C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
693
other forms of dietary fibers, and unabsorbed sugars.
Fermentation of these residues is a major source of
energy for the microorganisms in the colon. The met-
abolic end point is the generation of short-chain fatty
acids, which play a major role in intestinal epithelial cell
growth and differentiation. Colonic microorganisms
also play a role in vitamin synthesis and in absorption of
calcium, magnesium, and iron. Vitamin K, for example,
is synthesized by the intestinal flora. Lastly, the resident
gut flora provides a crucial line of resistance to coloni-
zation by exogenous microbes, and therefore is highly
protective against invasion of tissues by pathogens. The
administration of broad-spectrum antibiotics can dis-
rupt the microbial balance and allow overgrowth of spe-
cies with potential pathogenicity, such as
Clostridium
difficile
(see Chapter 29).
SUMMARY CONCEPTS
■■
Digestion is the process of dismantling foods
into their constituent parts, and absorption is the
process of moving nutrients and other materials
from the GI tract into the internal environment.
■■
Carbohydrates must be broken down into
monosaccharides before they can be absorbed, a
process that involves both salivary and pancreatic
amylase and the brush border enzymes of the
small intestine.
■■
Digestion of proteins begins in the stomach with
the action of pepsin and continues in the small
intestine facilitated by the action of the proteolytic
pancreatic enzymes, which split the protein
molecules into dipeptides, tripeptides, or amino
acids that are absorbed through the wall of the
intestine.
■■
Fats are emulsified by bile into small droplets,
which are broken down by pancreatic lipase into
triglycerides containing medium- and long-chain
fatty acids. Bile salts form micelles that transport
these substances to the surface of intestinal villi,
where they are absorbed by lymphatics.
■■
Fluid not absorbed during movement of a meal
through the small intestine is absorbed in the
colon, and food residues are fermented by the
intestinal flora, resulting in short-chain fatty
acids, which support epithelial cell proliferation
and differentiation, and protects the colonized
host against invasion by pathogenic organisms.
Colonic microorganisms also play a role in
vitamin synthesis and in absorption of calcium,
magnesium, and iron and the synthesis of
vitamins, including vitamin K.
Anorexia, Nausea, and Vomiting
Anorexia, nausea, andvomitingarephysiologic responses
common to many GI disorders. These responses are pro-
tective to the extent that they signal the presence of dis-
ease and, in the case of vomiting, remove noxious agents
from the GI tract. They also can contribute to impaired
intake or loss of fluids and nutrients.
Anorexia
Anorexia represents a loss of appetite. Several factors
influence appetite. One is hunger, which is stimulated
by contractions of the empty stomach. Appetite or the
desire for food intake is regulated by the hypothalamus
and other associated centers in the brain. Smell plays an
important role, as evidenced by the fact that appetite
can be stimulated or suppressed by the smell of food.
Loss of appetite is associated with emotional factors,
such as fear, depression, frustration, and anxiety. Many
drugs and disease states cause anorexia. For example,
in uremia the accumulation of nitrogenous wastes in
the blood contributes to the development of anorexia.
Anorexia often is a forerunner of nausea, and most con-
ditions that cause nausea and vomiting also produce
anorexia.
Nausea and Vomiting
Nausea is an ill-defined and unpleasant subjective sensa-
tion. It stimulates the vomiting center in the brain stem
and often precedes or accompanies vomiting. Anorexia
usually precedes nausea, and stimuli such as foods and
drugs that cause anorexia in small doses usually pro-
duce nausea when given in larger doses. A common
cause of nausea is distention of the duodenum or upper
small intestinal tract. Nausea frequently is accompa-
nied by autonomic nervous system manifestations such
as watery salivation and vasoconstriction with pallor,
sweating, and tachycardia. Nausea may function as an
early warning signal of a pathologic process.
As a basic physiologic protective mechanism, vomit-
ing limits the possibility of damage from ingested nox-
ious agents by emptying the contents of the stomach
and portions of the small intestine. Nausea and vomit-
ing may also represent a total-body response to drug
therapy, including overdose, cumulative effects, toxicity,
and side effects.
The act of vomiting consists of taking a deep breath,
closing the airways, and producing a strong, forceful
contraction of the diaphragm and abdominal muscles
along with relaxation of the gastroesophageal sphinc-
ter. Respiration ceases during the act of vomiting.
Vomiting may be accompanied by dizziness, lighthead-
edness, a decrease in blood pressure, and bradycardia.
Retching
is the term used to describe the rhythmic spas-
modic movements of the diaphragm, chest wall, and
abdominal muscles without the expulsion of vomitus
(dry heaves).
