874
UNIT 4
Maintenance of the Body
23
enterogastric reflex and the hormonal (enterogastrone) mecha-
nisms that inhibit acid and pepsin secretion as we described
earlier. Tese mechanisms inhibit gastric secretion and prevent
further duodenal filling by reducing the force of pyloric con-
tractions
(Figure 23.20)
.
A carbohydrate-rich meal moves through the duodenum
rapidly, but fats form an oily layer at the top of the chyme and
are digested more slowly by enzymes acting in the intestine.
For this reason, when chyme entering the duodenum is fatty,
reflexes slow stomach emptying, and food may remain in the
stomach six hours or more.
Homeostatic Imbalance
23.9
Vomiting
, or
emesis
, is an unpleasant experience that emp-
ties the stomach by a different route. Many factors signal the
stomach to “launch lunch,” but the most common are extreme
stretching of the stomach or intestine or irritants such as bacte-
rial toxins, excessive alcohol, spicy foods, and certain drugs.
Bloodborne molecules and sensory impulses stream from
the irritated sites to the
emetic center
(e-met
9
ik) of the medulla
where they initiate a number of motor responses. Before vomit-
ing, an individual typically feels nauseated, is pale, and salivates
excessively. A deep inspiration directly precedes vomiting. Te
diaphragm and abdominal wall muscles contract, increasing
intra-abdominal pressure, the gastroesophageal sphincter re-
laxes, and the so± palate rises to close off the nasal passages. As a
result, the stomach (and perhaps duodenal) contents are forced
upward through the esophagus and pharynx and out the mouth.
Excessive vomiting can cause dehydration and severely dis-
rupt the body’s electrolyte and acid-base balance. Since large
amounts of HCl are lost in vomitus, the blood becomes alkaline
as the stomach attempts to replace its lost acid.
Check Your Understanding
27.
Name the three phases of gastric secretion.
28.
How does the presence of food in the small intestine inhibit
gastric secretion and motility?
29.
How does the pH of venous blood leaving the stomach
change during a meal?
For answers, see Appendix H.
The Small Intestine
and Associated Structures
Identify and describe structural modifications of the wall of
the small intestine that enhance the digestive process.
Differentiate between the roles of the various cell types of
the intestinal mucosa.
Describe the function of intestinal hormones and
paracrines.
In the small intestine, usable food is finally prepared for its jour-
ney into the cells of the body. However, this vital function can-
not happen without the aid of secretions from the liver (bile)
smooth muscle and increase gastric motility. For this reason,
the more food there is in the stomach, the more vigorous the
stomach mixing and emptying movements will be—within cer-
tain limits—as we describe next.
Regulation of Gastric Emptying
Te stomach usually empties completely within four hours a±er
a meal. However, the larger the meal (the greater the stomach
distension) and the more liquid its contents, the faster the stom-
ach empties. Fluids pass quickly through the stomach. Solids
linger, remaining until they are well mixed with gastric juice and
converted to the liquid state.
Te rate of gastric emptying also depends as much—and
perhaps more—on the contents of the duodenum as on what
is happening in the stomach. Te stomach and duodenum act
in tandem like a “coupled meter” that functions at less than
full capacity. As chyme enters the duodenum, receptors in its
wall respond to chemical signals and to stretch, initiating the
Presence of fatty, hypertonic,
acidic chyme in duodenum
Duodenal entero-
endocrine cells
Chemoreceptors and
stretch receptors
Enterogastrones
(secretin,
cholecystokinin,
vasoactive intestinal
peptide)
Duodenal
stimuli
decline
Via short
reflexes
Via long
reflexes
Enteric
neurons
Initial stimulus
Physiological response
Result
Secrete
Target
Contractile force and
rate of stomach
emptying decline
CNS centers
sympathetic
activity;
parasympathetic
activity
Stimulate
Inhibit
Figure 23.20
Neural and hormonal factors that inhibit
gastric emptying.
These controls ensure that the food is well
liquefied in the stomach and prevent the small intestine from being
overwhelmed.
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