The Digestive System
The primary function of the alimentary tract is to absorb nutrient molecules from digested foods. Once food has been taken in, churned and broken down into tiny bits of organic nutrients by enzymes, it can be absorbed into the body. 90 percent of which is absorbed in the small intestine, which lies between the stomach and large intestine.
The small intestine has three parts: the duodenum, where most digestion occurs, and the jejunum and the ileum, where most absorption takes place.
Digested food, or chyme, moves rapidly through the small intestine while it is being absorbed before it empties into the large intestine at the ileoceal valve. The journey through the large intestine is where water is reabsorbed from the watery chyme left after nutrients have been absorbed in the small intestine. This turns the slushy contents into firmer feces, which are made at the rear end of the large intestine. The feces consist of undigested cellulose fibers from plant-based foods, also known as roughage, and of the bacteria that thrive in the large intestine. These bacteria make up about 30 percent of the dry weight of the feces. Feces in the last part of the intestines - the rectal region - are stored until a convenient moment. During defecation, the sphincter that closes off the anal canal is relaxed under voluntary control, and the feces are voided.