Page 144 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

6 Stomach, Intestines, Liver S. Silbernagl

Function of the Gastrointestinal Tract
To cover the material and energy demands of rectum, are also storage places for the feces,
the organism food must be swallowed, pro- so that defecation is necessary relatively rare-
cessed and broken down (digestion) as well ly, despite frequent food intake.
as taken up (absorption) by the intestine. The two plexuses in the wall of the esopha-
Solid foods are chewed by the teeth, each gus, stomach, and intestine serve to control
bite being mixed with saliva from the sali- motility and secretion, with superregional
vary glands. Saliva contains mucin, a lubri- reflexes and modulating influences of the
cant, and antibodies as well as α-amylase to central nervous system transmitted via the
digest polysaccharides. It is the task of the autononomic nervous system and visceral–af-
esophagus to rapidly transport the food ferent nerve tracts. In addition, the gastroin-
from the throat to the stomach. The lower testinal tract secretes numerous peptide hor-
esophageal sphincter briefly opens, but mones and transmitters that participate in
otherwise prevents reflux of the potentially controlling and regulating the gastrointesti-
harmful gastric juice. The proximal stomach nal tract and its accessory glands.
primarily serves to store food taken up dur- There are many nonspecific and specific
ing a meal. Its muscle tone determines the mechanisms which defend against patho-
supply to the distal stomach, where the food genic organisms on the inner surface (ca.
is processed (broken up further and emulsi- 100 m ) of the gastrointestinal tract. Begin-
2
fied). Proteins are denatured and broken ning at the mouth, components in saliva,
down by the gastric acid and pepsins, and li- such as mucins, immunoglobulin A (IgA), and
pases begin fat digestion. The distal stomach lysozyme, inhibit microorganisms invading.
also has the task of apportioning chyme. In Hydrochloric acid and pepsins have a bacteri-
addition, the stomach secretes the intrinsic cidal effect, and Peyer’s patches in the gas-
factor that is essential for the absorption of trointestinal tract are its own immunocom-
cobalamines (vitamin B 12 ). petent lymph tissue. Special M cells (“mem-
The breakdown of food particles is com- branous cells”) of the mucosa provide lumi-
pleted in the small intestine by means of en- nal antigens with access to Peyer’s patches,
zymes from the pancreas and the mucosa of which can respond with release of IgA (oral
–
the small intestine. The HCO 3 ions of the immunization or, as an abnormal process, al-
pancreatic juice are needed to neutralize the lergization). IgA is combined in the intestinal
acidic chyme. Fat digestion in addition re- epithelium with the secretory component
quires bile salts supplied in bile. The pro- which protects the secreted IgA against di-
ducts of digestion (monosaccharides, amino gestive enzymes. Macrophages in the intes-
acids, dipeptides, monoglycerides, and free tinal wall and in the sinusoids of the liver
fatty acids) as well as water, minerals, and vi- (Kupffer cells) form a further barrier against
tamins are absorbed in the small intestine. invading pathogenic organisms.
Together with the bile secreted by the liv-
er, excretory products (e.g., bilirubin) reach
the stool. The liver has numerous additional
metabolic functions: it is the obligatory in-
termediate station for almost all substances
absorbed from the small intestine, and it is
able to detoxify numerous foreign substances
and metabolic end-products and to bring
about their excretion.
The large intestine is the last station for
water and ion absorption. It is colonized by
134 bacteria with physiological functions. The
large intestine, especially the caecum and
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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