Diarrhea is very frequent and/or watery and loose bowel movements. True diarrhea occurs when an excess of water remains in the stool. This is different from conditions that cause only an increase in the number of bowel movements per day, but that do not result in loose, watery stools.
The colon, also known as the large intestine, controls the formation and passage of solid waste (stool) from the body. The colon absorbs water from solid waste and uses muscles to move the waste along toward the rectum and anus, for passage out of the body.
If too little water is absorbed from the waste as it passes through the colon, diarrhea can result. Another cause of diarrhea may be changes in the muscle contractions of the colon, so that stool moves through much too quickly. This quick passage prevents sufficient time for the water to be absorbed.
Diarrhea can be caused by infections, medications, or damage to the digestive tract.