Page 160 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Diarrhea
The term diarrhea is used if stool has lost its tion in the upper small intestine (diminished
normal firm consistency. This is usually asso- Na + symport with glucose and galactose)
ciated with an increase in its weight (in leads to reduced water absorption. The os-
males > 235; in females > 175 g/d) and its fre- motic activity of the nonabsorbed carbohy-
quency (> 2 per day). Diarrhea can have var- drates additionally results in water secretion.
ious causes: However, bacteria in the large intestine can
Osmotic diarrhea results from the intake metabolize up to 80 g/d (divided over four
of a large number of substances that are meals) of nonabsorbed carbohydrates into
poorly absorbable even normally, or in mal- organic acids useful for providing energy
absorption (→ p.152ff.). Among the first that together with water are absorbed in the
Liver group are sorbitol (in “sugar-free” medica- colon (→ B). It is only the large amounts of
tions and sweets or certain fruits), fructose
marked gas produced (flatulence) that pro-
Stomach, Intestines, sium salts (antacids, laxatives) as well as tion. However, if > 80 g/d (i.e., > ⁄4 of normal
vide evidence of carbohydrate malabsorp-
(in lemonades, diverse fruits, honey), magne-
1
carbohydrate supply) is not absorbed or the
poorly absorbed anions such as sulfate, phos-
intestinal bacteria are decimated by antibi-
phate, or citrate.
otics, diarrhea occurs.
Nonabsorbed substances are osmotically
active in the small intestine and therefore
Secretory diarrhea (in the narrow sense)
–
tinal mucosa is activated (→ C). Within the
→ B, left). Table A illustrates this in a simulat-
–
ed experiment. Intake of, for example,
mucosal cells Cl is secondarily actively en-
6 “suck” water into the lumen. (H 2 O secretion; occurs when Cl secretion of the small intes-
–
+
+
150 mmol of a nonabsorbable substance (in riched by a basolateral Na -K -2 Cl symport
–
this example, polyethyleneglycol, PEG) in carrier and is secreted via luminal Cl chan-
250 mL water (PEG concentration = [PEG] = nels. These open more frequently when the
600 mmol/L) starts osmotic water secretion intracellular concentration of cAMP rises.
in the duodenum so that the volume is in- cAMP is formed in greater amounts in the
creased to 750 mL ([PEG] falls to 200 mmol/ presence of, for example, certain laxatives
L). The osmolality has adjusted tothat of plas- and bacterial toxins (Clostridium difficile,
ma (290 mosm/L), 90 mosm/L now being Vibrio cholerae). Cholera toxin causes mas-
+
+
contributed by Na , K and the accompanying sive diarrhea (up to 1000 mL/h) that can ra-
anions (ion secretion into the lumen because pidly become life-threatening because of the
+
of the high chemical gradients). The volume loss of water, K , and HCO 3 – (hypovolemic
in the middle of the small intestine has risen shock, hypokalemia, nonrespiratory acidosis).
to 1000 mL. [PEG] has fallen to 150 mmol/L, Overproduction of VIP (vasoactive intes-
and the entering ions contribute 140 mosm/ tinal peptide) by pancreatic islet cell tumors
L. Because of the high active absorption, espe- also causes high cAMP levels in intestinal
–
+
cially of Na (plus anions ) in ileum and colon mucosa cells leading to copious, life threat-
(denser epithelium than in the jejunum), the ening diarrhea: pancreatic “cholera” or wa-
osmolality contributed by the ions falls to 90 tery diarrhea syndrome.
and 40 mosm/L, respectively. The main cation There are several reasons why diarrhea oc-
+
+
in stool is K (marked Na absorption in ileum curs after resection of the ileum and of part of
and colon). The result is that given 150 mmol the colon (→ D). Bile salts, normally absorbed
PEG in 250 mL H 2 O, the volume of diarrhea in the ileum, cause accelerated passage
will be 600 mL. Without ion absorption in through the colon (reducedwater absorption).
the ileum and colon (e.g., after resection, dis- In addition, the nonabsorbed bile salts are de-
ease), the volume of diarrhea could even rise hydroxylated by the bacteria in the colon. The
to 1000 mL. (PEG is, e.g., given to cleanse the resulting bile salt metabolites stimulate the
gut before a coloscopy). secretion of NaCl and H 2 O in the colon. Finally,
150 In malabsorption of carbohydrates (→ B, there is also a lack of active absorption of Na +
+
right and p.152ff.) the reduced Na absorp- in the resected intestinal segments.
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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