The colon, which connects the small intestine to the rectum and anus, absorbs water, nutrients, and salts from the partially digested food it receives from the small intestine. A few times each day, strong muscle contractions move down its length, pushing the waste matter towards the anus. Coordinated muscle movements then help the colon expel the waste from the body.
The colon, which is about five feet long, connects the small intestine with the rectum and anus. The major function of the colon is to absorb water, nutrients, and salts from the partially digested food that enters from the small intestine. Two pints of liquid matter enter the colon from the small intestine each day. Stool volume is a third of a pint. The difference in volume represents what the colon absorbs each day.
As you chew your food and swallow, the food travels through your esophagus -- which is the tube that connects your mouth to your stomach. In the stomach, strong acids and enzymes digest, or break down, the food into small particles. These particles are called proteins, fats, and carbohydrates.
After leaving your stomach, these food particles enter the small intestine. This is a long, continuous tube that slowly contracts, or squeezes, to push the food along through it. As the small intestine continues to digest your food, it absorbs nutrients that your body uses for energy, growth, and repairs. By the time the food reaches the end of the small intestine, almost all of its nutrients have been absorbed. At this point, what's left of the food is mostly water and indigestible waste products.
At the end of the small intestine, the material then enters the large intestine, or colon, which is also a long tube. Its main job is to remove water from the waste products as they pass through and recycle this water back to your body.
After traveling through this area, the waste is held at the end of the colon in the rectum. It will then leave your body through the anus as stool when you have a bowel movement. The firmness of the stool will depend on how long it has been in your colon. If the stool moves through quickly, it will be more watery. But if it moves through too slowly, you can become constipated. This is because the longer stool stays in the colon, the more water is removed from it.
Colon motility (the contraction of the muscles and the movement of the contents) is controlled by nerves and hormones and by electrical activity in the muscle. Contractions move the contents slowly back and forth, but mainly toward the rectum. During this passage, water and nutrients are absorbed into the body. What remains is stool.
A few times each day, strong muscle contractions move down the colon, pushing the stool ahead of them. Some of these strong contractions result in a bowel movement. The muscles of the pelvis and anal sphincters have to relax at the right time to allow the stool to be expelled. If the muscles of the colon, sphincters, and pelvis do not contract in a coordinated way, the contents do not move smoothly, resulting in abdominal (stomach) pain, cramps, constipation or diarrhea, and a sense of incomplete bowel movement.