GI TBL Notes
From Iusmphysiology
Contents |
[edit] Pathophysiology of diarrhea
- Diarrhea can indicate gastrointestinal disease or primary disorders outside the GI tract.
- Small or large bowel can induce diarrhea.
- 2 million people die of diarrhea each year--mostly children.
- There are four basic mechanisms: osmotic diarrhea, secretory diarrhea, inflammatory / infectious diarrhea, and deranged motility darrhea.
[edit] Osmotic diarrhea
- Caused by excessive amounts of solutes in the lumen which keep water from being absorbed.
- Characterized by arrest of diarrhea upon fasting or ceasing to injest the causative agent.
- Can occur because of poorly absorbed solute like a carbohydrate or a divalent ion.
- Think mannitol, sorbitol, epson salt and some antacids.
- Can occur because of malaborption.
- Inability to absorb is the the common deficit in osmotic diarrhea.
- Think of lactose intolerance.
[edit] Secretory diarrhea
- Water is normally reabsorbed before reaching the large intestine; diarrhea results if reabsorption is not achieved.
- Cholera toxin from Vibrio cholerae activates adenylyl cyclase, increases cAMP levels in enterocytes, which results in opening of chloride channels.
- Water follows the chloride ion to cause water loss.
- Cholera toxin also causes the enteric nervous system to stimulate secretion.
- Other agents of secretory diarrhea:
- laxatives
- hormones like VIP (from tumors)
- many different drugs
- metals, organic toxins, and plant products
- Secretory diarrhea does not usually resolve during a 2-3 day fast.
[edit] Inflammatory and infectious diarrhea
- Pathogens can interrupt the GI epithelium and thus cause diarrhea.
- Destruction leads to blood and serum exudation into the lument.
- Destruction leads to poor water absorption.
- Destruction leads to white blood cell activation; they can release cytokines which can increase secretion.
- Destruction leads to ROS which kill epithelial cells such that new, young cells are generated; these cells are bad at absorbing nutrients and therefore aid in generation of osmotic diarrhea.
- Classic agents:
- Bacteria like Salmonella, E. coli, and Campylobacter
- Viruses like rotaviruses, coronaviruses, parvoviruses, and norovirus
- Protozoa like coccidia, Cryptosporium, and giardia.
[edit] Deranged motility diarrhea
- Disorders that affect motility often move food along too quickly for proper absorption and thus lead to osmotic diarrhea.
- It is hard to tell if increased motility causes diarrhea or is an effect of diarrhea.
[edit] Absorption of water and electrolytes
- Diet contains 1-2 L of water / day; body secretes 6-7 L / day.
- There is a tight coupling between water and solute absorption.
- The two main ways the GI tract epithelial cells absorb Na are by coupling it with glucose and by coupling it with aa transport.
- And the main way the GI tract epithelial cells absorb H20 is by absorbing Na and letting water follow through the epithelial cells (mostly) or (a little) through tight junctions between epithelial cells.
- Absorbed Na is quickly pumped out into the intercellular space--that is, between epithelial cells, in the ECF, thus generating a high osmolarity.
- We call water absorption from the lumen into the blood a three compartment model.
- The ability of the gut to reabsorb water (Na) is different at any given location:
- Upper SI is not so good at absorption which makes sense b/c the body is still secreting stuff in order to neutralize the chyme.
- The ileum and especially the colon are much more capable of absorbing water--even against an osmotic gradient of several hundred milliosmols.
