After spending 3 to 10 hours in the large intestine, chyme loses a lot of water and becomes feces, the final product of digestion. Feces consist of undigested dietary fiber such as cellulose, mucus, sloughed-off epithelial cells, and microbes. The descending and sigmoid colon stores feces and uses haustral contractions to dry it out but retains enough water to give it a semi-solid texture.

The mass peristalsis then pushes the feces into the rectum, which stretches the rectal walls to activate the stretch receptors. This results in the defecation reflex, an involuntary response provoked by distention of the rectum.

The myenteric plexus triggers the short reflex to stimulate contractions in the sigmoid colon and rectum. At the same time, the parasympathetic motor neurons in the sacral region activate the long reflex to enhance mass peristalsis. Both reflexes work together to allow the feces to move into the anal canal by relaxing the internal anal sphincter.

Finally, the pudendal nerves of the somatic nervous system relax the skeletal muscles of the external anal sphincter for voluntary expulsion of feces.