defecation

Defecation is the final step of digestion in which feces are expelled from the body through the anus. In humans, stool forms as the colon absorbs water and electrolytes from indigestible matter, then is stored in the rectum until a defecation reflex prompts the act. The process involves the rectum and anal canal, including two sphincters: the internal anal sphincter (involuntary) and the external anal sphincter (voluntary), as well as the pelvic floor muscles and the surrounding tissues that control anal opening.

During defecation, rectal stretch receptors trigger a reflex that relaxes the internal anal sphincter and causes the rectum to contract to move stool toward the anal canal. The external sphincter can be voluntarily relaxed to allow passage, sometimes with abdominal muscles increasing intra-abdominal pressure. A coordinated action by the pelvic floor and abdominal muscles allows stool to pass; the process ends with the stool exiting via the anus.

Frequency and stool form vary; normal bowel habits range roughly from three times a day to three times a week, influenced by diet, hydration, physical activity, and medications. Stool consistency is described on the Bristol Stool Chart (types 3-4 commonly considered normal).

Common disorders include constipation (difficulty passing stool), fecal incontinence (involuntary leakage), diarrhea (loose stool), fecal impaction, and hemorrhoids. Most issues are manageable with dietary fiber, fluids, exercise, and regular toilet routines; persistent symptoms, blood in stool, weight loss, or severe pain warrant medical evaluation.