It is assumed that the reader is familiar with general physiological osmotic concepts.
Diarrhea results mainly from excess fecal water, water that makes up 60 to 90% of the mass of the stool. Typically, about 9 liters of fluid enters the GI tract per day, and 1 L reaches the colon. The usual uptake of the colon is 0.8 L/day, but the colon can recover up to four times that volume. The stool weight of the typical adult is 100 to 300 grams per day, and diarreah may occur when the stool weight is more than 200-300 g/day; however, this does not tend to happen when that is the normal weight. It is important to note that chronic diarrhea is most commonly caused not by excessive deposition of fluid into the bowels, but by failure of the bowels to take up fluid that exists within their lumena. In the following, we will refer to loss and failure to take up without distinction.
Of course, excess loss of water may cause dehydration. Loss of water alone causes a concentration of the intra- and extracellular fluids, and this is often the case in acute diarrhea. However, we assume that a patient with chronic diarrhea has adapted to the necessary intake of water, or has physiologically adapted to having a higher mean fluid concentration; we will therefore not discuss dehydration further.
Loss of fluid which does not result in dehydration usually results in electrolytic imbalance, since ingestion of isoosmolar fluid is unusual in a chronic condition. The colonic epithelium takes in sodium in most situations (then passing it through to the serosa). In healthy patients, the colon is quite efficient at conserving sodium and water, and even works well when the small intestine alone is unable to maintain a sodium balance. Typically, this fluid is mostly absorbed in the ascending and transverse colon, which has an approximate transit time of 15 hours. Only a small amount is absorbed in the descending colon, which moves matter through in only 3 hours. Diarrhea often occurs by shortening the longer time, but may occur with a hyperactive descending colon as well.
Sodium is not the only electrolyte lost in chronic diarrhea. Commonly also lost are potassium, magnesium, and chloride. Bicarbonate loss can also occur, and may cause metabolic acidosis. Hypmagnesemia may cause tetany. In some cases, feces may have higher than normal mucus content, exacerbating hypokalemia.