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By the time food reaches the colon from the small intestine, it has been completely digested. All of the nutrients that will be absorbed from it have already been absorbed into the body. The colon's main job is to absorb almost all of the remaining water and to store the leftover contents until the time of defecation.
Lawrence Friedman, Gastroenterology, answeredAfter a journey through the rest of the digestive system, what's left of the food arrives in the colon, or large intestine, a four-foot-long muscular tube about the diameter of your fist, where the walls act like a sponge and soak up 80% to 90% of the remaining water. In fact, the colon accepts about a quart of liquid from the ileum each day. Once inside the colon, food residue travels up the right side (the ascending colon), across the transverse colon, down the left side (the descending colon), through the sigmoid colon to the rectum (behind the left side of the groin), and out of the body. The time required for food to move through the colon varies widely, but is generally in the range of four to 72 hours.
Bacteria that reside in the colon help in the digestive process, feeding off whatever remains of your meal. The bacteria produce fatty acids as well as hydrogen, carbon dioxide, and, in some people, methane gas. Some of these gases are consumed as nutrients by the cells of the colon, while others are expelled as waste. Undigested matter, such as fiber, is propelled along by contractions of the colon wall and settles as solids in the rectum, the final six inches of the colon.
The end of the rectum is guarded by sphincter muscles that help control what goes out. The waste accumulates until the rectal wall becomes so distended that it signals the internal anal sphincter to relax, triggering the urge for a bowel movement. Fortunately, the external anal sphincter, which is under voluntary control, keeps the rectal contents in place until a convenient time.
What comes out is primarily water and colon bacteria, plus bile (fluid secreted by the liver), mucus, and cells normally shed from the intestinal lining. Undigested food makes up very little of the average quarter- to half-pound stool. The exception is fiber: The more fiber you ingest, the greater the quantity of your stool.