Diarrhea and Vomitting
The GIT produces large volumes of isotonic fluid to assist digestion and absorption of food
(±8 litres/day in an adult)
Note: isotonic fluid having the same tonicity or osmotic pressure as blood.
This fluid (and accompanying electrolytes) is ‘borrowed’ from the ECF and is later returned.
This ‘borrowed’ fluid (along with electrolytes) is not returned in 2 situations:
1.) Vomiting
2.) Diarrhea
The result/consequence of these thieves of the body is the following:
- Loss of water
- Loss of important ions, including: Na+, K+, Cl-, HCO3
Which ions are lost depends on?
This depends on the nature of the clinical presentation:
- diarrhoea loses HCO3-, Na+ and K+ → acidosis
- vomiting loses H+, Cl- and K+ → alkalosis
, The different types of diarrhea:
There are 4 main pathophysiological mechanisms of diarrhea:
1.) Osmotic Diarrhea
2.) Secretory Diarrhea
3.) Dysmotility Diarrhea
4.) Exudative Diarrhea
1.) Osmotic Diarrhea:
- Ingestion of poorly absorbed sugars or alcohols (e.g. mannitol, sorbitol) or ions (as found in
laxatives - magnesium, sulphate, and phosphate) leads to osmotic diarrhea
- Since the intestines cannot maintain an osmotic gradient, these unabsorbed ions in the
intestinal lumen cause retention of water to maintain an intraluminal osmolality equal to
that of body fluids (about 290 mOsm/kg) (i.e. water does not get absorbed but
instead remains or gets retained in the lumen)
- The diarrhea disappears with fasting or cessation of ingestion of the
offending substance
Examples
o Lactose intolerance, results due to deficiency of disaccharidase lactase
Lactulose, a synthetic disaccharide cannot be hydrolyzed by the human
intestine and cannot be absorbed intact in greater than trace amounts (thus
causing increased intraluminal osmolality).
After prolonged periods of bowel rest, mucosal atrophy develops resulting
in malabsorption and diarrhea
o Fat malabsorption
Hydrolysed to fatty acids (ions) by colonic bacteria
2.) Secretory Diarrhea:
- Net secretion of Cl- or HCO3- or inhibition of net Na+ absorption is the mechanism for this
diarrhea
Causes:
- The most common cause for secretory diarrhoea is infection
- Infectious agents (bacteria, parasites and viruses) produce enterotoxins
that interact with receptors and lead to increased anion secretion (e.g.
this is the case of cholera toxin – see biochem notes).
Or these enterotoxins may block specific absorptive pathways (e.g. Na-H exchange)
- Peptides produced by endocrine tumours such as vasoactive
intestinal peptide or calcitonin cause secretory diarrhea by
stimulating secretion by epithelial cells
- Significant loss of surface area of intestines (e.g. after resective
surgery, inflammatory bowel disease), may compromise water
absorption and cause diarrhea
The GIT produces large volumes of isotonic fluid to assist digestion and absorption of food
(±8 litres/day in an adult)
Note: isotonic fluid having the same tonicity or osmotic pressure as blood.
This fluid (and accompanying electrolytes) is ‘borrowed’ from the ECF and is later returned.
This ‘borrowed’ fluid (along with electrolytes) is not returned in 2 situations:
1.) Vomiting
2.) Diarrhea
The result/consequence of these thieves of the body is the following:
- Loss of water
- Loss of important ions, including: Na+, K+, Cl-, HCO3
Which ions are lost depends on?
This depends on the nature of the clinical presentation:
- diarrhoea loses HCO3-, Na+ and K+ → acidosis
- vomiting loses H+, Cl- and K+ → alkalosis
, The different types of diarrhea:
There are 4 main pathophysiological mechanisms of diarrhea:
1.) Osmotic Diarrhea
2.) Secretory Diarrhea
3.) Dysmotility Diarrhea
4.) Exudative Diarrhea
1.) Osmotic Diarrhea:
- Ingestion of poorly absorbed sugars or alcohols (e.g. mannitol, sorbitol) or ions (as found in
laxatives - magnesium, sulphate, and phosphate) leads to osmotic diarrhea
- Since the intestines cannot maintain an osmotic gradient, these unabsorbed ions in the
intestinal lumen cause retention of water to maintain an intraluminal osmolality equal to
that of body fluids (about 290 mOsm/kg) (i.e. water does not get absorbed but
instead remains or gets retained in the lumen)
- The diarrhea disappears with fasting or cessation of ingestion of the
offending substance
Examples
o Lactose intolerance, results due to deficiency of disaccharidase lactase
Lactulose, a synthetic disaccharide cannot be hydrolyzed by the human
intestine and cannot be absorbed intact in greater than trace amounts (thus
causing increased intraluminal osmolality).
After prolonged periods of bowel rest, mucosal atrophy develops resulting
in malabsorption and diarrhea
o Fat malabsorption
Hydrolysed to fatty acids (ions) by colonic bacteria
2.) Secretory Diarrhea:
- Net secretion of Cl- or HCO3- or inhibition of net Na+ absorption is the mechanism for this
diarrhea
Causes:
- The most common cause for secretory diarrhoea is infection
- Infectious agents (bacteria, parasites and viruses) produce enterotoxins
that interact with receptors and lead to increased anion secretion (e.g.
this is the case of cholera toxin – see biochem notes).
Or these enterotoxins may block specific absorptive pathways (e.g. Na-H exchange)
- Peptides produced by endocrine tumours such as vasoactive
intestinal peptide or calcitonin cause secretory diarrhea by
stimulating secretion by epithelial cells
- Significant loss of surface area of intestines (e.g. after resective
surgery, inflammatory bowel disease), may compromise water
absorption and cause diarrhea
