894
P A R T 1 1
Drugs acting on the gastrointestinal system
stomach, this movement is known as retching, and it can
be quite tiring and uncomfortable. This action causes a
backward peristalsis and movement of stomach contents
up the oesophagus and out the mouth. The body thus
rids itself of offending irritants.
The vomiting reflex is complex and protective, but
it can be undesirable in certain clinical situations, when
the stimulant is not something that can be vomited or
when the various components of the vomiting reflex
could be detrimental to a person’s health status.
■■
Swallowing, a centrally mediated reflex important
in delivering food to the GI tract for processing, is
controlled by the medulla. It involves a complex series
of timed reflexes.
■■
Vomiting is controlled by the chemoreceptor trigger
zone (CTZ) in the medulla or by the emetic zone in
immature or injured brains. The CTZ is stimulated
by several different processes and initiates a complex
series of responses that first prepare the system
for vomiting and then cause a strong backward
peristalsis to rid the stomach of its contents.
CHAPTER SUMMARY
■■
The gastrointestinal (GI) system is composed of
one long tube that starts at the mouth, includes the
oesophagus, the stomach, the small intestine and the
large intestine, and ends at the anus. The GI system is
responsible for digestion and absorption of nutrients.
■■
Secretion of digestive enzymes, acid, bicarbonate
and mucus facilitates the digestion and absorption of
nutrients.
■■
The GI system is controlled by a nerve plexus, which
maintains a basic electrical rhythm and responds
to local stimuli to increase or decrease activity. The
sympathetic nervous system, if stimulated, slows GI
activity; stimulation of the parasympathetic nervous
system increases activity. Initiation of activity
depends on local reflexes.
■■
A series of local reflexes within the GI tract
helps to maintain homeostasis within the system.
Overstimulation of any of these reflexes can
result in constipation (underactivity) or diarrhoea
(overactivity).
KEY POINTS
■■
Swallowing, a centrally mediated reflex important
in delivering food to the GI tract for processing, is
controlled by the medulla. It involves a complex series
of timed reflexes.
■■
Vomiting is controlled by the chemoreceptor trigger
zone (CTZ) in the medulla or by the emetic zone in
immature or injured brains. The CTZ is stimulated
by several different processes and initiates a complex
series of responses that first prepare the system
for vomiting and then cause a strong backward
peristalsis to rid the stomach of its contents.
Knowing your strengths and weaknesses helps you to
study more effectively. Take a PrepU Practice Quiz
to find out how you measure up!
ONLINE RESOURCES
An extensive range of additional resources to enhance teaching
and learning and to facilitate understanding of this chapter may
be found online at the text’s accompanying website, located on
thePoint at
These include Watch and
Learn videos, Concepts in Action animations, journal articles,
review questions, case studies, discussion topics and quizzes.
BIBLIOGRAPHY
Barrett, K. E. & Ganong, W. F. (2010).
Ganong’s Review of Medical
Physiology
(23rd edn). New York: McGraw-Hill.
Goodman, L. S., Brunton, L. L., Chabner, B. & Knollmann, B. C.
(2011).
Goodman and Gilman’s Pharmacological Basis of
Therapeutics
(12th edn). New York: McGraw-Hill.
Guyton, A. & Hall, J. (2011).
Textbook of Medical Physiology
(12th edn)
.
Philadelphia: Saunders Elsevier.
Johnson, L. R. (2007).
Gastrointestinal Physiology
(7th edn).
St Louis, MO: Mosby.
Parkman, H. & Fisher, R. S. (2006).
The Clinician’s Guide to
Acid/Peptic Disorders and Motility Disorders of the GI Tract.
Thorofare, NJ: Slack.
Porth, C. M. (2011).
Essentials of Pathophysiology: Concepts
of Altered Health States
(3rd edn). Philadelphia: Lippincott
Williams & Wilkins.
Porth, C. M. (2009).
Pathophysiology: Concepts of Altered Health
States
(8th edn). Philadelphia: Lippincott Williams & Wilkins.
Rhoades, R. A. & Bell, D. R. (2008).
Medical Physiology: Principles
of Clinical Medicine.
Philadelphia: Lippincott Williams &
Wilkins.
Seidel, E. (2006).
Crash Course: GI System.
St Louis, MO: Mosby.
Seifter, J., Rafnon, A. & Sloane, D. (2005).
Concepts in Medical
Physiology.
Philadelphia: Lippincott Williams & Wilkins.
