What pathologic processes can cause diarrhea?
Excessive stool water is due to the presence of some solute that osmotically obligates water retention within the lumen. This solute can be some poorly absorbed, osmotically active substance, such as magnesium ions, or can be an accumulation of ordinary electrolytes, such as sodium or potassium, that normally are absorbed easily by the intestine. When excess stool water is due to ingestion of a poorly absorbed substance, the diarrhea is called osmotic diarrhea. Examples of this include lactose malabsorption and diarrhea induced by osmotic laxatives. When the excessive stool water is due to the presence of extra electrolytes resulting from reduction of electrolyte absorption or stimulation of electrolyte secretion, the diarrhea is known as secretory diarrhea. Causes of secretory diarrhea include infection, particularly infections that produce toxins that reduce intestinal fluid electrolyte absorption; reduction of mucosal surface area resulting from disease or surgery; absence of an ion transport mechanism; inflammation of the mucosa; ingestion of drugs or poisons; endogenous secretagogues such as bile acids; dysfunction caused by abnormal regulation by nerves and hormones; and tumors producing circulating secretagogues.