Page 162 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Maldigestion and Malabsorption
A defect in the processing and enzymatic 6. Transportation into blood and lymph,
splitting within the gastrointestinal tract is through which the absorbed substances
called maldigestion; a disorder of absorption reach the liver and the systemic circula-
is called malabsorption. As both of them are tion, respectively.
closely intertwined, they are grouped to- The causes of malabsorption can affect all
gether here as malabsorption (in the wider these steps (→ C, D):
sense). ! After gastric resection and/or vagotomy
Malabsorption may affect the three ener- (see also p.148), the stimulation of enteral
gy carriers of food, i.e., fats, proteins, and car- hormone secretion (CCK, e.g.) is reduced and
bohydrates, as well as vitamins, iron, cal- the synchronization of chyme apportioning
Liver cium, magnesium, and trace elements, for with pancreatic secretion, gallbladder emp-
example, zinc (→ C). Malabsorption of the
tying, and choleresis is disturbed. Further-
Stomach, Intestines, also clinically significant (→ D). The respec- accelerated and the pH in the duodenal lu-
more, passage through the small intestine is
enterohepatically circulating bile salts is
tive site of absorption (→ A) of these sub-
men is too acidic, so that the digestive pro-
cess may be greatly disturbed (enzyme inac-
stances is determined by: 1) the number
tivation, bile salt precipitation). A gastrino-
and duration of preceding steps of proces-
ma (Zollinger–Ellison syndrome) can cause
sing and splitting; and 2) the provision in
! Pancreatic diseases, for example, chronic
nisms of absorption.
pancreatitis (→ p.160), carcinoma of the
Thus, monosaccharides such as glucose
6 the intestinal segments of specific mecha- malabsorption for the same reason.
and galactose can be absorbed at the begin- pancreas, cystic fibrosis (→ p.162), or resec-
ning of the duodenum; disaccharides must tion of the pancreas may lead to malabsorp-
first be split by the enzymes of the brush bor- tion due to a lack of important enzymes (li-
der; polysaccharides (just like proteins and pase, colipase, trypsin, chymotrypsin, amy-
–
fats) must first come into contact with pan- lase, etc.) as well as of HCO 3 which is neces-
creatic juice, with the result that they may sary for buffering acidic chyme.
not be absorbed until they reach the jejunum ! Atrophic gastritis with achlorhydria
(→ A) Rapid emptying of the stomach can (→ p.142) will firstly diminish gastric diges-
mean that the place of absorption is moved tion and secondly favor colonization of the
distally (→ p.148), i.e. intestinal segments small intestine with bacteria. This may also
which lie further downstream can take over be caused by stasis in the small intestine
absorption that, in the long term, can lead to due to diverticulosis or a small-intestine
a change in the mucosa. The ileum, for exam- shunt (blind loop syndrome, → p.148). The
ple, may take on jejunum-like properties. bacteria deconjugate bile salts (→ D) and
This is not possible with substances for split the binding between cobalamine and
which only the terminal ileum possesses intrinsic factor. The resulting cobalamine
specific absorption mechanisms (cobala- malabsorption leads to cobalamine deficien-
mine, bile salts). cy, as does a reduced intake (strictly vegetar-
Normal digestion and absorption consists ian diet; it is true also for breastfed infants of
of the following serial steps (→ B): such mothers, because their milk also lacks
1. Mechanical processing of food cobalamine), intrinsic factor deficiency
(chewing, distal gastric peristalsis); (achlorhydria; see also p.142), lack of enzy-
2. Luminal digestion (gastric, intestinal, matic liberation of cobalamine from its bind-
and pancreatic juices; bile); ing with other proteins (high gastric pH,
3. Mucosal digestion by enzymes of trypsin deficiency), or resection of the termi-
the brush border; nal ileum, the site of absorption of the cobal-
4. Absorption by the mucosal epithelium; amine–intrinsic factor complex.
152 5. Processing in the mucosal cell;
"
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
All rights reserved. Usage subject to terms and conditions of license.