C H A P T E R 5 6
Introduction to the gastrointestinal system
891
cells to release
hydrochloric acid
and the chief cells to
release pepsin. Parasympathetic stimulation also leads
to acid release. Gastrin and the parasympathetic system
stimulate
histamine-2 (H
2
) receptors
near the parietal
cells, causing the cells to release hydrochloric acid into
the lumen of the stomach. Proteins, calcium, alcohol and
caffeine in the stomach increase gastrin secretion. High
levels of acid decrease the secretion of gastrin. Other
digestive enzymes are released appropriately, in response
to proteins and carbohydrates, to begin digestion. Peptic
ulcers can develop when there is a decrease in the pro-
tective mucosal layer or an increase in acid production.
Digestive:
Digestion of carbohydrates
As the now-acidic bolus leaves the stomach and
enters the small intestine, secretin is released, which
stimulates the pancreas to secrete large amounts of
sodium bicarbonate (to neutralise the acid bolus), the
pancreatic enzymes
chymotrypsin and trypsin (to break
down proteins to smaller amino acids), other lipases (to
break down fat) and amylases (to break down sugars).
These enzymes are delivered to the GI tract through the
common bile duct, which is shared with the gallbladder.
Digestive:
Metabolism of amino acids
If fat is present in the bolus, the gallbladder contracts
and releases
bile
into the small intestine. Bile contains a
detergent-like substance that breaks apart fat molecules
so that they can be processed and absorbed. The bile in
the gallbladder is produced by the liver during normal
metabolism. Once delivered to the gallbladder for
storage, it is concentrated; water is removed by the walls
of the gallbladder. Some people are prone to developing
gallstones
in the gallbladder when the concentrated bile
crystallises. These stones can move down the duct and
cause severe pain or even blockage of the bile duct.
In response to the presence of food, the small and
large intestines may secrete various endocrine hormones,
including growth hormone, aldosterone and glucagon.
They also secrete large amounts of mucus to facilitate
the movement of the bolus through the rest of the GI
tract.
Digestion
Digestion is the process of breaking food into usable,
absorbable nutrients. Digestion begins in the mouth,
with the enzymes in the saliva starting the process of
breaking down sugars and proteins. The stomach con-
tinues the digestion process with muscular churning,
breaking down some foodstuffs while mixing them thor-
oughly with hydrochloric acid and enzymes. The acid
and enzymes further break down sugars and proteins
into building blocks and separate vitamins, electrolytes,
minerals and other nutrients from ingested food for
absorption. The beginning of the small intestine intro-
duces bile to the food bolus, which is now called
chyme
.
Bile breaks down fat molecules for processing and
absorption into the bloodstream. Digestion is finished at
this point, and absorption of the nutrients begins.
Digestive:
General digestion
Absorption
Absorption is the active process of removing water,
nutrients and other elements from the GI tract and deliv-
ering them to the bloodstream for use by the body. The
portal system drains all of the lower GI tract, where
absorption occurs, and delivers what is absorbed into
the venous system directly to the liver. The liver filters,
clears and further processes most of what is absorbed
before it is delivered to the body (see Figure 56.1). Some
absorption occurs in the lower end of the stomach,
most commonly absorption of water and alcohol. The
majority of absorption occurs in the small intestine.
It about 8500 mL/day, including nutrients, drugs and
anything that is taken into the GI tract, as well as any
secretions. The small intestine mucosal layer is specially
designed to facilitate this absorption, with long villi
on the epithelial layer providing a vast surface area for
absorption. The large intestine absorbs approximately
350 mL/day, mostly sodium and water.
Motility
The GI tract depends on an inherent motility to keep
things moving through the system. The nerve plexus
maintains a basic electrical rhythm (BER), much like
the pacemaker rhythm in the heart. The cells within
the plexus are somewhat unstable and leak electrolytes,
leading to the regular firing of an action potential. This
rhythm maintains the tone of the GI tract muscles and
can be affected by local or autonomic stimuli to increase
or decrease the rate of firing.
The basic movement seen in the oesophagus is
peri-
stalsis
, a constant wave of contraction that moves from
the top to the bottom of the oesophagus. The act of
swallowing
, a response to a food bolus in the back of the
throat, stimulates the peristaltic movement that directs
the food bolus into the stomach. The stomach uses its
three muscle layers to produce a churning action. This
action mixes the digestive enzymes and acid with the
food to increase digestion. A contraction of the lower
end of the stomach sends the chyme into the small
intestine.
The small intestine uses a process of
segmentation
with an occasional peristaltic wave to clear the segment.
Segmentation involves contraction of one segment
of small intestine while the next segment is relaxed.
The contracted segment then relaxes, and the relaxed
segment contracts. This action exposes the chime to a
