a rapid turnover
rate and are replaced every 4 to 5 days. Because of the
regenerative capabilities of the mucosal layer, injury to
this layer heals rapidly without leaving scar tissue.
Submucosa
The submucosal layer, or second layer, consists of dense
connective tissue, aggregates of adipose tissue, and
occasional glands. This layer contains the large blood
vessels that send branches to the mucosa, muscular
externa, and adventitia. It also contains the lymphatic
vessels, as well as nerves that control motility and the
secretory activity of glands in the mucosal layer.
Muscularis Externa
The
muscularis externa
consists of two concentric and
relatively thick layers of smooth muscle: an inner layer
made up of circularly arranged smooth muscle cells
and an outer layer of longitudinally arranged smooth
muscle. Located between the two muscle layers is a con-
nective tissue layer that contains nerves that control
smooth muscle movement, as well as blood and lym-
phatic vessels. Contraction of the smooth muscle in this
layer mixes and churns the GI contents and facilitates its
movement along the GI tract.
Serosa and Adventitia
The serosa and adventitia constitute the outermost layer
of the GI tract. The
serosa
is a serous membrane consist-
ing of a layer of simple squamous epithelium, called the
mesothelium
, and a small amount of underlying con-
nective tissue. It is equivalent to the visceral peritoneum
and is continuous with the mesentery and omentum that
enclose and support the abdominal viscera. Instead of
the serosa, an
adventitia
consisting only of connective
tissue is found where the wall of the GI tract is directly
attached to the body wall.
The peritoneum is a continuous transparent serous
membrane that lines the abdominopelvic cavity and
invests the stomach and intestines. It is the largest serous
membrane in the body, having a surface area approxi-
mately equal to that of the skin. The peritoneum con-
sists of two continuous layers: the
visceral peritoneum
and the
parietal peritoneum
, which line the wall of the
abdominopelvic cavity. Between the two layers is the
peritoneal cavity
, a potential space containing fluid
secreted by the serous membranes. This serous fluid
forms a moist and slippery surface that prevents friction
between the continuously moving abdominal structures.
The
mesentery
is the double layer of peritoneum that
encloses a portion or all of one of the abdominal vis-
cera and attaches it to the abdominal wall (Fig. 28-4A).
The mesentery contains the blood vessels, nerves, and
lymphatic vessels that supply the intestinal wall (Fig.
28-4B). It also holds the organs in place and stores fat.
There are dorsal as well as ventral mesenteries; how-
ever, in most places the mesentery is dorsal and attaches
to the posterior abdominal wall. The mesentery that
encloses the jejunum and ileum is gathered in folds that
Longitudinal
muscle
Circular
muscle
Submucosa
Serosa
(mesothelium)
Serosa
(connective
tissue)
Muscularis externa
Mucosa
Epithelium
Lamina propria
Muscularis
mucosa
Mesentery
FIGURE 28-3.
Transverse section
of the gastrointestinal system.
