686
U N I T 8
Gastrointestinal and Hepatobiliary Function
Secretory Functions of the
Gastrointestinal Tract
The GI tract produces a number of secretions, including
mucus, hydrochloric acid, digestive enzymes, and hor-
mones. Secretory activity, like motility, is influenced by
local, humoral, and neural influences. Neural control of
GI secretory activity is mediated through the ANS, with
parasympathetic stimulation increasing secretory activ-
ity and sympathetic stimulation inhibiting secretory
activity. Many local influences, including the pH and
osmolarity of the GI contents, act as stimuli for neural
and hormonal mechanisms.
Gastrointestinal Secretions
Throughout the GI tract, secretory glands serve two basic
functions: production of mucus to lubricate and protect
the mucosal layer of the GI tract wall and secretion of
fluids and enzymes to aid in the digestion and absorption
of nutrients. Each day, approximately 7000 mL of fluid
is secreted into the GI tract (Table 28-1). Approximately
50 to 200 mL of this fluid leaves the body in the stool; the
remainder is reabsorbed in the small and large intestines.
These secretions are mainly water and have sodium and
potassium concentrations similar to those of extracel-
lular fluid. Because water and electrolytes for digestive
tract secretions are derived from the extracellular fluid
compartment, excessive secretion or impaired absorp-
tion can lead to extracellular fluid deficit.
Salivary Secretions
Saliva is secreted by the salivary glands. The salivary glands
consist of the parotid, submaxillary, sublingual, and buccal
glands. Saliva has three functions. The first is protection
and lubrication. Saliva is rich in mucus, which protects the
oral mucosa and coats the food as it passes through the
mouth, pharynx, and esophagus. The sublingual and buc-
cal glands produce only mucus-type secretions. The second
function of saliva is its protective antimicrobial action. The
saliva cleans the mouth and contains the enzyme lysozyme,
which has an antibacterial action. Third, saliva contains
ptyalin and amylase, which initiate the digestion of dietary
starches. Secretions from the salivary glands are primarily
regulated by the ANS. Parasympathetic stimulation
increases flow and sympathetic stimulation decreases flow
of saliva. The dry mouth that accompanies anxiety attests
to the effects of sympathetic activity on salivary secretions.
Gastric Secretions
The otherwise smooth lining of the stomach is dot-
ted with
gastric pits
that form the openings for two
types of glands: pyloric and oxyntic or gastric glands
(Fig. 28-7). The pyloric glands secrete gastrin and mucus
for protection of the pyloric mucosa, and are located in
the antral portion, or distal 20%, of the stomach. The
oxyntic glands are located on the inside surfaces of the
body and fundus of the stomach, which occupies about
80% of the stomach. They contain three types of cells:
mucous neck cells, which secrete mainly mucus; peptic
(chief cells), which secrete large quantities of pepsinogen;
and parietal (oxyntic cells), which secrete HCl and intrin-
sic factor, which is needed for vitamin B
12
absorption
(Fig. 28-7). The pepsinogen that is secreted by the peptic
cells is rapidly converted to pepsin (a protein-digesting
enzyme) when exposed to the low pH of the gastric juices.
A few stem cells are also found in the gastric pits. These cells
are the parent cells for all new cells of the gastric mucosa.
The cellular mechanism for HCl secretion by the pari-
etal cells in the stomach involves the hydrogen ion (H
+
)/
potassium ion (K
+
)-adenosine triphosphatase (ATPase)
transporter and chloride ion (Cl
–
) channels located on
their luminal membrane (Fig. 28-8). During the process
of HCl secretion, carbon dioxide (CO
2
) produced by aer-
obic metabolism combines with water (H
2
O), catalyzed
by the enzyme carbonic anhydrase, to form carbonic acid
(H
2
CO
3
), which readily dissociates into H
+
and bicarbon-
ate (HCO
3
–
). The HCO
3
–
moves out of the cell and into
blood from the basolateral membrane. At the luminal
side of the membrane, H
+
is secreted into the stomach by
the H
+
/K
+
-ATPase transporter (also known as the
proton
pump
). Chloride follows H
+
into the stomach by diffus-
ing through Cl
–
channels in the luminal membrane.
Three substances stimulate HCl secretion by the pari-
etal cells: acetylcholine, gastrin, and histamine. Although
each substance binds to different receptors on the pari-
etal cell and has a different mechanism of action, they
all serve to stimulate an increase in H
+
secretion through
the H
+
/K
+
-ATPase transporter. Acetylcholine is released
TABLE 28-1
Secretions of the
Gastrointestinal Tract
Secretions
Amount Daily (mL)
Salivary
1200
Gastric
2000
Pancreatic
1200
Biliary
700
Intestinal
2000
Total
7100
secretions, and absorptive functions of the
different segments of the GI tract.The activity
of the neurons in the enteric nervous system
is regulated by both local influences, such as
stretch and distention of the GI tract wall, and
input from the autonomic nervous system.
■■
Autonomic signals to the gut are carried from
the brain and spinal cord by the parasympathetic
and sympathetic nervous systems, with
parasympathetic stimulation increasing the
activity of the enteric nervous system and
sympathetic stimulation inhibiting its activity.
SUMMARY CONCEPTS
(continued)
