The intestine is divided into two parts: Large and Small. The Small Intestine is what we think of when we hear the word intestine: a collection of looped tubes, passing feces to the anus. However, the Small Intestine is actually at the top of the chain; it connects to the Stomach and passes food along while digesting fats, proteins, and carbohydrates. The Large Intestine has a larger diameter but is shorter, and absorbs vitamins and water while compacting wasted into the rectum, where it waits for extrection. Both intestines are lined with powerful muscles that move material along by Peristalsis, or muscle contration in series.
Intestinal Flora are the largest collection of cells in the body. Composed primarily of Anaerobes, or non-oxygen-using cells, Intestinal Flora is responsible for breaking down and releasing parts of food and nutrients that cannot be harvested by the stomach flora. In the Large Intestine, the flora is mainly involved in breaking down undigested fiber, releasing and producing vitamins, and creating Cross-Reactive Antibodies, which protect the vulnurable anal opening against outside infection. Without these flora, fiber would continue to the rectum and cause constipation, vitamins would be excreted with waste instead of absorbed, and any bacteria entering the body via the anus would be able to attack from inside.
Intestinal Flora is difficult to culture and as a result not all species have been identified. They are an example of Friendly Bacteria living in symbiosis inside the human body. Antibiotics and chemotherapy can inadvertantly kill off large quantities of Intestinal Flora, leaving the body open to digestion issues and infection.