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P A R T 1 1
Drugs acting on the gastrointestinal system
vast surface area to increase the absorption. The small
intestine maintains a BER of 11 contractions per minute.
This regular movement is assessed when listening for
bowel sounds.
The large intestine uses a process of mass movement
with an occasional peristaltic wave. When the beginning
segment of the large intestine is stimulated, it contracts
and sends a massive peristaltic movement throughout
the entire large intestine. The end result of the mass
movement is usually excretion of waste products.
Rectal distension after mass movement stimulates a
defecation reflex that causes relaxation of the external
and internal sphincters. Control of the external sphinc-
ter is a learned behaviour. The receptors in the external
sphincter adapt relatively quickly and will stretch and
require more and more distension to stimulate the reflex
if the reflex is ignored.
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The GI system begins at the mouth and ends at
the anus; a long tube extends between them and
comprises the oesophagus, the stomach, the small
intestine and the large intestine. Essential functions
are digestion and absorption of nutrients.
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The GI system secretes enzymes, acid, bicarbonate
and mucus to facilitate the digestion and absorption
of nutrients.
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The small intestine is the organ where most
absorption occurs. The veins of the small intestine
carry the absorbed products to the liver for filtering,
cleaning and metabolism, or the breaking down of
absorbed products into usable substances.
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The nerve plexus controls the GI system by
maintaining electrical rhythm and responding to
local stimuli (increasing or decreasing activity). The
autonomic nervous system influences GI activity,
with the sympathetic system slowing and the
parasympathetic system increasing activity.
GASTROINTESTINAL REFLEXES
To function effectively, several local and central reflexes
occur. Local reflexes involve stimulation of the nerves in
the GI tract and cause movement and secretion. Central
reflexes, which include swallowing and vomiting, are
controlled by the medulla.
Local reflexes
Stimulation of local nerves within the GI tract causes
increased or decreased movement within the system,
maintaining homeostasis. Loss of reflexes or stimulation
can result in constipation and the lack of movement of
the bolus along the GI tract or diarrhoea with increased
KEY POINTS
motility and excretion. The longer a faecal bolus remains
in the large intestine, the more sodium and water are
absorbed from it and the harder and less mobile it can
become. There are many
local gastrointestinal reflexes
.
Some knowledge of how these reflexes operate makes
it easier to understand what happens when the reflexes
are blocked or overstimulated and how therapeutic
measures are often used to cause reflex activity.
•
Gastroenteric reflex
: Stimulation of the stomach
by stretching, the presence of food or cephalic
stimulation (the body’s response to smelling, seeing,
tasting or thinking about food) causes an increase
in activity in the small intestine. It is thought that
this prepares the small intestine for the coming
chyme.
•
Gastrocolic reflex
: Stimulation of the stomach also
causes increased activity in the colon, again preparing
it to empty any contents to provide space for the new
chyme.
•
Duodenal–colic reflex
: The presence of food or
stretching in the duodenum stimulates colon activity
and mass movement, again to empty the colon for the
new chyme.
It is important to remember the gastroenteric, gastro
colic and duodenal reflexes when helping people to
maintain GI movement. Taking advantage of stomach
stimulation (e.g. having the person drink prune juice or
hot water or eat bran) and providing the opportunity
of time and privacy for a bowel movement encourage
normal reflexes to keep things in control.
Other local GI reflexes include the following:
•
Ileogastric reflex
: The introduction of chyme or
stretch to the large intestine slows stomach activity,
as does the introduction of chyme into the small and
large intestine, allowing time for absorption. In part,
this reflex explains why people who are constipated
often have no appetite: The continued stretch on the
ileum that comes with constipation continues to slow
stomach activity and makes the introduction of new
food into the stomach undesirable.
•
Intestinal–intestinal reflex
: Excessive irritation to
one section of the small intestine causes a cessation
of activity above that section to prevent further
irritation, and an increase in activity below that
section, which leads to a flushing of the irritant. This
reflex is active in “Montezuma’s revenge” (traveller’s
diarrhoea): Local irritation of the intestine causes
increased secretions and movement below that
section, resulting in watery diarrhoea and a cessation
of movement above that section. Loss of appetite or
even nausea may occur. An extreme reaction to this
reflex can be seen after abdominal surgery, when the
handling of the intestines causes intense irritation
and the reflex can cause the entire intestinal system
to cease activity, leading to a paralytic ileus.
