begins acutely, lasts less than 14 days and involves passage of frequent watery stools without visible blood. Acute diarrhea causes dehydration and when food intake is reduced it contributes to malnutrition.
Normally absorption and secretion of water and electrolytes occur through out the intestine. About 90% in small intestine and 10% in the large intestine, leaving 5-10gm/kg being excreted in formed stools.
Water and electrolytes are simultaneously absorbed by the villi and secreted by the crypts. “two-directional flow”
Any change in the two-directional flow of water and electrolytes in the small intestine results in either reduced net absorption or actual net secretion and causes increased volume of fluid to enter the large intestine. When this volume exceeds the limited absorptive capacity of the large bowel, diarrhea occurs.
After being absorbed Na is transported out of the epithelial cells by the Na K ATPase pump this transfers sodium into the ECF elevating its osmolality and causing passive flow of water and other electrolytes from the small bowel lumen through intercellular channels into the ECF
Secretion of water and electrolytes normally occurs in the crypts of the small bowel epithelium where NaCl is transported from the ECF into the epithelial cells across the basolateral membrane. The Sodium is then pumped back into the ECF by the Na K ATPase, while the Chloride ion pass through the luminal membrane of the crypt cells into the bowel lumen, followed by water passively
Secretory diarrhea: is caused by abnormal secretion of water and electrolytes into the small bowel lumen. This occurs when the absorption of Sodium by the villi is impaired while the secretion of Chloride in the crypt cells continues or is increased. The net result is fluid secretion which leads to loss of water
In infectious diarrhea secretory effect on the bowel mucosa can result from bacterial toxins like E.coli, and Vibrio cholerae OR viruses such as rotavirus.
Osmotic diarrhea: The small bowel mucosa is a porous epithelium across which water and electrolytes move rapidly to maintain osmotic balance between the bowel contents and the ECF. Diarrhea can occur when a
Poorly absorbed osmotically active substance is ingested, if the substance is isotonic solution the water and solute will simply pass through the gut unabsorbed causing diarrhea like MgSO4(used as purgative), Lactose or glucose. When the substance taken as hypertonic solution water will move from ECF into gut lumen until osmolality of intestinal contents and ECF is equalized.