To understand constipation, it helps to know how the colon, or large intestine, works. As food moves through the colon, the colon absorbs water from the food while it forms waste products, or stool. Muscle contractions in the colon then push the stool toward the rectum. By the time stool reaches the rectum it is solid, because most of the water has been absorbed.
Constipation occurs when the colon absorbs too much water or if the colon's muscle contractions are slow or sluggish, causing the stool to move through the colon too slowly. As a result, stools can become hard and dry. Common causes of constipation are
- not enough fiber in the diet
- lack of physical activity (especially in the elderly)
- medications
- milk
- irritable bowel syndrome
- changes in life or routine such as pregnancy, aging, and travel
- abuse of laxatives
- ignoring the urge to have a bowel movement
- dehydration
- specific diseases or conditions, such as stroke (most common)
- problems with the colon and rectum
- problems with intestinal function (chronic idiopathic constipation)
Source :
http://digestive.niddk.nih.gov/ddiseases/pubs/constipation/