Pathophysiology of Acute Diarrhea and Absorption Flashcards
Increased volume and weight of daily stool
diarrhea
Diarrhea can result when the maximum colonic absorptive capacity of __liters a day is exceeded .
4
T/F Reduction of net water absorption by as little as 1% may be sufficient to cause diarrhea, and thus even relatively modest compromise of absorptive function can lead to loose stools.”
T
We arbitrarily define diarrhea as being either acute or chronic based upon a duration of ____ weeks.
4
The incidence and rank order of etiologies of acute and chronic diarrhea are dependent upon what three factors?
patient age, socioeconomics, geography
Can the gut alter the osmolarity of its luminal contents?
NO
How does the Osm of stool fluid compare to that of blood?
= b/c gut cannot concentrate fluid
Two most prevalent solutes in our blood
NaCl, KCl
The osmolarity of blood may be closely approximated by what formula?
Osm = 2[Na+K]
What does an increase in Osm gap mean?
there are other soluble substances in teh stool (eg if taking a laxative or if undigested carbs are in the stool)
What is the osmolar gap?
measured Osm - calculated Osm –> exists because 2[Na+K] doesn’t account for all solutes
What is a normal osmolar gap?
<50mOsm
The Osm of stool =
Osm blood = 290mOsm
What happens if an increased osmotic load is presented to the colon?
increased out put will occur to maintain blood osm=stool osm
How can viral gastroenteritis cause diarrhea?
damage to the brush border can impair carb absorption/digestion, increasing osmotic load
Water absorption in colon surface vs crypt
crypt pumps cl/water out and surface absorbs Na/water –> net = fluid absorption
Cryptosporidium mechanism of diarreah
injury of surface cells in small bowel can lead to loss of mucin, brush borders, and cytoplasm loss resulting in diarrhea
Celiac disease mechanism of diarrhea
may cause malabsorption and diarrhea due to loss of small bowel surface area. In this disease, a component of wheat (gliadin) binds to the surface eneterocytes and this complex is viewed as foreign by the patient’s immune system. Cellular mediated immunity comes into play and there is a infiltration of lymphocytes which attack and kill the surface epithelium. The base of the crypts, where the proliferative stem cell compartment resides, responds to the loss of the surface by increases the mitotic activity of the crypt cells. Thus, this disease is characterized by 1) villous blunting, 2) crypt hyperplasia, 3) intra-epithelial lymphocytes.
Dx of celiac sprue
SB biopsy (villous atrophy, crypt hyperplasia, intraepithelial lymphocytosis), anti gliadin antibodies, response to gluten free diet, iron deficiency anemia+dirreha+malabsorption
Can Short Bowel cause diarrhea?
yes –> surgical, trauma, crohn’s fistula/stricture
Ulcerative colitis diarrhea
Inflamed colonic surface -> leaky gut. May lead to a bloody diarrhea.
Interestingly, some patients with ulcerative colitis even when the mucosa is not terribly inflamed may have diarrhea. In this situation, there may be two mechanisms:
The colon, in addition to the terminal ileum, takes up bile acids by specific transporters. These surface transporters are actually down regulated in the surface colon cells in ulcerative colitis by unknown factors.
In the inflamed state, cytokines are released which may effect secretion of electrolytes and motilty.
C. difficile Colitis and diarrhea
Toxins are liberated which cause epithelial injury and death in the colon. Interestingly, despite the cell death, bleeding does not occur. These patients present with mucoid, watery diarrhea.
Lymphocytic Colitis and diarrhea
Surface damage due to intra-epithelial lymphocytes. –> Lymphocytic colitis is a rare cause of diarrhea but is important because 1) it illustrates the effect of damaging the surface of the colon in a subtle, non-ulcerative manner. 2) on inspection of the colon by endoscopy, the surface may appear “normal.” Thus, all that appears to be normal may not be so!
Melanosis Coli: Iatrogenic Diarrhea
Melanosis coli or “brown bowel” is caused by the use of certain types of laxatives. The mechanism of action involves, in part, surface epithilial damage of the colon leading to decreased ability to absorb water. With continued use, a melanin like pigment (which is brownish in color) accumulates in the lamina propria macrophages (arrows). These brown cells may be seen on colon biopsy and when of significant numbers may give the surface of the colon a brown color which may be seen on endoscopic examination and gross inspection of the colon.
E.coli enterotoxin effects on diarrhea
luminal toxin –> guanylyl cyclase C –> increase in cGMP –> activates PKG –> opens channels/inhibits Na/H and Cl/HCO3 exchangers –> HCO3-rich secretion
Cholera toxin effects on diarrhea
Cholera toxin activates the adenylate cyclase enzyme in cells of the intestinal mucosa leading to increased levels of intracellular cAMP, and the secretion of H20, Na+, K+, Cl-, and HCO3- into the lumen of the small intestine.
2 mechanisms of neuroendocrine tumors and diarrhea
- Stimulating ion secretion causing a net water flux out into the gut lumen. VIP (vasoactive intestinal polypeptide) secreting tumors exemplify this mechanism.
- Gastrin secreting tumors may cause massively increased gastric acid out put due to stimulation of the parietal cells. This acid dumps into the duodenum and may create an acid Ph which inactivates pancreatic enzymes. Thus, the fats and proteins will not be broken down for absorption.
