How the Digestive System Works

The Large and Small Intestines

David Scharf/Science Faction/Getty Images

Your now unidentifiable sandwich squirts into the duodenum, the first part of the small intestine. The breakdown process continues with enzymes from the pancreas and bile from the liver. Again, peristalsis helps mix up these juices. The next small intestine section is the coiled jejunum, followed by the ileum, which leads straight to the large intestine. These two sections absorb nutrients and water more than they break down food.

The small intestine has a smaller circumference than the large intestine, but it's actually the longer of the two sections -- it has the surface area of a tennis court! You may wonder how all this fits into your body. The answer is simple: The surface of the small intestine has many tight folds that can absorb nutrients and water -- they greatly increase the surface area. These folds are covered with villi, or tiny projections that have even smaller microvilli on them. Villi and microvilli have affinities for specific nutrients. That means that several different kinds of villi will grab the nutrients, electrolytes and dietary molecules in your sandwich (like the carbohydrates and folic acid from the bread, protein and sodium from the ham, and calcium and vitamin B12 from the cheese). The absorbed nutrients move through the wall of the intestines and into blood vessels that take them throughout the body.


Once all the good stuff is taken from the food, the indigestible parts are transported into the large intestine, the final stretch of the digestive process. The large intestine absorbs extra fluid to produce the solid waste we know as feces. To move the waste, the colon uses the same involuntary muscular movements that we learned about earlier. Unlike the stomach and small intestines, though, whose movements take a matter of hours, it takes days for waste to move through the large intestine -- the waste moves at a pace of about 1 centimeter per hour [source:].

The large intestine has three main parts. First is a pouch called the cecum. (The cecum is home to the appendix, the small fingerlike pouch that can become inflamed and extremely painful in some people.) Next comes the colon, which has three sections: ascending, transverse and descending. In the first two sections, salts and fluids are absorbed from the indigestible food. Billions of bacteria that normally live in the colon help to ferment and absorb substances like fiber. While these tracts absorb, they also produce mucus that helps feces move easily through the descending colon and into the third part of the large intestine: the rectum. Here, your feces wait to be excreted through the anus in your next bowel movement.

While your sandwich moves through the system, other organs, glands, hormones and nerves are pitching in as well. We'll find out what they do and why you should care.