The motility of the small intestine gradually pushes its contents through another sphincter—the ileocaecal sphincter—and into the large intestine, or colon. Most of the available nutrients have been removed from the chyme that enters the colon, but it contains a lot of water and inorganic ions. A major type of motility in the colon is called segmentation movements. These movements are mostly due to contractions of circular muscles that repeatedly divide the colon into separate segments. Functionally, segmentation movements cause the chyme to slosh around in the colon and promote absorption of ions and water by increasing contact between the chyme and the epithelium of the colon. As ions and water are absorbed, the chyme becomes a semisolid mass that forms feces. Absorption of too much water from the colon can cause hard feces and constipation. The opposite condition, diarrhea, results if too little water is absorbed. Excessive diarrhea caused by diseases such as cholera can produce such rapid loss of water and electrolytes that death can occur in hours. Simple rehydration therapy with electrolytes can save lives.

Fecal matter is stored in the descending colon and in the rectum until defecation occurs. This is usually once a day and is preceded by strong peristaltic activity. The distension of the walls of the rectum by fecal matter initiates a parasympathetic reflex, causing the rectal muscle to contract and the internal anal sphincter to relax. In addition, there is an external anal sphincter that is under conscious control so that defecation is not entirely an involuntary act.