Diarrhea is both a sign (Ý stool weight) and a symptom (Ý frequency of defecation, Ý volume, ß consistency).
Acute vs. Chronic
Duration – acute: <2-3 weeks, chronic: > 3 weeks
Etiology – acute: usually infection, chronic: multiple
Course – acute: self-limited, chronic: variable
Normal Fluid Balance
9000 mL fluid presented to the intestines/day, 8800 mL absorbed, 200 mL excreted
Absorption from: duodenum/jejunum > ileum > colon
Although ingested food has wide range of osmolality, intraluminal fluid becomes isosmotic to plasma in the upper small intestine and remains so further distally.
Major cations of stool: Na+, K+
Major anions of stool: short chain fatty acids (SCFA) produced by intestinal bugs
Stool osmolality = 2 x (Na+ + K+) – always within 50 mOsm of this.
Mechanisms Producing Diarrhea
one or all of the following may coexist to produce diarrhea:
– either increased secretion or diminished absorption of fluids result in secretory diarrhea. May originate at any level within GI tract. Stool fluid is isosmolar to plasma (osmolar gap is normal). Fasting has no effect.
: cholera toxin
Þ Ý Cl- secretion and ß sodium choloride absorption in villus cells Þ marked fluid accumulation in lumen, Zollinger-Ellison syndrome causes Ý gastrin Þ stimulated gastric acid secretion and fluid load, stimulated pancreatic bicarb and fluid secretions, acidifed duodenum and fat maldigestion, Malabsorption of bile salts in terminal ileum Þ impaired absorption and increased secretion from colon.
– ingestion of a poorly absorbed solute osmotically draws water into the GI tract lumen to maintain isosmolality to plasma. This type of diarrhea disappears upon fasting/stopping ingestion of solute. The resulting stool osmolality will be abnormally high (osmolar gap is high).
: ions (sulfate, magnesium, phoshpate), lactose in people with lactase deficiency, failure to absorb most SCFAs, sorbitol from chewing gum.
: osmotic diarrhea usually stops when fasted stool osmolality > than 2 x (Na+ + K+).
– blood or plasma leaking from ulcerated mucosa produce bloody diarrhea. Proximal gut leakage will cause less fluid loss than distally gut leakage.
(4) Reduced mucosal contact time
– rapid GI transit due to increased motility will ß time fluid is in contact with absorbing mucosa. Less efficient fluid absorption results in increased fluid content in the stool.
intestinal resections/bypasses caused by disease or surgical operation can result in less mucosa being in contact with luminal fluid.
Examples of Mixed Mechanisms:
– small intestinal mucosal inflammation, partial shortening of villi