what a good question! it sent me to pages 1324-5 of pathophysiology: the biologic basis for disease in adults and children
, 3rd edition, by kathryn l. mccance and sue e huether and pages 226-7 of fluid and electrolyte balance: nursing considerations
, 4th edition, by norma m. metheny. ms. heuther who wrote that particular chapter in the book states that there are three major causes of diarrhea: osmotic, secretory and motile. i know you are interested in the osmotic causes, but let me also give you some information about the other two as well.
(1) secretory diarrhea is primarily due to bacterial enterotoxins from cholera, e.coli and tumors that secrete hormones that stimulate mucosal secretions in the intestine. large volume diarrhea results and it is due to fluid and electrolytes that are secreted by the mucous cells in the intestinal tract. it also results in sodium being prevented from being absorbed thus resulting in its loss as the fluids accumulate in the bowel and eventually expelled from the body.
(note: fluids that accumulate in the intestine are considered fluid losses because they do not get reabsorbed back into the body and have only one place left to go--out the front or back end (vomiting or diarrhea). in bowel obstructions, in particular, you will find electrolyte imbalances due to fluid losses. the fluid losses come from fluids that have accumulated in the intestine from a shut down of peristalsis.)
(2) motile diarrhea is where the contents of the intestinal tract are just moved along much more rapidly than normal so that the reabsorption of water doesn't have time to occur. as a result, there can be problems with nutrient absorption and digestion if this occurs over long periods of time. the reasons this occurs is due to shortening of the length of the intestine as a result of surgery, lesion(s) or tumors that interrupt the autonomic control of bowel motility, fecal impactions that can block or cause an impasse to intestinal fluids and aggravate peristalsis resulting in small volume diarrhea, inflammatory bowel disease, and collagen vascular disease.
and, what you want to know about, (3) osmotic diarrhea is due to substances that are just poorly absorbed in the intestine and cause more fluids to be drawn into it by osmosis. ms. heuther states that lactase deficiency is the most common cause of diarrhea due to osmosis. when milk sugar, or lactose, remains in the intestine and is not absorbed, it draws more fluid into the bowel by osmosis which leads to fluid build up and resulting diarrhea. other nonabsorbable substances that will cause osmotic diarrhea are sorbitol, mannitol, magnesium sulfate, magnesium hydroxide and sodium phosphate. remember from chemistry that osmosis is the movement of fluid across a semi-permeable membrane from an area of low concentration to one of high concentration in an attempt to equalize the concentrations of the solutions on both sides of the membrane.
lactulose, or cephulac, is a commonly prescribed drug used as a laxative. it is a synthetic lactose derivative. nonabsorbable, once ingested it works it's laxative effects by pulling fluid into the large intestine by osmosis. if you've ever munched on more than just a couple of the sugarless candies you will have a good understanding of how this laxative effect works since sorbitol is used as the sweetener in them! and, you usually don't make the mistake of eating so many a second time!
with diarrhea, a person can develop fluid volume losses of up to 10 liters a day, metabolic acidosis (especially in pediatric patients) because of the loss of the primarily alkaline fluid (due to bicarbonates) of the intestine, hypokalemia, sodium imbalances and hypomagnesaemia when there is prolonged diarrhea.
hope that gives you the information (and more) that you were looking for.
welcome to allnurses!