The Digestion and Absorption of Food
563
XI. Sodium, chloride, bicarbonate, and water are secreted by the
small intestine. However, most of these secreted substances,
as well as those entering the small intestine from other
sources, are absorbed back into the blood.
XII. Intestinal motility is coordinated by the enteric nervous
system and modifi ed by long and short refl exes and
hormones.
a. During and shortly after a meal, the intestinal contents
are mixed by segmenting movements of the intestinal
wall.
b. After most of the food has been digested and absorbed,
the migrating myoelectric complex (MMC), which
moves the undigested material into the large intestine
by a migrating segment of peristaltic waves, replaces
segmentation.
XIII. The primary function of the large intestine is to store and
concentrate fecal matter before defecation.
a.
Water is absorbed from the large intestine secondary
to the active absorption of sodium, leading to the
concentration of fecal matter.
b. Flatus is produced by bacterial fermentation of undigested
polysaccharides.
c. Three to four times a day, mass movements in the colon
move its contents into the rectum.
d. Distension of the rectum initiates defecation, which is
assisted by a forced expiration against a closed glottis.
e. Defecation can be voluntarily controlled through
somatic nerves to the skeletal muscles of the external anal
sphincter.
Pathophysiology of the Gastrointestinal Tract
I. The factors that normally prevent breakdown of the mucosal
barrier and formation of ulcers are (1) secretion of an
alkaline mucus, (2) tight junctions between epithelial cells,
and (3) rapid replacement of epithelial cells.
a. The bacterium
Helicobacter pylori
is a major cause of
damage to the mucosal barrier, leading to ulcers.
b. Drugs that block histamine receptors or inhibit the H
+
/
K
+
-ATPase pump inhibit acid secretion and promote ulcer
healing.
II. Vomiting is coordinated by the vomiting center in the
brainstem medulla oblongata. Contractions of abdominal
muscles force the contents of the stomach into the esophagus
(retching); if the contractions are strong enough, they force
the contents of the esophagus through the upper esophageal
sphincter into the mouth (vomiting).
III. Precipitation of cholesterol or, less often, bile pigments
in the gallbladder forms gallstones, which can block
the exit of the gallbladder or common bile duct. In the
latter case, the failure of bile salts to reach the intestine
causes decreased fat digestion and absorption, and the
accumulation of bile pigments in the blood and tissues
causes jaundice.
IV. Lactase activity, which is present at birth, undergoes a
genetically determined decrease during childhood in many
individuals. In the absence of lactase, lactose cannot be
digested, and its presence in the small intestine can cause
diarrhea and increased fl atus production when milk products
are ingested.
V. Infl ammatory bowel disease (IBD) is comprised of Crohn’s
disease and ulcerative colitis. They result from a genetic
predisposition to having an inappropriate immune response
to infection.
VI. Constipation is primarily the result of decreased colonic
motility. The symptoms of constipation are produced by
overdistension of the rectum, not by the absorption of toxic
bacterial products.
VII. Diarrhea can be caused by decreased fl
uid absorption,
increased fl
uid secretion, or both.
KEY TERMS
absorption
529
aminopeptidase
537
amylase
530
antrum
545
anus
557
appendix
556
area postrema
559
aspiration
543
basic electrical rhythm
549
bile
532
bile canaliculi
553
bile pigment
554
bile salt
532
bilirubin
554
body (of stomach)
545
brush border
536
carboxypeptidase
537
cecum
556
cephalic phase
543
chief cell
545
cholecystokinin (CCK)
541
chylomicron
538
chyme
532
chymotrypsin
537
circular muscle
535
colipase
537
colon
556
common bile duct
532
defecation
532
defecation refl ex
557
digestion
529
duodenum
532
emulsifi cation
537
enteric nervous system
541
enterochromaffi n-like (ECL)
cell
545
enteroendocrine cell
545
enterogastrone
548
enterohepatic circulation
553
enterokinase
552
epiglottis
543
esophagus
530
external anal sphincter
557
feces
529
fi ber
536
fl atus
557
fundus
545
gallbladder
532
gastric
532
gastric phase
543
gastrin
541
gastroileal refl ex
556
gastrointestinal (GI)
system
528
gastrointestinal tract
528
ghrelin
542
glottis
543
glucose-dependent insulinotropic
peptide (GIP)
541
gluten
539
hepatic
532
hepatic portal vein
536
hepatocyte
553
hydrochloric acid
530
ileocecal sphincter
556
ileum
532
internal anal sphincter
557
intestinal phase
543
intestino-intestinal refl ex
556
intrinsic factor
540
jejunum
532
lactase
560
lacteal
536
large intestine
532
leptin
542
lipase
537
liver
532
longitudinal muscle
535
long refl ex
541
lower esophageal sphincter
544
mass movement
557
micelle
538
microvillus
536
migrating myoelectric complex
(MMC)
555
motilin
556
motility
529
mouth
530
mucosa
535
muscularis externa
535
myenteric plexus
535
pancreas
532
parietal cell
545
pepsin
530
pepsinogen
545
peristaltic waves
544
Peyer’s patches
536
pharynx
530
potentiation
541
pyloric sphincter
549
receptive relaxation
548
rectum
532
saliva
530
salivary gland
530
secondary peristalsis
545
secretin
541
secretion
529
segmentation
555
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