egestion

Egestion is the biological process by which undigested or unabsorbed material is expelled from the digestive tract. In most animals, egestion results in the discharge of feces through the anus and is considered the final step in the processing of a meal. Some sources distinguish egestion from defecation, using egestion to refer to the removal of indigestible matter from the gut, while defecation is the act of expelling that material from the body.

Mechanism and control involve coordinated gut motility. Peristaltic movements transport the contents toward the rectum, and stretch receptors trigger defecation reflexes. In many vertebrates, the internal anal sphincter relaxes involuntarily and the external anal sphincter is under voluntary control, allowing conscious initiation of defecation.

Distinction from excretion is important. Excretion refers to the elimination of metabolic wastes produced by the body's cells, such as urea, bile pigments, and excess minerals, often via the kidneys, liver, lungs, and skin. Egestion, by contrast, deals specifically with non-digested portions of a meal and the bodily expulsion of solid waste from the digestive tract.

In broader biology, egestion can be described across different organisms. In single-celled organisms and some invertebrates, undigested material may be expelled by exocytosis or through a dedicated opening. In humans and many mammals, egestion culminates in bowel movements that reflect dietary intake, gut transit time, and overall digestive health. Abnormalities in egestion, such as constipation or diarrhea, can indicate disruptions in gut function or disease.